Extensive research has gone into understanding the response of global temperatures to increased CO2 emissions. However, not enough quantitative studies have investigated the time it takes for this response to manifest itself. Uncertainties in these estimates are due to uncertainties in the understanding of the effects of equilibrium climate sensitivity, carbon cycling and thermal inertia of oceans, which all act to modulate the temperature response to increased greenhouse gases. A recent study published in a current issue of Env. Research Letters by Ricke and Caldeira (2014), uses a suite of carbon cycle and physical-climate models from two model inter comparison projects (IRF-MIP for carbon cycle models; and CMIP5 archive for physical climate models) to evaluate the temperature response to a pulsed input of CO2 into the system.
The carbon-cycle models ranged in complexity from simple box models to complex earth system models. The structure of temperature response resulting from a pulsed input of 100 GT of carbon is evaluated. Ensemble memberscccc of IRF-MIP are used to estimate the uncertainty in the temperature response due to uncertainty in the carbon cycle. For physical-climate models from the CMIP5 archive, simulations conducted with 4xCO2 are used. A linear interpolation is used to translate results from 4xCO2 runs in the CMIP5 archive to estimate the duration and magnitude of climate response to 100 GT of C. These results are used to calculate the uncertainties in the temperature response from equilibrium climate sensitivity and the thermal inertia of the oceans. Combined approximations of the climate system’s response to a present-day CO2 pulse emission from all the models are calculated using a convoluted integral approach.
There is broad agreement between the various models. A pulse emission of CO2 results in a step increase in atmospheric CO2 levels, followed by a slow decline due to uptake by the biosphere and oceans. Temperature response lags the CO2 rise due to the thermal inertia in the upper layers of the ocean. The median duration between emission and maximum warming is ~10 years (with a range between 6.6 and 30.7 years). This suggests that the temperature effects of increased CO2 will be felt much quicker than generally assumed (i.e., in decades, rather than centuries). Additionally, models also show that the response lasts for over a century. The median temperature, a century after the emission, is 87% (with a range from 65-97%) of the maximum observed temperature, indicating that the effects can last for a long period of time. Therefore, benefits from reducing emissions today can have a large impact affecting both current and future generations, rather than future generations alone. In terms of uncertainties, while equilibrium climate sensitivity is the reason for the largest uncertainty of these estimates, it is noted that simply improving any single uncertainty term will not change the total uncertainty significantly. Therefore, efforts should be undertaken to resolve all three uncertainties in parallel.
Ricke, K.L., and Caldiera, K., 2014. Maximum warming occurs about one decade after a carbon dioxide emission, Environmental Research Letters, 9, doi:10.1088/1748-9326/9/12/124002.