New study directly measures greenhouse effect at Earth’s surface

  • 25 Feb 2015, 18:00
  • Robert McSweeney

Cloudy skies | Shutterstock

Scientists know that greenhouse gases in the atmosphere cause the Earth to warm. But measuring exactly how much heat they trap is harder than you might think.

Previous studies using satellites have established that more heat is entering the atmosphere than leaving it. But a new study goes a step further and directly measures the amount of warming greenhouse gases are producing at Earth's surface.

The paper provides the critical link between rising carbon dioxide concentrations and the extra energy trapped in the climate system, the researchers say.

Greenhouse effect

Joseph Fourier first suggested in the 1820s that gases in the Earth's atmosphere trap heat and help keep the planet warm, coining the term greenhouse effect. Physicist John Tyndall later extended the theory by identifying the gases, such as carbon dioxide and methane, that were responsible for the warming.

Jumping forward a century and a half, we now know a lot more. Using satellites to measure how much of the sun's energy enters the Earth's atmosphere, and how much is reflected or re-emitted back into space, scientists have shown that the difference between the two is increasing. This means the Earth is trapping more heat than it used to, and therefore must be warming.

But while those studies show a widening gap between the energy reaching and leaving Earth, they are unable to directly measure how much warming greenhouse gases are causing at a particular point in time. New research, published today in Nature, shows how scientists have directly been able to measure the warming effect of greenhouse gases at Earth's surface.

Measuring energy

The researchers used a set of instruments to take thousands of measurements at the Earth's surface. The instruments record the longwave energy that is re-emitted by greenhouse gases back towards the Earth's surface, which causes the warming.

Making these sorts of measurements on the ground is difficult, says lead author Dr Daniel Feldman, a geological scientist at the Lawrence Berkeley National Laboratory in the US. With weather systems passing overhead, and temperatures and humidity changing frequently, it's tricky to take energy measurements without other factors getting in the way.

To overcome this problem, the researchers measured temperature and water vapour at the same locations so that their influence on warming could be eliminated from the calculations, leaving just the impact of greenhouse gases.

The scientists used data from 2000 to 2010, collected from two sites in the US: the southern Great Plains and northern Alaska. They chose these sites because of their very different climates, says Feldman. This meant the researchers could investigate both a mid-latitude and a high-latitude location.

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Surface warming 'hiatus' could stick around another five years, say scientists

  • 23 Feb 2015, 16:30
  • Roz Pidcock

Don't be surprised if the slower pace of warming we're seeing at the Earth's surface lasts for another five years, scientists say.

new paper out today puts the chances of the so-called "hiatus" staying until the end of the decade at about 15 per cent, or one in six.

But the heat hasn't gone away. The scientists say most of it is lurking in the deep ocean and we can expect the pace of warming to pick up when this heat gets released again.

Slower surface warming

Since 2000, the temperature at the Earth's surface  hasn't warmed as quickly as it has in previous decades, despite greenhouse gas emissions rising  faster than they were before.

A growing body of evidence is  homing in on the  Pacific Ocean as the main culprit for why we're seeing "unexpectedly modest" warming, as the Nature Climate Change paper puts it.

Scientists think a natural fluctuation is causing heat to find its way to the deep ocean in the Pacific, where it doesn't warm the atmosphere as much it would if it stayed at the surface.

A number of recent studies have found that periods of faster and slower warming  aren't unusual in Earth's temperature record. It's what scientists expect as these natural cycles flip-flop between their  different phases, superimposed on top of greenhouse gas warming.

But what are the chances of natural variability being strong enough to offset some, or even all of the warming expected from greenhouse gases?

The new paper by Dr Chris Roberts, an ocean and climate specialist at the Met Office Hadley Centre, and colleagues at the University of Exeter sheds some new light on this question.

Odds of a 'hiatus'

The new paper uses a suite of climate models to examine past temperatures with and without greenhouse gas forcing. The authors find there's a 28 per cent chance natural variability could cause a five-year long 'hiatus'.

The scientists define 'hiatus' as a period during which the observed temperature rise is less than the warming expected from greenhouse gases of 0.2 degrees Celsius per decade.

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Uncertainty behind climate projections could be cut in half by 2030, study shows

  • 23 Feb 2015, 16:00
  • Robert McSweeney

Smoke stacks | Shutterstock

Scientists will soon be able to forecast climate change more accurately, according to new research. Projections of future temperature rely on estimates of how sensitive the Earth's climate is to rising emissions, and the uncertainty in those estimates could be halved within 15 years.

More certainty about the climate's sensitivity to emissions means a better assessment of our chances of keeping global temperature rise below the two-degree limit, the researchers say.

Climate sensitivity

Climate sensitivity is the amount of warming we can expect when carbon dioxide in the atmosphere reaches double the level before the industrial revolution. On current emission trends, we're set to reach that point shortly after 2050.

There are two ways to express climate sensitivity. Equilibrium Climate Sensitivity (ECS) refers to the total amount of warming once the Earth has had time to adjust fully to the extra carbon dioxide. ECS allows for 'feedbacks' in the climate system that can amplify or slow the pace of warming, many of which act over decades or even centuries.

An alternative option is the Transient Climate Response (TCR), which is the warming at earth's surface we can expect at the point of doubling. This doesn't take into account long term feedbacks, and so estimates of TCR are lower than for ECS.

In its 2013 report, the IPCC estimates TCR is likely to lie between 1.0 and 2.5 degrees Celsius. The new research, published in Nature Geoscience, suggests scientists will be able to reduce the uncertainty around these estimates by about 50 per cent by 2030.

Bigger proportion

The new paper deals with one way to estimate TCR, which is to compare how much greenhouse gases have risen over the industrial period with observations of how much the temperature has changed in that time.

But factors such as aerosols and other greenhouse gases have contributed to the observed temperature change, making it difficult to calculate TCR from historical observations.

Aerosols are released into the atmosphere when fossil fuels are burned. These tiny particles have a direct effect on temperature by scattering sunlight, and an indirect effect by stimulating cloud formation, preventing sunlight reaching Earth's surface.

Although scientists know that aerosols have an overall cooling effect on the climate, they aren't as certain about the size of the temperature effect as they are for carbon dioxide.

But uncertainty over aerosols is set to be less of a problem in the near future, the study says.

While emissions of carbon dioxide are expected to rise in the next few decades, emissions of aerosols and other greenhouse gases are expected to slow, or fall. This means carbon dioxide will make up a bigger proportion of the human-caused factors affecting the climate.

You can see this in the graph below: the influence of carbon dioxide on the climate (red line) is projected to increase more rapidly than aerosols and other gases over the next 15 years (blue and green lines).

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New satellite reveals places on Earth most at risk from ocean acidification

  • 17 Feb 2015, 13:15
  • Roz Pidcock

Images beamed back from space are helping scientists monitor how vulnerable the world's oceans are to human pressures. As the level of carbon dioxide in the atmosphere rises, it gets absorbed into the oceans, making them more acidic.

Ocean acidification is a serious but often overlooked concern facing the world's oceans and the ecosystems that depend on them, say the researchers.

The international team of scientists published some of their early findings and images in the journal Environmental Science and Technology today.

Acidifying oceans

quarter of the carbon dioxide released into the atmosphere dissolves into our seas. This changes the seawater chemistry, making it more acidic. This is known as ocean acidification.

Since the start of the industrial era, the pH of ocean surface water has dropped by 0.1, equivalent to a  26 per cent increase in acidity.

But acidification isn't happening at the same pace everywhere, some places are acidifying faster than others. Observing the earth from space using satellites can help identify which regions on Earth are most at risk from ocean acidification.

Alkalinity From Space

A global map of total ocean alkalinity, an indication of the sea surface's ability to buffer itself against ocean acidification. Credit: Ifremer/ESA/CNES

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Reaction: Geoengineering is no substitute for cutting carbon emissions, conclude US researchers

  • 11 Feb 2015, 16:30
  • Robert McSweeney

Above and below clouds | Shutterstock

On Tuesday, the US National Research Council published two new reports on 'climate interventions', or what's more commonly known as 'geoengineering'.

Geoengineering is the deliberate large-scale intervention into the Earth's climate system to try and limit the effects of human-caused global warming, and it can be divided into two main areas Removing carbon dioxide from the atmosphere, sometimes known as Carbon Dioxide Removal (CDR), is one approach. The other is reflecting some sunlight away from the Earth before it can be trapped by greenhouse gases, referred to as 'albedo modification' in the reports, but more commonly known as Solar Radiation Management (SRM).

The new reports are the result of an 18-month study into the potential impacts, benefits and costs of geoengineering. The study produces a set of recommendations, which call for more research and development, but also caution that sunlight-reflecting technologies "should not be deployed at this time".

Geoengineering Summary Table

Overview of general differences between carbon dioxide removal approaches and albedo modification approaches. Source: US National Research Council ( 2015)

While the reports make clear that geoengineering is not a substitute for global action to reduce carbon emissions, it recognises that some action may be necessary to avoid the worst impacts of climate change.

The reports have prompted a flurry of reaction, particularly in the US. Here are some of the selected highlights.

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Aerosols dampen pace of Arctic warming for now, say scientists

  • 11 Feb 2015, 11:00
  • Robert McSweeney

Arctic sea landscape | Shutterstock

As the Earth warms under increasing greenhouse gas emissions, temperatures have risen more quickly in the Arctic than the rest of the world. But particles emitted as fossil fuels are burned mask a lot of that warming. Without them, the temperature rise in the Arctic would be more than double what we've seen in the past century, a new study finds.

But it's no good news story, those same particles are responsible for causing air pollution in cities across the world.

And with air pollution set to fall while greenhouse gases continue to rise, we could soon see a faster rate of warming in the Arctic, the lead author tells Carbon Brief.

Twice as fast

Temperatures in the Arctic are increasing more than twice as fast as the global average. This is known as Arctic amplification. A primary cause is diminishing Arctic sea ice: as the ice melts, energy from the sun that would have been reflected away is instead absorbed by the ocean.

A new study, just published in Nature Climate Change, works out the specific contributions from different influences on temperatures in the Arctic, including natural factors.

Humans can affect the climate in contrasting ways. The planet warms as extra greenhouse gases in the atmosphere trap more heat. But at the same time, other tiny particles and gases known as aerosols are having an overall cooling effect.

The particles have a direct effect on temperature by scattering sunlight, and an indirect effect by stimulating clouds to form, preventing sunlight reaching Earth's surface.

Some sources of aerosols are natural, such as volcanoes and chemicals released by tiny sea creatures. However, since the industrial revolution, humans have been emitting more and more aerosols through vehicle exhausts and burning fossil fuels and wood.

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US flooding on the rise in a changing climate, study shows

  • 09 Feb 2015, 16:05
  • Robert McSweeney

The central US is experiencing flooding more often now than it was 50 years ago, new research shows. The study across 14 states finds rivers over much of the region are breaching their banks more frequently, leading to a greater number of floods.

The researchers attribute the increase in flooding to rising temperatures in the region and more days with heavy rainfall.

Serious flooding

In recent decades, the central US has been hit by a number of serious and widespread floods. Flooding in the spring of 1993 and summer of 2008 affected as many as ten states, for example. The disaster saw hundreds of counties declared Presidential Disaster Areas, giving them access to emergency relief funding.

Scientists have since been trying to work out whether floods are getting worse or if what we're seeing in this part of the US is down to natural variability.

To investigate changes in river flooding, scientists look at historical records of river flow and the maximum amount of water they can hold before overflowing. The flow, or 'discharge', of a river is measured by instruments at different points along its course.

Studies in the past have found the maximum flow through rivers hasn't changed much over the late twentieth and early twenty-first centuries. But a new study, published in Nature Climate Change, finds that rivers are hitting these high flows more often.

More frequent peaks

The researchers analysed records of river flows from 774 instrument stations across the central US, from North Dakota to West Virginia. They counted how many times each river hit a point where there was so much water flowing through it that it was likely to cause a flood.

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Pachauri: IPCC should take official role in assessing country pledges to curb climate change

  • 09 Feb 2015, 15:45
  • Roz Pidcock

The outgoing chair of the Intergovernmental Panel on Climate Change (IPCC), Dr Rajendra Pachauri, has a new vision for the organisation's future.

Traditionally focused on collating the science underpinning climate change, Pachauri's proposals would seem to take the IPCC in a distinctly more political direction.

Suggesting the panel "moves forward with the times and responds to changing expectations", Pachauri wants the IPCC to take an official role in assessing countries' pledges to reduce greenhouse gas emissions, and in totting up whether they add up to enough to meet global climate change targets.

Stepping down

Pachauri has chaired the IPCC for the past 13 years, overseeing the publication of two  major assessment reports. Published every five or six years, the job of these reports is to pull all the latest scientific evidence on how and why the climate is changing into one definitive tome.

With the IPCC's Fifth Assessment Report (AR5) completed last October, the IPCC is in self-reflection mode. This is standard practice after every major report, but this time is perhaps the last formal opportunity for Pachauri to make his thoughts known before stepping down as Chair later this year.

The IPCC has posted  several documents on its website, containing a number of proposals due for consideration when the panel meets at the end of the month in Nairobi, Kenya. One such document is the Chairman's own " Vision Paper on the Future of the IPCC".

Screenshot 2015-02-09 15.43.01

The IPCC's Fifth Assessment Report (AR5)

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What a three-million year fossil record tells us about climate sensitivity

  • 04 Feb 2015, 18:00
  • Robert McSweeney

Sunrise on icy beach | Shutterstock

Understanding how the climate reacts to increasing atmospheric carbon dioxide, known as 'climate sensitivity', is crucial to making accurate projections of global temperature rise.

One way scientists estimate climate sensitivity is to look at Earth's distant past to see how the planet responded to changing greenhouse gas levels. A new study, published in Nature, uses fossilised marine organisms to reconstruct carbon dioxide levels as far back as three million years ago.

And the results suggest climate sensitivity during both these warm and cold climates is in line with the Intergovernmental Panel on Climate Change's (IPCC) estimates. This means we can have high confidence in its projections of future warming, say the scientists.

Climate sensitivity

Climate sensitivity is the amount of warming we can expect when carbon dioxide in the atmosphere reaches double what it was before the industrial revolution. Pre-industrial levels were about 280 parts per million (ppm), we're at nearly 400 ppm now and at current emission rates, we're due to hit 560 ppm soon after 2050.

Warming won't just stop with a doubling of carbon dioxide, however. Under the highest of the IPCC's emissions scenarios, greenhouse gas concentrations by the end of the century would be equivalent to 1370 ppm of carbon dioxide - over four times pre-industrial levels.

Working out the precise value of climate sensitivity is tricky. Estimates come with a range, a lower and upper limit within which the real value could reasonably lie.

The IPCC estimates the value of climate sensitivity is likely to lie between 1.5 and 4.5 degrees Celsius. This takes into account different lines of evidence, including recent observations, records of temperature in earth's distant past, and climate models. You can read more about how scientists calculate climate sensitivity in Carbon Brief's FAQ.


Understanding how sensitive the climate is to 'forcings' such as carbon dioxide has important implications for the long-term projections of future warming, says co-author Dr Gavin Foster, associate professor in geochemistry at the University of Southampton.

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Satellite measurements of the troposphere confirm warming trend, data shows

  • 04 Feb 2015, 14:55
  • Roz Pidcock

Hot on the heels of the news that 2014 was likely the  warmest year on record at Earth's surface, scientists have confirmed the lower part of Earth's atmosphere is warming too.

Since 1979, the troposphere has warmed by 0.14 degrees per decade, scientists at the University of Alabama in Huntsville (UAH)  conclude.

Whether you choose to look at temperatures at Earth's surface or higher up in the atmosphere, nine of the 10 hottest years on record have now occurred in the 21st century.

A tropospheric record

Earlier this week, the World Meteorological Organisation (WMO)  confirmed Earth's surface in 2014 - that's the air above land and the top of the ocean - was  most likely the warmest it's been during the modern temperature record, stretching back to at least the late 19th century.

It's natural that we would be interested in Earth's surface as that's where humans live. But there's another way scientists keep tabs on warming. Satellites monitor air temperature up to 10 kilometres above our heads in a part of the atmosphere known as the troposphere.


Path of a polar orbiting satellite. Source: US National Oceanic and Atmospheric Administration (  NOAA)

Several satellites have been circling Earth since the late 1970's, courtesy of the US National Oceanic and Atmospheric Administration (NOAA). These satellites circle the Earth at the poles, carrying microwave instruments that measure how much heat is given off by oxygen in the troposphere. From this, scientists can  work out the air temperature.

These polar-orbiting satellites send more than 16,000 measurements daily, achieving 100 per cent coverage of the globe every 3 to 4 days.

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