Improved calculation of warming-equivalent emissions for short-lived climate pollutants

• Published in: NPJ climate and atmospheric science 2019-09, Vol.2 (1), p.29, Article 29
• Main Authors: Cain, Michelle, Lynch, John, Allen, Myles R., Fuglestvedt, Jan S., Frame, David J., Macey, Adrian H
• Format: Article
• Language: English
• Subjects: 704/106/694/1108; 704/106/694/682; 704/844/2175; 704/844/682; Anthropogenic factors; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Carbon dioxide; Carbon dioxide emissions; Climate change; Climate change mitigation; Climate Change/Climate Change Impacts; Climate policy; Climate system; Climatology; Earth and Environmental Science; Earth Sciences; Emissions; Environmental policy; Equivalence; Global warming; Greenhouse effect; Greenhouse gases; Human influences; Masks; Mathematical analysis; Methane; Paris Agreement; Pollutants; Radiative forcing; Surface temperature; Time
• ISSN: 2397-3722; 2397-3722
• DOI: 10.1038/s41612-019-0086-4

Anthropogenic global warming at a given time is largely determined by the cumulative total emissions (or stock) of long-lived climate pollutants (LLCPs), predominantly carbon dioxide (CO 2 ), and the emission rates (or flow) of short-lived climate pollutants (SLCPs) immediately prior to that time. Under the United Nations Framework Convention on Climate Change (UNFCCC), reporting of greenhouse gas emissions has been standardised in terms of CO 2 -equivalent (CO 2 -e) emissions using Global Warming Potentials (GWP) over 100-years, but the conventional usage of GWP does not adequately capture the different behaviours of LLCPs and SLCPs, or their impact on global mean surface temperature. An alternative usage of GWP, denoted GWP*, overcomes this problem by equating an increase in the emission rate of an SLCP with a one-off “pulse” emission of CO 2 . We show that this approach, while an improvement on the conventional usage, slightly underestimates the impact of recent increases in SLCP emissions on current rates of warming because the climate does not respond instantaneously to radiative forcing. We resolve this with a modification of the GWP* definition, which incorporates a term for each of the short-timescale and long-timescale climate responses to changes in radiative forcing. The amended version allows “CO 2 -warming-equivalent” (CO 2 -we) emissions to be calculated directly from reported emissions. Thus SLCPs can be incorporated directly into carbon budgets consistent with long-term temperature goals, because every unit of CO 2 -we emitted generates approximately the same amount of warming, whether it is emitted as a SLCP or a LLCP. This is not the case for conventionally derived CO 2 -e. Climate mitigation: new metric for comparing greenhouse gases Effective climate policy necessitates being able to compare the impact of different greenhouse gases on warming. This is commonly accomplished by converting non-CO 2 emissions to CO 2 equivalent emissions using Global Warming Potentials (GWP). However, the conventional GWP approach masks the true behaviour of short-lived climate pollutants (SLCPs) and an alternative GWP approach has recently been developed. Michelle Cain of the University of Oxford and a team of international colleagues extend this alternative GWP approach that slightly underestimates the impact of SLCPs on warming. They develop a new metric that approximates both the short-timescale climate response to changing SLCP emission rates and the l