Figure: Idealized carbon dioxide emission scenarios (a) and global mean temperature responses (b).
In recent years, the ida of measuring climate change as a function of cumulative CO2 emissions has emerged as a simple and effective tool to understand and quantify how global temperature response to anthropogenic carbon emission. Climate model simulations and theoretical arguments indicate an almost linear relationship between cumulative carbon emissions and projected global mean temperature to the year 2100 and beyond in a wide set of future carbon emissions scenarios.
Using millennial simulations with complex Earth System Models, we show for the first time that even if carbon dioxide emissions came to a sudden halt, the carbon dioxide already in the Earth's atmosphere could continue to warm the Earth for hundreds of years. This result suggest that the coefficient linearly relating global warming to cumulative carbon emissions may increase after carbon emission ceases and that it might take a lot less carbon than previously thought to remain below a policy-driven global temperature target.
Global mean temperature changes simulated (phase I) and estimated (phases II and III) by (a) 12 CMIP5–ESMs and (b) 8 EMICs. Gray lines in phase I indicate the simulated temperature responses of the individual models, whereas the green, red and blue lines in phase I–III are the simulated and estimated multi-model mean temperature responses. The simulated (phase I and II) and estimated (phase III) temperature response in GFDL ESM2M is shown in black color.
This initial 2014 work was followed by a paper confirming that the initial model results also hold under a more standard carbon emissions scenario and under a wide-range of different complex Earth System Models. Surprisingly, simple intermediate complexity models show a decrease in global temperature after zero carbon emissions. Because the IPCCs 'carbon budget' estimates were based on these simpler models, this has interesting implications. If you want to stay below 2 degrees global warming, for example, you may need to keep to a stricter carbon budget.