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High altitude view of the Earth in space with stars. The desert in the western United States.
Credit: MarcelClemens/Shutterstock
CLIMATE SENSITIVITY
23 February 2015 16:00

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

Robert McSweeney

Robert McSweeney

02.23.15
Robert McSweeney

Robert McSweeney

23.02.2015 | 4:00pm
Climate sensitivityUncertainty behind climate projections could be cut in half by 2030, study shows

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).

Myhre Et Al (2015) Climate Sensitivity Fig 3a

Graph of the influence (‘forcing’) of human-caused substances on the climate, using observations of the past and projections for the future. Includes carbon dioxide (red line), other greenhouse gases (blue), aerosols (bright green), other substances (pale green), and a total of all influences (black). Positive influences have a warming effect, while negative influences have a cooling effect. Projections for the future are based on high (RCP8.5) and low (RCP2.6) emissions scenarios. Source: Myhre et al. (2015)

Headache ‘will eventually go away’

Since scientists have a better idea of carbon dioxide’s impact on temperature than the other factors, the fact that the gas will make up a larger proportion of the total influence of human activity on temperature will make it easier to calculate TCR based on historical observations.

This, in turn, will mean scientists are able to make more accurate projections of the future climate response to human activity, says lead author Dr Gunnar Myhre, a senior research fellow at the Center for International Climate and Environmental Research in Oslo (CICERO). This would also give scientists a better idea of how far emissions must fall to stay below the internationally-accepted limit of two degrees warming above pre-industrial levels, he adds.

Co-author Prof Piers Forster, director of research at the University of Leeds, says he’s looking forward to getting past his frustrations with uncertainty in climate projections:

“Uncertainty in climate sensitivity has hampered our ability to make accurate climate projections. We show that the headache of trying to diagnose climate sensitivity from past changes will eventually go away, even if it does last another 10 years.”

So better estimates of future temperature rise may not be far away, but we ought to be well on our way to cutting emissions by then, says Myhre:

“Climate research has shown over several decades that the current large and increasing emissions of greenhouse gases will result in substantial global warming, so actions to reduce the emissions should anyway be made. Even though we will be more certain in the future about the climate system and climate sensitivity, the current knowledge is very solid.”

Main image: High altitude view of the Earth in space with stars. The desert in the western United States.

Mhyre, G. et al. (2015) Declining uncertainty in transient climate response as CO2 forcing dominates future climate change, Nature Geoscience, doi:10.1038/ngeo2371

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