Abstract
We investigate the extent to which global mean temperature, precipitation, and the carbon cycle are constrained by cumulative carbon emissions throughout four experiments with a fully coupled climate–carbon cycle model. The paired experiments adopt contrasting, idealised approaches to climate change mitigation at different action points this century, with total emissions rising to more than two trillion tonnes of carbon (TtC). For each pair, the contrasting mitigation approaches—capping emissions early versus reducing them to zero a few decades later—cause their cumulative emissions trajectories to diverge initially, then converge, cross, and diverge again. We find that global mean temperature is linear with cumulative emissions across all experiments, although differences of up to 1.5 K exist regionally when the trajectories of total carbon emitted during the course of the two scenarios coincide, for both pairs of experiments. Interestingly, although the oceanic precipitation response scales with cumulative emissions, the global precipitation response does not, due to a decrease in precipitation over land above emissions of around one TtC. Most carbon fluxes are less well constrained by cumulative emissions as they reach two trillion tonnes. The opposing mitigation approaches have different consequences for the Amazon rainforest, which affects the linearity with which the carbon cycle responds to cumulative emissions. The average Transient Climate Response to cumulative carbon Emissions (TCRE) is 1.95 K TtC−1, at the upper end of the Intergovernmental Panel on Climate Change’s range of 0.8–2.5 K TtC−1.
Highlights
Since the beginning of the industrial age, human activities have resulted in the release to the atmosphere of more than half a trillion tonnes of carbon (TtC) in the form of carbon dioxide (Ciais et al 2013), the greenhouse gas primarily responsible for the change in climate the planet has experienced since that time (Myhre et al 2013).An important question which has been addressed through the use of climate models over the last two decades is, at what level should atmospheric CO2 be limited to in order to prevent dangerous climate change (Hansen et al 2008)? Answering this question is difficult due to uncertainty in the amount of warming the planet would undergo following stabilisation of atmospheric CO2
In the event that a CO2 concentration target could be agreed upon, the task of deciding how emissions should be controlled in order to achieve the target is itself compounded by the existence of feedbacks between the carbon cycle and climate; the changing climate undermines the capacity of the land surface and the oceans to absorb carbon from the atmosphere, leading to further climate change (Friedlingstein et al 2006)
The cumulative emissions of 2012E10 coincide with those of 2050E0 in year 2065; their decadal mean temperatures centred on that year differ by only 0.05 °C
Summary
Since the beginning of the industrial age, human activities have resulted in the release to the atmosphere of more than half a trillion tonnes of carbon (TtC) in the form of carbon dioxide (Ciais et al 2013), the greenhouse gas primarily responsible for the change in climate the planet has experienced since that time (Myhre et al 2013).An important question which has been addressed through the use of climate models over the last two decades is, at what level should atmospheric CO2 be limited to in order to prevent dangerous climate change (Hansen et al 2008)? Answering this question is difficult due to uncertainty in the amount of warming the planet would undergo following stabilisation of atmospheric CO2. A simple linear relationship between emissions and global mean warming has been established, with the potential to simplify advice to policymakers. Acknowledging this development, the Intergovernmental Panel on Climate Change (IPCC) for the first time expressed climate change in terms of cumulative carbon emissions in its 5th Assessment Report (AR5), through the transient climate response to cumulative CO2 emissions (TCRE) metric, defined as the global mean warming per TtC emitted (Collins et al 2013, IPPC 2013). The work of Leduc et al (2016) suggests that linearity of surface temperature change with cumulative carbon emissions is applicable regionally
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