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Narrowing down long-term global warming estimates
A new constraint on a key metric that is used to gauge Earth’s response to increasing carbon dioxide — the equilibrium climate sensitivity (ECS) — is presented in a paper published this week in Nature. The analysis suggests that extremely high estimates of this sensitivity can be ruled out.
ECS is defined as the global mean warming that would occur if the atmospheric carbon dioxide concentration doubled. It is a key tool for discussing and comparing climate models and an important point of policy discussions, including international climate change agreements. However, a range of ECS estimates have been calculated, which have been hard to reconcile.
Many attempts to constrain ECS have used either the historical warming record or reconstructions of past climates. Methods based on historical warming are affected by multiple factors — including uncertainties in ocean heat uptake and the contribution of aerosols to net radiative forcing — which can affect the accuracy of the estimate. Peter Cox and colleagues now present a probability distribution of ECS, based on the observed historical variability in temperature, rather than the warming trend itself. The authors determine a likely range for ECS of 2.2–3.4 degrees Celsius, which is a 60% reduction in uncertainty compared to the estimate from the Intergovernmental Panel on Climate Change of 1.5–4.5 degrees Celsius. In addition, this approach allows the authors to almost exclude ECS estimates above 4.5 degrees Celsius or below 1.5 degrees Celsius
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Professor Steven Sherwood is ARC Laureate Fellow at the ARC Centre for Climate System Science and UNSW Climate Change Research Centre and Chief Investigator at the ARC Centre of Excellence for 21st Century Weather
This important and interesting new study by Cox et al. addresses how sensitive the Earth’s climate is to human influence. Climate contrarians say it's not sensitive, while some models say it is very sensitive; most scientists figure it's unlikely to be too far from a middle value (around 3C per doubling of CO2). Middle implies about 4C of warming by 2100 if we continue on a high-emissions trajectory, which would not be good. Low sensitivity would imply less to worry about, while high sensitivity would be really alarming.
What this paper argues is that size and duration of natural year-to-year changes in global temperature can tell us the answer. These natural changes are captured by all climate models, but the study shows that the stronger and more persistent they are, the more the longer-term warming. If their approach is right it basically means the mainstream view has been correct: there is essentially no chance that contrarians have been right, but also hardly any chance of the worst-case scenario. Both the lowest and highest previous predictions are contradicted by this new approach.
The new study has an appealing basis in statistical mechanics, adding credibility compared to previous studies of this type, which pointed more to higher sensitivity. However no single study so far is without caveats—for example, the Cox approach mixes up natural variability due to El Nino, decadal variations, volcanic eruptions and air pollutants, and we know that models have different biases with respect to each of these. There are also theoretical problems with applying their statistical approach in this way even though it seems to work. So it is not clear whether to put more weight on this study, or the previous ones suggesting even higher sensitivity. But one thing all the studies agree on is that climate sensitivity is not very low, i.e., it is above 2C per doubling. As such, the new study adds to the already very strong evidence in favour of the central predictions made over the years by the IPCC, though also offering the good news of further reducing the chance of the worst-case possibilities.
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