A stark warning that unmitigated emissions would cause even more profound warming of 8 °C globally and 17 °C in the Arctic by 2300, has been made in a new study.
From Nature Climate Change
Burning the Earth’s remaining fossil fuel resources, equivalent to five trillion tonnes of CO2 emissions (5 EgC), could lead to an average temperature increase of about 8 °C globally and 17 °C in the Arctic by 2300, reports a paper published online this week in Nature Climate Change. The modelling study shows that the linear relationship between global warming and cumulative CO2 emissions holds up to 5 EgC, and suggests that, in the absence of mitigation, warming is likely to be considerably larger than previously anticipated.
The relationship between warming and cumulative CO2 emissions has previously been shown to be approximately linear up to 2 EgC, but simulations run using simple climate models suggest that this relationship breaks down at higher cumulative emissions.
Katarzyna Tokarska and colleagues use a set of comprehensive, complex Earth system models to simulate long-term warming in response to 5 EgC. These models show that warming continues to increase approximately linearly under high cumulative emissions, with an estimated global mean warming range between 6.4 and 9.5 °C, and a mean Arctic warming ranging between 14.7 and 19.5 °C. In comparison, Earth system models that are not as comprehensive or complex show that there will be less warming, possibly due to the incorrect representation of certain processes and feedbacks, such as the efficiency of ocean heat uptake.
The models also suggest that 5 EgC will increase the average regional precipitation by more than a factor of four in the tropical Pacific, but lower it by more than a factor of two over parts of Australia, the Mediterranean, southern Africa and the Amazon, and by a factor of three in central America and North Africa.
In an accompanying News & Views article, Thomas Frölicher writes: “the work by Tokarska and colleagues highlights that the regulatory framework based on cumulative CO2 emissions is probably robust over a much wider range of plausible CO2 emissions than previously thought. This implies that the unregulated exploitation of fossil fuel resources could result in significant, more profound climate change.”
Concrete actions to curtail greenhouse gas emissions have so far been limited on a global scale, and therefore the ultimate magnitude of climate change in the absence of further mitigation is an important consideration for climate policy. Estimates of fossil fuel reserves and resources are highly uncertain, and the amount used under a business-as-usual scenario would depend on prevailing economic and technological conditions. In the absence of global mitigation actions, five trillion tonnes of carbon (5 EgC), corresponding to the lower end of the range of estimates of the total fossil fuel resource, is often cited as an estimate of total cumulative emissions. An approximately linear relationship between global warming and cumulative CO2 emissions is known to hold up to 2 EgC emissions on decadal to centennial timescales; however, in some simple climate models the predicted warming at higher cumulative emissions is less than that predicted by such a linear relationship. Here, using simulations from four comprehensive Earth system models, we demonstrate that CO2-attributable warming continues to increase approximately linearly up to 5 EgC emissions. These models simulate, in response to 5 EgC of CO2 emissions, global mean warming of 6.4–9.5 °C, mean Arctic warming of 14.7–19.5 °C, and mean regional precipitation increases by more than a factor of four. These results indicate that the unregulated exploitation of the fossil fuel resource could ultimately result in considerably more profound climate changes than previously suggested.
Katarzyna B. Tokarska, Nathan P. Gillett, Andrew J. Weaver, Vivek K. Arora and Michael Eby; The climate response to five trillion tonnes of carbon; Nature Climate Change (2016) doi:10.1038/nclimate3036
Nature Climate Change news release.