In a new perspective published on 17 July in Nature, climate scientists identify several important factors affecting carbon budget estimates, including historical warming, transient climate response to cumulative carbon emissions (TCRE), zero-emissions commitments, and non-carbon dioxide (CO2) contributions to global warming (1). By weeding out these uncertainties, decision-makers may be able to use carbon budget estimates more effectively to limit global warming.
Global warming is proportional to the total amount of carbon dioxide released into the atmosphere, that authors explain, which is just a fancy way of saying that increasing CO2 emissions lead to more global warming.
But from this simple idea comes the carbon budget — a useful tool in climate science that simplifies the response of the complex Earth system to CO2 emissions into a simple linear relationship. And this allows scientists to more easily communicate the implications of climate change to policymakers and the public.
The remaining carbon budget can be defined as the total global amount of anthropogenic carbon dioxide produced by human activities that can still be emitted into the atmosphere without the global average temperature increase exceeding the targets laid out in the internationally agreed Paris Accord — to keep warming well below 1.5 or 2 degrees Celcius
However, at present, a wide range of reported remaining carbon budget estimates limits the effectiveness of this important tool. Therefore, more accurate estimates would help policymakers develop policies to meet stringent climate targets.
So, to make carbon budgets more comparable, the researchers identified a number of relevant factors affecting these estimates. Moreover, they describe a new framework that could be used to more effectively track estimates of the remaining carbon budget. In addition, they derive an equation for estimating the remaining carbon budget that incorporates the five key factors.
“Bringing CO2 emissions, from industry to transport, to net-zero requires urgent action”, says co-author Dr Elmar Kriegler of the Potsdam Institute for Climate Impact Research (PIK) in Germany. “Yet, untangling different carbon budget calculations is more than just an academic issue. It tells us about the risks.”
The authors explain that carbon budget estimates often ignore importing tipping processes such as permafrost thawing and therefore, the associated long-term release of large quantities of CO2 and methane. Differences in carbon budgets may also arise due to the way temperature is measured. For example, the Earth’s Surface air temperature (SAT) — frequently used in estimates —.versus sea surface temperatures, which warm more slowly than air and could lead to underestimates.
Teasing out uncertainties and making different estimates more comparable will help policymakers to make more informed choices. In particular, the authors suggest weeding out uncertainties in carbon budget estimates could be invaluable to the forthcoming Sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, and could represent an important milestone in climate policy.
(1) Rogelj, J. et al. Estimating and tracking the remaining carbon budget for stringent climate targets. Nature (2019) DOI: 10.1038/s41586-019-1368-z