Research Gate talked to lead author Katarzyna Tokarska from the University of Victoria to find out why her results are so much more extreme than previous estimates.
RG: Could you explain the main findings of your study and their significance?
Katarzyna Tokarska: If we continue to burn the Earth’s remaining fossil fuel resources, the Earth will encounter a profound climate change, with the global mean temperature rise ranging between 6.4 and 9.5 degrees Celsius. The relationship between warming and total amount of carbon emitted continues to be linear even for higher levels of total carbon emitted.
It is relevant to know what would happen if we do not take actions to mitigate climate change – for example, if we do not ever implement the Paris Agreement or other similar agreements. Our study shows a profound climate change in the absence of further mitigation actions.
RG: How did you go about reaching these findings?
Tokarska: We used comprehensive climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We analyzed climate change under a business-as-usual scenario, which represents a scenario of high levels of greenhouse gas forcing under no or very little mitigation actions.
RG: Why have the majority of previous estimates of climate change-induced temperature rises been so conservative in comparison to your findings?
Tokarska: The Intergovernmental Panel on Climate Change Fifth Assessment reports the linearity of the relationship between warming and cumulative carbon emissions up to 2000 PgC, the equivalent to two trillion tons of carbon, and very few studies looked beyond that. Only a few studies, using simpler climate models, have looked at this relationship for higher levels of cumulative carbon emissions.
Our study suggests the role of ocean heat uptake as one of the key processes contributing to the linearity of this relationship at high levels of total carbon emissions. The ocean takes up heat more slowly under those conditions, leading to a higher global mean warming.
In some of the simpler models, the ocean continues to take up heat despite increasing emissions and increasing warming, making the ‘net’ global temperature lower than it would be without ocean heat uptake. Other processes, such as cloud feedbacks or effects on the carbon cycle could also play a role, but we would need additional model simulations to be able to test for them.
RG: How did you come to the figure of five trillion tons of remaining carbon?
Tokarska: Five trillion tons of carbon (5 EgC) has often been cited as an amount of CO2 which would be ultimately emitted without mitigation actions – for example in the IPCC Third Assessment Report. As we note in the paper, this number is actually at the low end of quoted estimates of the fossil fuel resource. Other studies estimate this number at approximately 10 trillion tons, which, if burned, could lead to even more extreme temperature rises.
RG: What are the next steps in this research?
Tokarska: We are currently working on exploring the role of non-CO2 forcing in that framework – or the impact of non-CO2 forcing on cumulative carbon budgets using a comprehensive Earth System Model – as this study mostly focused on CO2-attributable climate change.
RG: What was your personal motivation for this research?
Tokarska: On a personal level, part of the motivation for my research is a quote from Pope Francis: “What kind of world do we want to leave to those who come after us, to children who are now growing up?” Knowing how the Earth could become if we burn the Earth’s remaining fossil fuel resources, we should consider climate mitigation actions now before it is too late.
