by Makari Krause
Anthropogenic carbon emissions have been a large factor in climate change since the start of the industrial revolution. Scientists have become increasingly concerned with warming and other effects associated with the release of carbon into the atmosphere. Currently, most world governments have set a target that limits warming to two degrees Celsius since preindustrial times. With this target in place policies are then enacted to limit carbon emissions and hopefully to mitigate anthropogenic effects on earth’s climate. Steinacher et al. (2013) set out to show that setting a target temperature is not sufficient to control many other effects of climate change such as sea level rise and ocean acidification that also result from anthropogenic carbon emissions. They find that when targets are set for these other factors, the allowable carbon emissions are much lower than current targets based on temperature alone.
Steinacher, M., Joos, F., & Stocker, T. F., 2013. Allowable carbon emissions lowered by multiple climate targets. Nature 499(7457), 197–201. http://goo.gl/iSO7tn
Steinacher et al. looked at the goal of the United Nations Framework Convention on Climate Change and saw that it was; “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” Their climate system includes the hydrosphere, atmosphere, biosphere, and geosphere and the Steinacher et al. believe that their goal cannot be reached with respect to all of these different spheres through only a temperature target. They define six target variables that will be affected by climate change: global mean surface air temperature increase above preindustrial levels (∆SAT), steric sea level rise (SSLR), the fraction of the southern ocean surface area that undergoes a transition from supersaturation to undersaturation with respect to aragonitic calcium carbonate (ASO), the loss of global ocean surface area with at least threefold supersaturation (AΩ>3), the fraction of the global cropland area that suffers from substantial local net primary production reductions, and the percentage of carbon lost from cropland soils since 2005. The responses of these variables to carbon were determined using the highest and lowest representative concentration pathways from the IPCC’s Fifth Assessment Report. Steinacher et al. then ran their model for 55 economically plausible greenhouse gas concentration pathways.
Steinacher et al. found that ∆SAT and SSLR both increase with CO2 and non-CO2 radiative forcing. In order for ∆SAT to stay below the 3˚ C limit, atmospheric CO2 must not exceed 550-870 ppm. SSLR, on the other hand, will not exceed its limit of 60 cm in the next 100 years in any of the scenarios for climate change. Ocean acidification was not effected much by non-CO2 radiative forcing and was mainly driven by CO2 increase. ASO will be limited to 25% of the Southern Ocean surface by 2100 if CO2 is limited to 625 ppm. AΩ>3 will be heavily affected by CO2 and Steinacher et al. found that “it is unlikely (<33%) that less than 90% of these waters are lost during this century in scenarios with CO2 > 550ppm. There was not much of a connection between the two cropland targets and it is unlikely that the limits to these two variables will be exceeded during this century.
Steinacher et al. then combined the different variables and found the allowable cumulative twenty-first-century fossil-fuel CO2 emissions. AΩ>3 was the most restrictive variable when non-CO2 radiative forcing is high with a limit of 625 GtC. With low non-CO2 radiative forcing ∆SAT was the most restrictive with a limit of 1,600 GtC. Limits on the other variables would only start becoming important over much longer time scales.
These results imply that in order to meet the target for the most stringent variable we would have to limit atmospheric CO2 to about 500ppm. While it would seem that meeting the most stringent target would simultaneously meet the others, this may not necessarily be the case. When all of the variables are combined and looked at simultaneously we find that they are interdependent and may require lower targets so that they do not exceed their limits, Steinacher at al. estimate that this target is 490±20 ppm, significantly lower than the limit on carbon emission if temperature is the only variable used. In addition to new targets being set worldwide, Steinacher et al. believe that there need to be region-specific targets set so that anthropogenic effects on the climate can be accurately assessed and monitored globally. They currently argue that targets are inadequate and are going to greatly overestimate the amount of carbon that we can emit before climate change begins to have drastic effects.