Near-term climate impacts are roughly linear in cumulative CO2 emissions

Christopher Callahan, Lyssa M Freese, Marissa Childs, Marshall Burke

Understanding the relationship between CO2 emissions and human well-being is central to informing climate policy. While global temperature change is proportional to cumulative CO2 emissions, many human outcomes respond nonlinearly to climate variation. As a result, the magnitude of the effect of a quantum of CO2 on these outcomes is ambiguous. Here we demonstrate that the impact of climate change on many important outcomes is, in fact, approximately linear in cumulative CO2 emissions. Analyzing established response functions across economic growth, disease, mortality, and crop yields, we show that local warming is generally insufficient to alter the local derivatives of these response functions. Therefore, the marginal effect of CO2 emissions does not substantially change over policy-relevant ranges of emissions. Thus, there is no level of CO2 emissions that is irrelevant to human well-being: each ton emitted directly translates into reduced GDP growth, increased disease, altered temperature-related mortality, and reduced crop yields.