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October 6, 2008

14 min read

The Physical Science behind Climate Change

Why are climatologists so highly confident that human activities are dangerously warming Earth? Members of the IPCC, the 2007 peace winner, write on climate change

By William Collins , Robert Colman , James Haywood , Martin R. Manning & Philip Mote

Editor's note: This story was originally posted in the July 2007 issue, and has been reposted to highlight the long history of Nobelists publishing in Scientific American.

For a scientist studying climate change, “eureka” moments are unusually rare. Instead progress is generally made by a painstaking piecing together of evidence from every new temperature measurement, satellite sounding or climate-model experiment. Data get checked and rechecked, ideas tested over and over again. Do the observations fit the predicted changes? Could there be some alternative explanation? Good climate scientists, like all good scientists, want to ensure that the highest standards of proof apply to everything they discover.

And the evidence of change has mounted as climate records have grown longer, as our understanding of the climate system has improved and as climate models have become ever more reliable. Over the past 20 years, evidence that humans are affecting the climate has accumulated inexorably, and with it has come ever greater certainty across the scientific community in the reality of recent climate change and the potential for much greater change in the future. This increased certainty is starkly reflected in the latest report of the Intergovernmental Panel on Climate Change (IPCC), the fourth in a series of assessments of the state of knowledge on the topic, written and reviewed by hundreds of scientists worldwide.

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The panel released a condensed version of the first part of the report, on the physical science basis of climate change, in February. Called the “Summary for Policymakers,” it delivered to policymakers and ordinary people alike an unambiguous message: scientists are more confident than ever that humans have interfered with the climate and that further human-induced climate change is on the way. Although the report finds that some of these further changes are now inevitable, its analysis also confirms that the future, particularly in the longer term, remains largely in our hands—the magnitude of expected change depends on what humans choose to do about greenhouse gas emissions.

The physical science assessment focuses on four topics: drivers of climate change, changes observed in the climate system, understanding cause-and-effect relationships, and projection of future changes. Important advances in research into all these areas have occurred since the IPCC assessment in 2001. In the pages that follow, we lay out the key findings that document the extent of change and that point to the unavoidable conclusion that human activity is driving it.

Drivers of Climate Change Atmospheric concentrations of many gases—primarily carbon dioxide, methane, nitrous oxide and halocarbons (gases once used widely as refrigerants and spray propellants)—have increased because of human activities. Such gases trap thermal energy (heat) within the atmosphere by means of the well-known greenhouse effect, leading to global warming. The atmospheric concentrations of carbon dioxide, methane and nitrous oxide remained roughly stable for nearly 10,000 years, before the abrupt and rapidly accelerating increases of the past 200 years. Growth rates for concentrations of carbon dioxide have been faster in the past 10 years than over any 10-year period since continuous atmospheric monitoring began in the 1950s, with concentrations now roughly 35 percent above preindustrial levels (which can be determined from air bubbles trapped in ice cores). Methane levels are roughly two and a half times preindustrial levels, and nitrous oxide levels are around 20 percent higher.

How can we be sure that humans are responsible for these increases? Some greenhouse gases (most of the halocarbons, for example) have no natural source. For other gases, two important observations demonstrate human influence. First, the geographic differences in concentrations reveal that sources occur predominantly over land in the more heavily populated Northern Hemisphere. Second, analysis of isotopes, which can distinguish among sources of emissions, demonstrates that the majority of the increase in carbon dioxide comes from combustion of fossil fuels (coal, oil and natural gas). Methane and nitrous oxide increases derive from agricultural practices and the burning of fossil fuels.

Climate scientists use a concept called radiative forcing to quantify the effect of these increased concentrations on climate. Radiative forcing is the change that is caused in the global energy balance of the earth relative to preindustrial times. (Forcing is usually expressed as watts per square meter.) A positive forcing induces warming; a negative forcing induces cooling. We can determine the radiative forcing associated with the long-lived greenhouse gases fairly precisely, because we know their atmospheric concentrations, their spatial distribution and the physics of their interaction with radiation.

Climate change is not driven just by increased greenhouse gas concentrations; other mechanisms— both natural and human-induced—also play a part. Natural drivers include changes in solar activity and large volcanic eruptions. The report identifies several additional significant human-induced forcing mechanisms—microscopic particles called aerosols, stratospheric and tropospheric ozone, surface albedo (reflectivity) and aircraft contrails—although the influences of these mechanisms are much less certain than those of greenhouse gases.

Investigators are least certain of the climatic influence of something called the aerosol cloud albedo effect, in which aerosols from human origins interact with clouds in complex ways and make the clouds brighter, reflecting sunlight back to space. Another source of uncertainty comes from the direct effect of aerosols from human origins: How much do they reflect and absorb sunlight directly as particles? Overall these aerosol effects promote cooling that could offset the warming effect of long-lived greenhouse gases to some extent. But by how much? Could it overwhelm the warming? Among the advances achieved since the 2001 IPCC report is that scientists have quantified the uncertainties associated with each individual forcing mechanism through a combination of many modeling and observational studies. Consequently, we can now confidently estimate the total human- induced component. Our best estimate is some 10 times larger than the best estimate of the natural radiative forcing caused by changes in solar activity.

This increased certainty of a net positive radiative forcing fits well with the observational evidence of warming discussed next. These forcings can be visualized as a tug-of-war, with positive forcings pulling the earth to a warmer climate and negative ones pulling it to a cooler state. The result is a no contest; we know the strength of the competitors better than ever before. The earth is being pulled to a warmer climate and will be pulled increasingly in this direction as the “anchorman” of greenhouse warming continues to grow stronger and stronger.

Observed Climate Changes The many new or improved observational data sets that became available in time for the 2007 IPCC report allowed a more comprehensive assessment of changes than was possible in earlier reports. Observational records indicate that 11 of the past 12 years are the warmest since reliable records began around 1850.

he odds of such warm years happening in sequence purely by chance are exceedingly small. Changes in three important quantities—global temperature, sea level and snow cover in the Northern Hemisphere—all show evidence of warming, although the details vary. The previous IPCC assessment reported a warming trend of 0.6 ± 0.2 degree Celsius over the period 1901 to 2000. Because of the strong recent warming, the updated trend over 1906 to 2005 is now 0.74 ± 0.18 degree C. Note that the 1956 to 2005 trend alone is 0.65 ± 0.15 degree C, emphasizing that the majority of 20th-century warming occurred in the past 50 years. The climate, of course, continues to vary around the increased averages, and extremes have changed consistently with these averages—frost days and cold days and nights have become less common, while heat waves and warm days and nights have become more common.

The properties of the climate system include not just familiar concepts of averages of temperature, precipitation, and so on but also the state of the ocean and the cryosphere (sea ice, the great ice sheets in Greenland and Antarctica, glaciers, snow, frozen ground, and ice on lakes and rivers). Complex interactions among different parts of the climate system are a fundamental part of climate change—for example, reduction in sea ice increases the absorption of heat by the ocean and the heat flow between the ocean and the atmosphere, which can also affect cloudiness and precipitation.

A large number of additional observations are broadly consistent with the observed warming and reflect a flow of heat from the atmosphere into other components of the climate system. Spring snow cover, which decreases in concert with rising spring temperatures in northern midlatitudes, dropped abruptly around 1988 and has remained low since. This drop is of concern because snow cover is important to soil moisture and water resources in many regions.

In the ocean, we clearly see warming trends, which decrease with depth, as expected. These changes indicate that the ocean has absorbed more than 80 percent of the heat added to the climate system: this heating is a major contributor to sea-level rise. Sea level rises because water expands as it is warmed and because water from melting glaciers and ice sheets is added to the oceans. Since 1993 satellite observations have permitted more precise calculations of global sea-level rise, now estimated to be 3.1 ± 0.7 millimeters per year over the period 1993 to 2003. Some previous decades displayed similarly fast rates, and longer satellite records will be needed to determine unambiguously whether sea-level rise is accelerating. Substantial reductions in the extent of Arctic sea ice since 1978 (2.7 ± 0.6 percent per decade in the annual average, 7.4 ± 2.4 percent per decade for summer), increases in permafrost temperatures and reductions in glacial extent globally and in Greenland and Antarctic ice sheets have also been observed in recent decades. Unfortunately, many of these quantities were not well monitored until recent decades, so the starting points of their records vary.

Hydrological changes are broadly consistent with warming as well. Water vapor is the strongest greenhouse gas; unlike other greenhouse gases, it is controlled principally by temperature. It has generally increased since at least the 1980s. Precipitation is very variable locally but has increased in several large regions of the world, including eastern North and South America, northern Europe, and northern and central Asia. Drying has been observed in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. Ocean salinity can act as a massive rain gauge. Near-surface waters of the oceans have generally freshened in middle and high latitudes, while they have become saltier in lower latitudes, consistent with changes in large-scale patterns of precipitation.

Reconstructions of past climate—paleoclimate— from tree rings and other proxies provide important additional insights into the workings of the climate system with and without human influence. They indicate that the warmth of the past half a century is unusual in at least the previous 1,300 years. The warmest period between A.D. 700 and 1950 was probably A.D. 950 to 1100, which was several tenths of a degree C cooler than the average temperature since 1980.

Attribution of Observed Changes Although confidence is high both that human activities have caused a positive radiative forcing and that the climate has actually changed, can we confidently link the two? This is the question of attribution: Are human activities primarily responsible for observed climate changes, or is it possible they result from some other cause, such as some natural forcing or simply spontaneous variability within the climate system? The 2001 IPCC report concluded it was likely (more than 66 percent probable) that most of the warming since the mid-20th century was attributable to humans. The 2007 report goes significantly further, upping this to very likely (more than 90 percent probable).

The source of the extra confidence comes from a multitude of separate advances. For a start, observational records are now roughly five years longer, and the global temperature increase over this period has been largely consistent with IPCC projections of greenhouse gas–driven warming made in previous reports dating back to 1990. In addition, changes in more aspects of the climate have been considered, such as those in atmospheric circulation or in temperatures within the ocean. Such changes paint a consistent and now broadened picture of human inter-vention. Climate models, which are central to attribution studies, have also improved and are able to represent the current climate and that of the recent past with considerable fidelity. Finally, some important apparent inconsistencies noted in the observational record have been largely resolved since the last report.

The most important of these was an apparent mismatch between the instrumental surface temperature record (which showed significant warming over recent decades, consistent with a human impact) and the balloon and satellite atmospheric records (which showed little of the expected warming). Several new studies of the satellite and balloon data have now largely resolved this discrepancy—with consistent warming found at the surface and in the atmosphere.

An experiment with the real world that duplicated the climate of the 20th century with constant (rather than increasing) greenhouse gases would be the ideal way to test for the cause of climate change, but such an experiment is of course impossible. So scientists do the next best thing: they simulate the past with climate models.

Two important advances since the last IPCC assessment have increased confidence in the use of models for both attribution and projection of climate changes. The first is the development of a comprehensive, closely coordinated ensemble of simulations from 18 modeling groups around the world for the historical and future evolution of the earth’s climate. Using many models helps to quantify the effects of uncertainties in various climate processes on the range of model simulations. Although some processes are well understood and well represented by physical equations (the flow of the atmosphere and ocean or the propagation of sunlight and heat, for example), some of the most critical components of the climate system are less well understood, such as clouds, ocean eddies and transpiration by vegetation. Modelers approximate these components using simplified representations called parameterizations. The principal reason to develop a multimodel ensemble for the IPCC assessments is to understand how this lack of certainty affects attribution and prediction of climate change. The ensemble for the latest assessment is unprecedented in the number of models and experiments performed.

The second advance is the incorporation of more realistic representations of climate processes in the models. These processes include the behavior of atmospheric aerosols, the dynamics (movement) of sea ice, and the exchange of water and energy between the land and the atmosphere. More models now include the major types of aerosols and the interactions between aerosols and clouds.

When scientists use climate models for attribution studies, they first run simulations with estimates of only “natural” climate influences over the past 100 years, such as changes in solar output and major volcanic eruptions. They then run models that include human-induced increases in greenhouse gases and aerosols. The results of such experiments are striking. Models using only natural forcings are unable to explain the observed global warming since the mid-20th century, whereas they can do so when they include anthropogenic factors in addition to natural ones. Large-scale patterns of tempera-ture change are also most consistent between models and observations when all forcings are included.

Two patterns provide a fingerprint of human influence. The first is greater warming over land than ocean and greater warming at the surface of the sea than in the deeper layers. This pattern is consistent with greenhouse gas–induced warming by the overlying atmosphere: the ocean warms more slowly because of its large thermal inertia. The warming also indicates that a large amount of heat is being taken up by the ocean, demonstrating that the planet’s energy budget has been pushed out of balance.

A second pattern of change is that while the troposphere (the lower region of the atmosphere) has warmed, the stratosphere, just above it, has cooled. If solar changes provided the dominant forcing, warming would be expected in both atmospheric layers. The observed contrast, however, is just that expected from the combination of greenhouse gas increases and stratospheric ozone decreases. This collective evidence, when subjected to careful statistical analyses, provides much of the basis for the increased confidence that human influences are behind the observed global warming. Suggestions that cosmic rays could affect clouds, and thereby climate, have been based on correlations using limited rec-ords; they have generally not stood up when tested with additional data, and their physical mechanisms remain speculative.

What about at smaller scales? As spatial and temporal scales decrease, attribution of climate change becomes more difficult. This problem arises because natural small-scale temperature variations are less “averaged out” and thus more readily mask the change signal. Nevertheless, continued warming means the signal is emerging on smaller scales. The report has found that human activity is likely to have influenced temperature significantly down to the continental scale for all continents except Antarctica.

Human influence is discernible also in some extreme events such as unusually hot and cold nights and the incidence of heat waves. This does not mean, of course, that individual extreme events (such as the 2003 European heat wave) can be said to be simply “caused” by human-induced climate change—usually such events are complex, with many causes. But it does mean that human activities have, more likely than not, affected the chances of such events occurring.

Projections of Future Changes How will climate change over the 21st century? This critical question is addressed using simulations from climate models based on projections of future emissions of greenhouse gases and aerosols. The simulations suggest that, for greenhouse gas emissions at or above current rates, changes in climate will very likely be larger than the changes already observed during the 20th century. Even if emissions were immediately reduced enough to stabilize greenhouse gas concentrations at current levels, climate change would continue for centuries. This inertia in the climate results from a combination of factors. They include the heat capacity of the world’s oceans and the millennial timescales needed for the circulation to mix heat and carbon dioxide throughout the deep ocean and thereby come into equilibrium with the new conditions.

To be more specific, the models project that over the next 20 years, for a range of plausible emissions, the global temperature will increase at an average rate of about 0.2 degree C per decade, close to the observed rate over the past 30 years. About half of this near-term warming represents a “commitment” to future climate change arising from the inertia of the climate system response to current atmospheric concentrations of greenhouse gases.

The long-term warming over the 21st century, however, is strongly influenced by the future rate of emissions, and the projections cover a wide variety of scenarios, ranging from very rapid to more modest economic growth and from more to less dependence on fossil fuels. The best estimates of the increase in global temperatures range from 1.8 to 4.0 degrees C for the various emission scenarios, with higher emissions leading to higher temperatures. As for regional impacts, projections indicate with more confidence than ever before that these will mirror the patterns of change observed over the past 50 years (greater warming over land than ocean, for example) but that the size of the changes will be larger than they have been so far.

The simulations also suggest that the removal of excess carbon dioxide from the atmosphere by natural processes on land and in the ocean will become less efficient as the planet warms. This change leads to a higher percentage of emitted carbon dioxide remaining in the atmosphere, which then further accelerates global warming. This is an important positive feedback on the carbon cycle (the exchange of carbon compounds throughout the climate system). Although models agree that carbon-cycle changes represent a positive feedback, the range of their responses remains very large, depending, among other things, on poorly understood changes in vegetation or soil uptake of carbon as the climate warms. Such processes are an important topic of ongoing research.

The models also predict that climate change will affect the physical and chemical characteristics of the ocean. The estimates of the rise in sea level during the 21st century range from about 30 to 40 centimeters, again depending on emissions. More than 60 percent of this rise is caused by the thermal expansion of the ocean. Yet these model-based estimates do not include the possible acceleration of recently observed increases in ice loss from the Greenland and Antarctic ice sheets. Although scientific understanding of such effects is very limited, they could add an additional 10 to 20 centimeters to sea-level rises, and the possibility of significantly larger rises cannot be excluded. The chemistry of the ocean is also affected, as the increased concentrations of atmospheric carbon dioxide will cause the ocean to become more acidic.

Some of the largest changes are predicted for polar regions. These include significant increases in high-latitude land temperatures and in the depth of thawing in permafrost regions and sharp reductions in the extent of summer sea ice in the Arctic basin. Lower latitudes will likely experience more heat waves, heavier precipitation, and stronger (but perhaps less frequent) hurricanes and typhoons. The extent to which hurricanes and typhoons may strengthen is uncertain and is a subject of much new research.

Some important uncertainties remain, of course. For example, the precise way in which clouds will respond as temperatures increase is a critical factor governing the overall size of the projected warming. The complexity of clouds, however, means that their response has been frustratingly difficult to pin down, and, again, much research remains to be done in this area.

We are now living in an era in which both humans and nature affect the future evolution of the earth and its inhabitants. Unfortunately, the crystal ball provided by our climate models becomes cloudier for predictions out beyond a century or so. Our limited knowledge of the response of both natural systems and human society to the growing impacts of climate change compounds our uncertainty. One result of global warming is certain, however. Plants, animals and humans will be living with the consequences of climate change for at least the next thousand years.

Climate change: How do we know it is happening and caused by humans?

• Published 25 October 2021

Scientists and politicians say we are facing a planetary crisis because of climate change.

But what's the evidence for global warming and how do we know it's being caused by humans?

How do we know the world is getting warmer?

Our planet has been warming rapidly since the dawn of the Industrial Revolution.

The average temperature at the Earth's surface has risen about 1.1C since 1850 . Furthermore, each of the last four decades has been warmer than any that preceded it, since the middle of the 19th Century.

These conclusions come from analyses of millions of measurements gathered in different parts of the world. The temperature readings are collected by weather stations on land, on ships and by satellites .

Multiple independent teams of scientists have reached the same result - a spike in temperatures coinciding with the onset of the industrial era.

Scientists can reconstruct temperature fluctuations even further back in time.

Tree rings, ice cores, lake sediments and corals all record a signature of the past climate.

This provides much-needed context to the current phase of warming. In fact, scientists estimate the Earth hasn't been this hot for about 125,000 years.

How do we know humans are responsible for global warming?

Greenhouse gases - which trap the Sun's heat - are the crucial link between temperature rise and human activities. The most important is carbon dioxide (CO2), because of its abundance in the atmosphere.

We can also tell it's CO2 trapping the Sun's energy. Satellites show less heat from the Earth escaping into space at precisely the wavelengths at which CO2 absorbs radiated energy.

Burning fossil fuels and chopping down trees lead to the release of this greenhouse gas. Both activities exploded after the 19th Century, so it's unsurprising that atmospheric CO2 increased over the same period.

There's a way we can show definitively where this extra CO2 came from. The carbon produced by burning fossil fuels has a distinctive chemical signature.

Tree rings and polar ice both record changes in atmospheric chemistry. When examined they show that carbon - specifically from fossil sources - has risen significantly since 1850 .

Analysis shows that for 800,000 years, atmospheric CO2 did not rise above 300 parts per million (ppm). But since the Industrial Revolution, the CO2 concentration has soared to its current level of nearly 420 ppm.

Computer simulations, known as climate models, have been used to show what would have happened to temperatures without the massive amounts of greenhouse gases released by humans.

• What does net zero mean?
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They reveal there would have been little global warming - and possibly some cooling - over the 20th and 21st Centuries, if only natural factors had been influencing the climate.

Only when human factors are introduced can the models explain increases in temperature.

What impact are humans having on the planet?

The level of heating Earth has experienced already is predicted to cause significant changes to the world around us.

Real-world observations of these changes match patterns scientists expect to see with human-induced warming. They include:

• The Greenland and Antarctic ice sheets melting rapidly
• The number of weather-related disasters has increased by a factor of five over 50 years
• Global sea levels rose 20cm (8ins) in the last century and are still rising
• Since the 1800s, the oceans have become about 40% more acid, affecting marine life

But wasn't it warmer in the past?

There have been several hot periods during the Earth's past.

Around 92 million years ago, for example, temperatures were so high that there were no polar ice caps and crocodile-like creatures lived as far north as the Canadian Arctic .

That should not comfort anyone, however, because humans were not around. At times in the past, sea level was 25m (80ft) higher than the present. A rise of 5-8m (16-26ft) is considered enough to submerge most of the world's coastal cities .

There is abundant evidence for mass extinctions of life during these periods. And climate models suggest that, at times, the tropics could have become "dead zones" , too hot for most species to survive.

These fluctuations between hot and cold have been caused by a variety of phenomena, including the way the Earth wobbles as it orbits the Sun over long periods, volcanic eruptions and short-term climate cycles such as El Niño.

For many years, groups of so-called climate "sceptics" have cast doubt on the scientific basis of global warming.

However, virtually all scientists who publish regularly in peer-reviewed journals now agree on the current causes of climate change.

A key UN report released in 2021 said it "is unequivocal that human influence has warmed the atmosphere, oceans and land".

The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost 200 countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.

• Six ways the UK could tackle climate change
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Top image credit: Getty Images. Climate stripes visualisation courtesy of Prof Ed Hawkins and University of Reading.

Related Topics

• Earth science
• Greenhouse gas emissions

ENCYCLOPEDIC ENTRY

Global warming.

The causes, effects, and complexities of global warming are important to understand so that we can fight for the health of our planet.

Earth Science, Climatology

Tennessee Power Plant

Ash spews from a coal-fueled power plant in New Johnsonville, Tennessee, United States.

Photograph by Emory Kristof/ National Geographic

Global warming is the long-term warming of the planet’s overall temperature. Though this warming trend has been going on for a long time, its pace has significantly increased in the last hundred years due to the burning of fossil fuels . As the human population has increased, so has the volume of fossil fuels burned. Fossil fuels include coal, oil, and natural gas, and burning them causes what is known as the “greenhouse effect” in Earth’s atmosphere.

The greenhouse effect is when the sun’s rays penetrate the atmosphere, but when that heat is reflected off the surface cannot escape back into space. Gases produced by the burning of fossil fuels prevent the heat from leaving the atmosphere. These greenhouse gasses are carbon dioxide , chlorofluorocarbons, water vapor , methane , and nitrous oxide . The excess heat in the atmosphere has caused the average global temperature to rise overtime, otherwise known as global warming.

Global warming has presented another issue called climate change. Sometimes these phrases are used interchangeably, however, they are different. Climate change refers to changes in weather patterns and growing seasons around the world. It also refers to sea level rise caused by the expansion of warmer seas and melting ice sheets and glaciers . Global warming causes climate change, which poses a serious threat to life on Earth in the forms of widespread flooding and extreme weather. Scientists continue to study global warming and its impact on Earth.

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Global warming is slowing Earth's spin and may affect how we keep time, study suggests

Science Global warming is slowing Earth's spin and may affect how we keep time, study suggests

Melting polar ice caps are a sad sign of our times, but they may have given global timekeepers an unexpected reprieve, according to new research.

For decades, experts have been trying to reconcile the precise time, as set by atomic clocks, with the somewhat unreliable speed at which the Earth spins.

Since 1972, authorities have added 27 leap seconds to our time standard – UTC, or Co-ordinated Universal Time – to compensate for a slow-down in the Earth's spin caused largely by the pull of the Moon on the oceans known as "tidal friction".

Keeping UTC in step with the Earth's rotation means, among other things, that we can expect the Sun to come up at a particular time in the morning, and astronomers can easily work out when to point their telescopes at a particular part of the sky.

But the addition of leap seconds happens at irregular intervals, with just six months' notice, causing a headache for global technology systems – from power grids to financial markets, and from satellites to social media networks.

Recently however,  Earth's spin has sped up  suggesting we might need to remove a second from UTC, rather than add one to it.

This has caused widespread concern that computers might not cope well with this "negative leap second" and there could be unprecedented disruption to global systems.

But what if this dreaded negative leap second was postponed – even for just a few years?

A new study, published in the journal Nature , suggests we may well get such a delay from the melting of Earth's ice caps, which are having a slowing effect on the planet's rotation.

Melting ice putting the brakes on Earth's spin

Duncan Agnew, a professor of geophysics at the University of California, San Diego, set out to model all the factors impacting the speed of Earth's rotation.

Some factors cause the Earth to spin faster, others cause it to slow down, and their net effect should explain the speed the planet spins at.

"It's almost a book-keeping exercise," Professor Agnew said.

His work adds weight to the idea the recent speeding up of Earth's rotation is due to changes in the Earth's liquid core. 

On the other hand, apart from tidal friction, he found another factor slowing the Earth's spin was the increased rate of polar melting, mostly from the Greenland ice cap but also from Antarctica.

"That was the most interesting result," he said.

"Global warming is … changing the rotation of the whole Earth."

So how does that work?

As polar ice melts, more water flows towards the equator and the impact can be likened to a figure skater who stretches out their arms and legs to slow down.

The slowing is due to a property of spinning systems called conservation of angular momentum.

"Because the shape of the Earth is changing, that causes the rotation to vary," Professor Agnew said.

He found the Earth was spinning ahead of the atomic clock and this would eventually require a leap second to be removed from UTC, in order to keep the two with a second of each other.

But Professor Agnew also found the impact of the Earth's melting polar ice has postponed the need for this negative leap second by three years.

According to his modelling, a second will need to be removed from UTC in 2029, rather than 2026.

"If global warming hadn't occurred over the last 30 years, we'd be very close to a negative leap second already."

The spectre of the negative leap second

The practice of adjusting UTC with leap seconds was put in place in a time before the internet.

In the '70s, it was important UTC matched the period of the Earth's rotation for purposes of celestial navigation, Patrizia Tavella, who is in charge of UTC at France's Bureau International des Poids et Measures (BIPM), said.

But since the advent of GPS and precise technologies that rely on atomic time, adjusting UTC has become a greater headache.

Adding leap seconds has led to computer glitches with widespread effects, including outages on social media site Reddit and Qantas servers in 2012, and web services company Cloudflare in 2017.

There's also little standardisation around how organisations adjust to UTC changes, with companies like Meta and Google taking different approaches.

"All that creates a great confusion on what time it is on the day of the leap seconds," Dr Tavella said.

Subtracting a second could cause unprecedented problems.

"A negative leap second has never been implemented.

"There are systems which have not been designed for this case, and the risk of failure is surely a concern for all the users and the metrologists."

Dr Tavella welcomed Professor Agnew's findings   that climate change has delayed the need for a negative leap second.

"If confirmed, it can be good news, as we have more time to study and take a sound decision on the future of the UTC." 

Michael Wouters from Australia's National Measurement Institute shared Dr Tavella's concerns.

But he emphasised the lack of certainty in predicting the required timing for negative leap seconds – which is why actual decisions about leap seconds are only made six months out.

"[Professor Agnew] really only looks at the effect of the polar melting on the timing of a leap second … and does not consider uncertainties in the other processes that affect Earth rotation," Dr Wouters said.

But he agreed with the key finding that polar melting may give some reprieve on the requirement for addition of a negative leap second. 

Studies shows reach of of global warming impact

Professor Agnew said his model involved the "simplest possible" extrapolation and acknowledged there was a "fairly large" amount of error, due mainly to the activity of the Earth's core being "fundamentally unpredictable".

Despite all this, an Australian expert in the study of the Earth's changing shape and rotation described the work as "robust" and "elegant" in its key finding.

"Enough ice is melting to change Earth's rotation rate and so our length of day," Matt King, a professor of polar geodesy at the University of Tasmania, said.

"We know [melting ice] also changes the location of the axis upon which Earth spins.

"These are profound things are happening outside the sight of most of us, but that doesn't mean they are trivial."

What happens next?

International time-keeping authorities are in the process of moving away from binding UTC so tightly to the rotation of the Earth.

By 2035 they hope to have a new system that requires fewer adjustments to UTC, but that will have no noticeable effects on our experience of daily things like sunrise and sunset times.

But what happens in the meantime?

Professor Agnew said the overall trend would be for Earth's rotation to slow, and the need for negative leap seconds in the long-term was "unlikely".

He said authorities should forget about introducing them and simply allow UTC and the speed of the Earth's rotation to diverge more before making adjustments.

And what if we gave up entirely on linking atomic clock time with the Earth's rotation?

Ultimately, it could result in some pretty strange phenomena.

Fancy a midday sunrise anyone?

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Climate change is changing how we keep time.

Melting ice sheets are slowing Earth's rotation speed, complicating global timekeeping

The rapidly accelerating melting of Earth’s polar ice sheets — including ice atop Greenland (shown here) — is slowing the planet’s spin, which affects global timekeeping.

KEREM YUCEL/AFP via Getty Images

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By Carolyn Gramling

March 27, 2024 at 1:34 pm

Climate change may be making it harder to know exactly what time it is.

The rapid melting of the ice sheets atop Greenland and Antarctica, as measured by satellite-based gravitational measurements, is shifting more mass toward Earth’s waistline. And that extra bulge is slowing the planet’s rotation , geophysicist Duncan Agnew reports online March 27 in Nature . That climate change–driven mass shift is throwing a new wrench into international timekeeping standards.

The internationally agreed-upon coordinated universal time, or UTC, is set by atomic clocks, but that time is regularly adjusted to match Earth’s actual spin. Earth’s rotation isn’t always smooth sailing — the speed of the planet’s spin changes depending on a variety of factors, including gravitational drag from the sun and the moon, changes to the rotation speed of Earth’s core, friction between ocean waters and the seafloor, and shifts in the planet’s distribution of mass around its surface. Even earthquakes can affect the spin: The magnitude 9.1 earthquake in Indonesia in 2004, for example, altered the land surface in such a way that it caused Earth to rotate a tiny bit faster, says Agnew, of the Scripps Institution of Oceanography in La Jolla, Calif.

But the impact of that quake is much smaller than that of the ice sheets’ melting — a point that Agnew says he finds particularly startling. Humankind “has done something that affects, measurably, the rotation rate of the entire Earth.”

The need for occasional tweaks to the synchronization of atomic clocks and Earth’s rotation gave birth in 1972 to the “leap second ,” an extra tick that international timekeepers agreed to add to UTC as needed ( SN: 1/19/24 ). Timekeepers have added 27 leap seconds to the clock since the idea was introduced.

Still, metrologists — measurement scientists — aren’t overly fond of this system. For one thing, it doesn’t happen on a regular schedule, but only whenever it seems to be needed. And financial markets and satellite navigation systems, which rely on precise timing, each have their own methodologies for incorporating a leap second. Those inconsistencies can, counterproductively, make it more challenging to have a universal time. So in 2022, an international consortium of metrologists voted to do away with leap seconds in favor of adding larger chunks of time, perhaps a minute, less frequently. The group resolved to settle those details at its next meeting, in 2026.

That may not come a second too soon. The slightly slower rotation has actually delayed the need for timekeeping adjustments by a few years, Agnew says — in fact, as a result of this change, the last time a leap second was required to be inserted was in 2016. At the moment, in fact, Earth’s rotation and atomic clocks are nearly in sync.

But that’s just a brief respite, Agnew’s calculations show. The biggest changes to Earth’s rotation right now are coming from its heart: slowing rotation of Earth’s core is actually speeding up the spin of the outer layers ( SN: 1/23/23 ). That slowdown will ultimately mean that timekeepers, under the current system, must begin removing leap seconds from the UTC, rather than inserting them, to keep things in sync.

That shift in strategy might have begun as soon as in 2026. But the study suggests that, thanks to climate change, global timekeepers now have an extra two or three years before they need to adjust, notes geophysicist Jerry Mitrovica of Harvard University. But no realistic projections of future melting can forestall the inevitable beyond 2030, Mitrovica adds: One way or another, the world is going to have to start losing time — or international timekeeping guidelines will need to change.

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Can We Engineer Our Way Out of the Climate Crisis?

Mammoth, a giant machine in Iceland that will pull planet-warming carbon dioxide out of the air. Credit... Francesca Jones for The New York Times

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By David Gelles

David Gelles reported from Reykjavik, Iceland, and Midland, Texas.

• Published March 31, 2024 Updated April 1, 2024

On a windswept Icelandic plateau, an international team of engineers and executives is powering up an innovative machine designed to alter the very composition of Earth’s atmosphere.

If all goes as planned, the enormous vacuum will soon be sucking up vast quantities of air, stripping out carbon dioxide and then locking away those greenhouse gases deep underground in ancient stone — greenhouse gases that would otherwise continue heating up the globe.

Just a few years ago, technologies like these, that attempt to re-engineer the natural environment, were on the scientific fringe. They were too expensive, too impractical, too sci-fi. But with the dangers from climate change worsening, and the world failing to meet its goals of slashing greenhouse gas emissions, they are quickly moving to the mainstream among both scientists and investors, despite questions about their effectiveness and safety.

First in a series on the risky ways humans are starting to manipulate nature to fight climate change. Once science fiction, today these ideas are becoming reality.

Researchers are studying ways to block some of the sun’s radiation. They are testing whether adding iron to the ocean could carry carbon dioxide to the sea floor. They are hatching plans to build giant parasols in space . And with massive facilities like the one in Iceland, they are seeking to reduce the concentration of carbon dioxide in the air.

As the scale and urgency of the climate crisis has crystallized, “people have woken up and are looking to see if there’s any miraculous deus ex machina that can help,” said Al Gore, the former vice president.

Since the dawn of the industrial age, humans have pumped huge volumes of heat-trapping gases into the atmosphere in pursuit of industry and advancement. It amounted to a remaking of the planet’s delicately balanced atmosphere that today has transformed the world, intensifying heat, worsening droughts and storms and threatening human progress.

As the risks became clearer, political and corporate leaders pledged to keep global average temperatures to no more than 1.5 degrees Celsius higher than before the Industrial Revolution . But for several months last year, the world briefly passed that symbolic threshold, sooner than many scientists expected.

Global temperatures are now expected to rise as much as 4 degrees Celsius, or more than 7 degrees Fahrenheit, by the end of the century. That has given new weight to what some people call geoengineering, though that term has become so contentious its proponents now prefer the term “climate interventions.” The hope is that taking steps like these might buy some time at a moment when energy consumption is on the rise, and the world isn’t quitting fossil fuels fast enough.

Many of the projects are controversial. A plant similar to the one in Iceland, but far larger, is being built in Texas by Occidental Petroleum, the giant oil company. Occidental intends to use some of the carbon dioxide it captures to extract even more oil, the burning of which is one of the main causes of the climate crisis in the first place.

Some critics say that other types of interventions could open up a Pandora’s box of new problems by scrambling weather patterns or amplifying human suffering through unintended consequences. In effect they are asking: Should humans be experimenting with the environment in this way? Do we know enough to understand the risks?

“We need more information so we can make these decisions in the future,” said Alan Robock, a professor of atmospheric science at Rutgers University. “Which is riskier: to do it, or not to do it?”

Others argue that fanciful or costly technologies will simply waste resources and time, or lull people with the false idea that it will be possible to slow global warming without phasing out fossil fuels.

There is also the risk of rogue actors barreling ahead with their own efforts to change the climate. Already, Mexico has banned what’s known as solar radiation modification after a start-up from California released sulfur dioxide into the atmosphere without permission.

And then there is the fact that, because these technologies are so new, there is relatively little regulation governing them.

“There are these much bigger questions around who decides how is this is all coordinated over time,” said Marion Hourdequin, a professor of environmental philosophy at Colorado College. “We don’t have a great track record of sustained global cooperation.”

With a subzero wind whipping down from the fjords, Edda Aradottir trudged through fresh snow to inspect the direct air capture plant in Iceland.

Ms. Aradottir is the chief executive of Carbfix, an Icelandic company that is working with the Swiss start-up that built the plant, Climeworks. Known as Mammoth, the project is a technological accomplishment, powered by clean geothermal energy and capable of capturing up to 36,000 metric tons of carbon dioxide per year and pumping it down into the bedrock.

That is just one one-millionth of annual global emissions. But unlike trees, which can be cut down or catch fire, Climeworks promises to store that carbon dioxide forever.

Inside a series of bunkerlike buildings, dozens of huge fans suck air into bins that contain chemical pellets that absorb carbon dioxide, then release the gas when they are heated up. The carbon dioxide is then mixed with water and pumped more than a mile below the surface, where extreme pressure turns it into a solid in a matter of years, a process known as “mineralization.” In effect, the gas becomes part of the rock.

“Over 99 percent of the carbon on Earth is already stored in rocks in the form of minerals,” Ms. Aradottir said. “Naturally, it happens over geologic time scales. We’re speeding it up.”

When Mammoth is turned on in the coming weeks, it will be the largest such facility in the world, even though the amount of carbon it can absorb is still just a drop in the bucket. Global carbon dioxide emissions hit an all-time high of 36 billion metric tons last year.

The Occidental plant, being built near Odessa, Texas, and known as Stratos, will be more than 10 times more powerful than Mammoth, powered by solar energy, and have the potential to capture and sequester 500,000 metric tons of carbon dioxide per year.

It uses a different process to extract carbon dioxide from the air, though the goal is the same: Most of it will be locked away deep underground. But at least some of the carbon dioxide, Occidental says, will also be used to extract more oil.

In that process, carbon dioxide is pumped into the ground to force out oil that might otherwise be too difficult to reach. Techniques like this have made Occidental a company worth more than $50 billion and helped send American crude production to a new high in recent years.

Of course, it is the world’s reliance on the burning of oil and other fossil fuels that has so dangerously sent carbon dioxide levels soaring. In the atmosphere, carbon dioxide acts as a blanket, trapping the sun’s heat and warming the world.

Today, Occidental says it is trying to become a “carbon management” company as well as an oil producer. Last year, it paid $1.1 billion for a start-up called Carbon Engineering that had developed a way to soak up carbon dioxide from the air, and began building the Stratos project. Today, what was a barren plot of dirt less than 12 months ago is a bustling construction site.

“It’s like the Apollo missions at NASA,” said Richard Jackson, who oversees carbon management and domestic oil operations at Occidental. “We’re trying to move as quickly as we can.”

In coming years, Occidental said it planned to build 100 facilities, each capable of capturing 1 million metric tons of carbon dioxide a year. It has struck up a financial partnership with BlackRock, the world’s largest asset manager, and made a deal to develop direct air capture plants with Adnoc, the United Arab Emirates’ state oil company.

In South Texas, it is planning to build 30 of the plants on the King Ranch, funded in part by $1.2 billion the Biden administration last year awarded to direct air capture projects .

Climeworks also has aggressive expansion plans. It secured a portion of the White House funds for a direct air capture plant in Louisiana, is working with a group of Kenyan entrepreneurs to build a large facility outside Nairobi and has plans to construct plants in Canada and other countries in Europe.

Driving the construction boom is the desire of many corporations to reduce their effect on global warming. Over the past decade, hundreds of multinational corporations have pledged to become carbon neutral by 2040. Rather than phasing out fossil fuels to meet those goals, most companies are finding that they have to pay for carbon credits, which can be acquired by preserving forests, supporting renewable energy projects or paying for carbon sequestration.

Microsoft, JPMorgan and UBS are some of the big companies that have signed long term agreements to buy credits from Climeworks. Amazon, AT&T and the Houston Astros are among those signed up with Occidental.

Bill Gates, the Microsoft co-founder, said last year that he was the largest individual customer of Climeworks, paying the company $10 million each year to offset his sizable carbon footprint.

Yet the grand plans envisioned by direct air capture companies, with hundreds of plants built in the years ahead, are predicated on a market that does not yet exist. Only a handful of big companies have so far proved willing to voluntarily spend millions of dollars on direct air capture credits, and there is no guarantee that the rest of the corporate world will follow suit anytime soon.

Even if more companies do decide to start offsetting their emissions, there are cheaper ways to do so, including by preserving forests and paying for renewable energy. For example, it currently costs between $500 and $1,000 to capture a metric ton of carbon dioxide with direct air capture, compared with just $10 to $30 per ton for most carbon credits today .

“ It’s very expensive,” said Mr. Robock. “And so it’s not going to be a solution in the short term or the long term.”

Still, the business world is bullish. Boston Consulting Group expects more companies to begin buying credits to pay for carbon dioxide removal, and more governments to encourage that buying. In the United States and Europe, governments have started subsidizing the construction of the plants. By 2040, BCG expects the market for carbon dioxide removal technologies could grow from less than $10 billion today to as much as $135 billion.

“Companies will face a rising price on carbon and regulatory pressures that will make them feel compelled to do this,” said Rich Lesser, the global chair of BCG.

Although the direct air capture market is still in its infancy, it already has vociferous detractors in academia, activist circles and beyond.

Some say it is little more than a ploy by oil and gas companies to prolong the very industries that are responsible for creating global warming. They point to the extensive evidence that fossil fuel interests for years worked to play down public awareness of climate change, and the fact that some of the captured carbon will be used for additional oil production.

Those concerns were magnified when Vicki Hollub, Occidental’s chief executive, last year said direct air capture could “preserve our industry.” She added, “This gives our industry a license to continue to operate for the 60, 70, 80 years that I think it’s going to be very much needed.”

Scientists say an urgent transition away from fossil fuels is necessary to avoid extreme global temperature increases. Last year, nearly 200 countries agreed to begin phasing out oil, coal and gas.

“This is a new wave of denial, deception and delay,” said Lili Fuhr, director of the fossil economy program at the Center for International Environmental Law. “You have the fossil fuel industry trying to say we can engineer our way out of this without any major changes to business as usual.”

A related line of reasoning holds that the enormous amounts of clean energy needed to power direct air capture plants would be better used powering homes and businesses, thereby displacing fossil fuels such as natural gas and coal that still provide much of the world’s electricity.

Ms. Fuhr added that the costs remained high in spite of a growing raft of government support, including a tax credit in the United States worth $180 for every metric ton of carbon dioxide that is captured and stored, a subsidy that is likely to significantly lower Occidental’s tax bill in the years ahead. “The industry has been successful in capturing subsidies, just not carbon,” she said.

And then there is the fact that even if Occidental and Climeworks make good on their ambitions to build hundreds of new plants in the coming years, they would still not come close to capturing even 1 percent of current annual global emissions.

Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University, said climate interventions in general, and carbon capture in particular, were dangerous distractions from the more urgent work of rapidly reducing the use of fossil fuels.

“There are many solutions that are just not helpful at all, that do not help an iota for climate and don’t help an iota for air pollution,” he said. “Among these are direct air capture.”

Proponents of the technology say that with fossil fuel emissions continuing to rise, the world is fast approaching the moment when any and all solutions to abate global warming should be on the table.

At the least, direct air capture has a role to play, they say, because there might always be at least some greenhouse gases being produced in the future, even if the vast majority of emissions are successfully eliminated. Sucking that carbon dioxide out of the air will be valuable, the argument goes.

But in the longer term, many scientists believe it will be necessary to go further and actually try to remove some of the excess carbon dioxide that has dangerously accumulated over the centuries. Proponents of direct air capture say their technology is suited for such a herculean task.

“No one is arguing you could solve all our carbon emissions with this,” Mr. Lesser said. “But it could be a meaningful part of solving a huge problem.”

The past year’s record temperatures, warming oceans and megafires are all evidence of a deeply uncomfortable reality: We’ve already been manipulating the planet for the past couple hundred years. In less time than it takes a redwood tree to reach its full height, humans have added enough carbon dioxide to the atmosphere to scramble weather patterns, melt glaciers and wipe out coral reefs.

Viewed this way, today’s attempts to slow down or even reverse the warming can be seen as efforts to undo the changes that have already taken place. Whether or not humans can succeed in this most ambitious feat is unclear. It is among the most consequential problems our species has faced.

Yet as people begin to deliberately tinker with the climate in new ways, there are profound questions that are only beginning to be contemplated. If the current extreme weather and temperature rise came about inadvertently, as the unintentional consequence of human development, what might happen when we begin actively trying to control the planet’s atmosphere?

“It’s true that we have been altering the climate through greenhouse gas emissions for centuries now,” said Ms. Hourdequin of Colorado College. “But trying to intentionally manage the climate through geoengineering would be a distinctive endeavor, quite different than the kind of haphazard interference that we’ve engaged in thus far.”

David Gelles reports on climate change and leads The Times’s Climate Forward newsletter and events series . More about David Gelles

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Melting polar ice is slowing the Earth's rotation, with possible consequences for timekeeping

Global warming has slightly slowed the Earth’s rotation — and it could affect how we measure time. 

A study published Wednesday found that the melting of polar ice — an accelerating trend driven primarily by human-caused climate change — has caused the Earth to spin less quickly than it would otherwise. 

The author of the study, Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California San Diego, said that as ice at the poles melts, it changes where the Earth’s mass is concentrated. The change, in turn, affects the planet’s angular velocity. 

Agnew compared the dynamic to a figure skater twirling on ice: “If you have a skater who starts spinning, if she lowers her arms or stretches out her legs, she will slow down,” he said. But if a skater’s arms are drawn inward, the skater will twirl faster. 

Less solid ice at the poles, then, means more mass around the equator — Earth’s waist.

“What you’re doing with the ice melt is you’re taking water that’s frozen solid in places like Antarctica and Greenland, and that frozen water is melting, and you move the fluids to other places on the planet,” said Thomas Herring, a professor of geophysics at the Massachusetts Institute of Technology who was not involved in the new study. “The water flows off towards the equator.” 

The study suggests, in other words, that human influence has monkeyed with a force that scholars, stargazers and scientists have puzzled over for millennia — something long considered a constant that was out of humanity’s control.

“It’s kind of impressive, even to me, we’ve done something that measurably changes how fast the Earth rotates,” Agnew said. “Things are happening that are unprecedented.”

His study, which was published in the journal Nature, suggests that climate change is playing a significant enough role in the Earth’s rotation to counteract an opposing trend. Because of a combination of factors, the Earth has begun to spin faster in recent decades, a temporary trend that has prompted scientists for the first time to consider subtracting a single “negative leap second” from clocks worldwide as soon as 2026. But the melting of polar ice has delayed that possibility by about three years, according to Agnew.

If timekeeping organizations do eventually decide to add a negative leap second, the adjustment could disrupt computer networks.

The reason leap second adjustments have historically been needed is that even without climate change, Earth’s daily rotation has trended slower over millions of years, even though it may seem constant. 

About 70 million years ago, days were shorter and lasted roughly 23.5 hours, a study in Paleoceanography and Paleoclimatology suggests . That means Cretaceous dinosaurs experienced a planet with 372 days in each year.  

Several key factors affect the planet’s spin — sometimes working in opposition. 

The friction of ocean tides, due in part to the moon’s gravitational pull, slows the Earth’s rotation. Meanwhile, since the last Ice Age, the Earth’s crust has been uplifting in some regions as it responds to the removal of ice sheets’ weight. That effect shifts where mass is distributed and speeds up the planet’s spin. Both of those processes are fairly constant and have predictable rates. 

Yet another factor is the movement of fluid within the Earth’s liquid inner core — a wild card that can either speed or slow how fast the Earth rotates, Agnew said.   Fluctuations in Earth's core are a primary reason that the planet has rotated faster than would be otherwise expected in recent decades.

That faster spin has led timekeepers to consider — for the first time since Coordinated Universal Time was officially adopted in the 1960s — whether it might make sense to subtract a leap second to keep universal time in lockstep with Earth’s rotation.  

But polar ice melt is counteracting that trend and has forestalled any decision point about whether to add a negative leap second. According to Agnew’s estimates, it has delayed that possibility from 2026 to 2029 —  if the current pace of Earth’s rotation holds. 

As climate change intensifies, researchers expect ice melt to have an even more profound effect on how the planet spins. 

“It will have a bigger contribution as time goes on and as melt accelerates, as we expect it’s likely to do,” Herring said. He added that the new study was a thorough, solid analysis that combined research from several disciplines of science.

The need for timekeepers to adjust universal time to stay in line with the Earth’s rotation is not a new phenomenon. But historically, that has involved adding leap seconds to the common standard for clocks when Earth’s slowing spin causes astronomical time to fall behind atomic time (which is measured by the vibration of atoms in atomic clocks). 

Adding or subtracting leap seconds is a pain, because they have the potential to disrupt satellite, financial and energy transmission systems that rely on extremely precise timing. Because of that, global timekeepers voted in 2022 to do away with the leap second additions and subtractions by 2035 and let universal time drift away from the pace of the Earth’s rotation. 

“There’s been a push since about 2000 to get rid of leap seconds,” Agnew said. 

Regardless of whether clocks wind up changing, the notion that melting polar ice is affecting the Earth’s rotation speaks to how significant an issue it has become. Research has already described the profound impact that ice loss will have on coastal communities.  

Scientists expect sea level rise to accelerate as the climate warms, a process that will continue for hundreds of years. Last year, top polar researchers warned in a report that parts of key ice sheets could collapse and that coastal communities should prepare for many feet of sea level rise. If humanity allows average global temperatures to rise by 2 degrees Celsius, the planet could be committed to more than 40 feet of sea level rise .

Evan Bush is a science reporter for NBC News. He can be reached at [email protected].

'Winners and losers' as global warming forces plants uphill

Some plant species will "win" and others will "lose" as global warming forces them to move uphill, new research shows.

Scientists examined the current range of more than 7,000 plant species in Brazil's Cerrado savanna, and estimated shifts based on warming by 2040.

The fate of plant species will depend on where they live: lowland species can move uphill for cooler conditions, but mountain plants have nowhere to go.

The study was carried out by the universities of Exeter and Campinas, the Royal Botanic Garden Edinburgh and Trinity College Dublin.

"Every plant and animal species has a 'geographical range' -- the area where conditions are suitable for it to live," said Mateus Silva, from the University of Exeter.

"As the climate warms, plants' ranges are shifting, with many species going uphill.

"This is the pattern we found in the Cerrado -- suggesting lowland areas may become local extinction hotspots, while mountains will host new combinations of plant species."

The Cerrado is just 0.4% of the planet's land surface, but it is home to 3.5% of all flowering plants -- approximately 12,000 species, a similar diversity level to the Brazilian Amazon rainforest.

The research team used Species Distribution Models to find the conditions in which each plant species is found, then contrasted this with fine-scale climate data including changes in temperature and rainfall.

The team chose a relatively short timescale -- focussing on change by 2040 -- to ensure the findings would be useful for current conservation efforts.

While more dramatic effects are anticipated over a longer period, the results suggest about 150 plant species face a "critical reduction" by 2040 -- losing more than 70% of their range.

"About half of Cerrado plant species will experience a net range loss due to climate change by 2040," Silva added.

"And more than two thirds (68-73%) of the Cerrado landscapes will see a net loss in species numbers.

"Virtually the entire Cerrado area will experience some level of species replacement due to climate change, and this will be most intense in highland areas.

"Meanwhile, lowland areas will have less plant diversity as some species become unable to tolerate the new conditions."

The researchers say their scenarios are "optimistic," as they only looked at climate change.

An additional threat is direct destruction of Cerrado savanna -- around half of which has already been cleared to grow crops or graze cattle.

"The range loss highlighted by our study will be compounded by this habitat loss and fragmentation, which makes it harder for species to move to new areas," Silva said.

"It's also important to note that our study did not examine interactions between species -- this complex ecology is also likely to play an important role.

"At this stage, we wanted to highlight the likely impact of large-scale patterns.

"The Cerrado highland areas reach 700-1,200m above sea level and they provide different conditions from the lowlands, highlighting the need for distinct conservation actions."

The study was funded by WWF, University of Exeter alumni, the Natural Environment Research Council (NERC) and the Brazilian research funding agencies FAPESP and CNPq.

• Endangered Plants
• New Species
• Endangered Animals
• Global Warming
• Environmental Awareness
• Exotic Species
• Global warming
• Larsen Ice Shelf
• Sulfur hexafluoride
• Global warming controversy
• Temperature record of the past 1000 years
• Climate change mitigation
• Attribution of recent climate change
• Effects of global warming

Story Source:

Materials provided by University of Exeter . Original written by Alex Morrison. Note: Content may be edited for style and length.

Journal Reference :

• Mateus C. Silva, Lucy Rowland, Rafael S. Oliveira, R. Toby Pennington, Peter Moonlight. Elevation modulates the impacts of climate change on the Brazilian Cerrado flora . Diversity and Distributions , 2024; DOI: 10.1111/ddi.13832

Cite This Page :

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