Scientists have presented a new, narrower estimate of the “climate sensitivity” – a measure of how much the climate could warm in response to the release of greenhouse gases
January 17, 2018 — The latest assessment report from the Intergovernmental Panel on Climate Change (IPCC) estimates that climate sensitivity is close to 3°C, with a “likely” range of 1.5 to 4.5°C.
The new study, published in Nature, refines this estimate to 2.8°C, with a corresponding range of 2.2 to 3.4°C. If correct, the new estimates could reduce the uncertainty surrounding climate sensitivity by 60%.
The narrower range suggests that global temperature rise is “going to shoot over 1.5°C” above pre-industrial levels, the lead author tells Carbon Brief, but “we might be able to avoid 2°C”. Meeting either limit will likely require negative emissions technologies that can remove CO2 from the atmosphere, he says.
The new estimate is another “brick in the wall” of scientists’ understanding of climate sensitivity, another scientist tells Carbon Brief, and “the best-informed views will be reached by multiple lines of evidence”.
Climate sensitivity is the amount of warming that can be expected in response to the concentration of CO2 in the atmosphere reaching double the level observed in pre-industrial times.
The research makes a new estimate of the “equilibrium” climate sensitivity (ECS) – that is, the amount of warming expected to occur once the full impact of the extra greenhouse gases release has played out. This measure includes the impact of warming on long-term climate feedback loops, which can take decades, or even centuries, to materialise.
The value of ECS is one of the big climate change questions that scientists are still trying to address.
It is important because understanding how sensitive the Earth is to CO2 could help us to estimate how much the planet could warm in response to greenhouse gases, explains Prof Peter Cox, lead author of the new paper and a climate scientist at the University of Exeter. He tells Carbon Brief:
The new findings indicate that the value of ECS could be close to 2.8°C, says Cox:
Capturing a signal
There are a number of techniques that scientists can use to work out what ECS could be.
One method is to look at how Earth has responded to natural greenhouse gas changes in its geological past to try to work out how it might respond to future global warming.
Another common technique involves using global climate models to estimate the theoretical effect of a doubling the amount of CO2 in the atmosphere.
A third method used by scientists involves matching global surface temperatures with the global warming trend over the past century to try and work out sensitivity from how the planet is responding. (This is what is known as the “energy budget model” approach.)
The new study uses a similar method to the energy budget model approach. However, instead of matching the global temperature record to global warming, the new research attempts to match temperature records to natural, long-term fluctuations in temperature.
Looking at natural variability rather than the warming trend allowed the scientists to exclude a range of uncertainties associated with human-caused climate change, Cox explains:
To understand how historical temperature fluctuations have changed over the past century, the researchers first removed the global warming trend from a set of observational temperature data.
They then compared this data to results from a series of 22 global climate models. Some models had lower climate sensitivity, while some some models had higher climate sensitivity.
The results are shown on the chart below. On the chart, black dots show natural fluctuations in temperature from 1940 to 2020. Each line represents the results from one model, with magenta lines showing results from higher sensitivity models and green showing the results from models with lower climate sensitivity.
The chart indicates that higher sensitivity models generally predict more warming than has been observed over the past 50 years, while lower sensitivity models either closely match the observed trend or estimate a lower amount of warming.
Together, these results allowed the researchers to produce their narrower range.
Understanding ECS could help scientists to work out how much the climate is likely to warm in the future, Cox says, which, in turn, could allow policymakers to estimate how easy it will be to meet the goals of the Paris Agreement.
Climate sensitivity is the amount of warming that will occur after CO2 concentrations become twice as high as they were in pre-industrial times. Pre-industrial CO2 concentration levels were about 280 parts per million (ppm) and levels are currently at around 404ppm.
This means that, if humans stopped releasing CO2 today, the world should expect to experience more than half of the warming dictated by the ECS. Cox explains:
If the results are correct and the climate sensitivity is 2.8°C, then it is likely that the world will fail to limit warming to 1.5°C above pre-industrial levels, which is the aspirational goal of the Paris Agreement, Cox adds:
Negative emissions technologies are a group of techniques – many of which still remain hypothetical – that aim to remove CO2 from the air in an attempt to tackle climate change.
The study’s results “reduce the probability of very high climate sensitivity”, which should “reassure” those taking steps to meet the goals of the Paris Agreement, says Prof Gabi Hegerl FRS, a climate system scientist from the University of Edinburgh, who was not involved in the research. She tells Carbon Brief:
Reducing uncertainty surrounding climate sensitivity should help policymakers to refocus their efforts on tackling climate change, says Cox:
‘Brick in the wall’
The new paper adds to the extensive research around the potential value for ECS.
Despite debate among scientists about the best way to estimate climate sensitivity, each new research paper can be seen as “brick in the wall” of our understanding, says Prof Andrew Dessler, an atmospheric scientist from Texas A&M University, who was not involved in the research. He tells Carbon Brief:
The methods used in the study are “credible”, says Prof Steven Sherwood, deputy director of the Climate Change Research Centre at the University of New South Wales. However, it does have its limitations, he says.
For example, the paper does not discuss how natural events, such as El Niño, could impact temperature fluctuations, he tells Carbon Brief:
In addition, the research may have made “significant” errors in its attempts to reduce uncertainty surrounding climate sensitivity, says Dr Patrick Brown, a climate scientist from the Carnegie Institution for Science in Stanford, California.
Last month, Brown was the lead author of a Nature paper which found that ECS could be relatively higher than previous estimates have suggested – their central estimate was 3.7°C. Brown tells Carbon Brief:
However, writing in an accompanying News & Views article, Prof Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds, who was also not involved in the research, argues that some published estimates for ECS have “depended on the researchers’ assumptions about ECS, rather than the evidence.” He writes:
Forster adds that the methods used in the present study are “enviably simple” and will leave climate scientists asking, “why didn’t I think of that?” He says: