Research proposes a new use of global warming potentials to compare the impact of cumulative pollutants such as carbon dioxide with short lived pollutants such as black carbon. Metric choice can affect the relative emphasis placed on reductions of cumulative climate pollutants versus short lived climate pollutants.
A new way of accounting for greenhouse gas emissions that would align government climate change commitments better with long-term temperature goals is suggested in a study published in Nature Climate Change.
The study by Myles Allen, professor of geosystem science at Oxford University and Piers Forster, professor of physical climate change at the University of Leeds, together with with colleagues from Reading, Oslo and Wellington, points out that current measures currently over-value the impact of “short lived climate pollutants”, such as methane and black carbon (soot).
For over 20 years, governments have used a “metric”, or exchange rate to compare the potency of different greenhouse gases, called the 100-year Global Warming Potential, or GWP100. This is defined as the amount of heat trapped in the atmosphere over 100 years by a given mass of an emitted gas, compared to the same mass emitted of CO2. The study shows that, despite its name, GWP100 actually indicates the relative impact on global temperature of different gases some 20 to 40 years after their time of emission.
If, the authors argue, temperatures approach their peak shortly after mid-century, as they need to if the Paris goal of a warming “well below 2C” is to be achieved, then GWP100 represents a rough-and-ready indicator of the relative importance of today’s emissions for peak warming.
But if they do not and temperatures continue to climb to the end of the century, then GWP100 substantially over-values “short lived climate pollutants”, such as methane and black carbon (soot). These substances are so-called because unlike CO2, which sticks around in the atmosphere for many hundreds of years, they typically have lifetimes of years or even days.
The authors propose the simplest solution is to avoid pretending there is any kind of equivalence between “cumulative pollutants”, such as CO2 and nitrous oxide, and any amount of short lived pollutants, until emissions of cumulative pollutants are falling fast enough that it is possible to predict when they will reach net zero. This is required to stabilise global temperatures at 2C, or any other target.
Parties to the United Nations Framework Convention on Climate Change (UNFCCC) have requested guidance on common greenhouse gas metrics in accounting for Nationally determined contributions (NDCs) to emission reductions. Metric choice can affect the relative emphasis placed on reductions of ‘cumulative climate pollutants’ such as carbon dioxide versus ‘short-lived climate pollutants’ (SLCPs), including methane and black carbon. Here we show that the widely used 100-year global warming potential (GWP100) effectively measures the relative impact of both cumulative pollutants and SLCPs on realized warming 20–40 years after the time of emission. If the overall goal of climate policy is to limit peak warming, GWP100 therefore overstates the importance of current SLCP emissions unless stringent and immediate reductions of all climate pollutants result in temperatures nearing their peak soon after mid-century, which may be necessary to limit warming to “well below 2 °C” . The GWP100 can be used to approximately equate a one-off pulse emission of a cumulative pollutant and an indefinitely sustained change in the rate of emission of an SLCP. The climate implications of traditional CO2-equivalent targets are ambiguous unless contributions from cumulative pollutants and SLCPs are specified separately.
Myles R. Allen, Jan S. Fuglestvedt, Keith P. Shine, Andy Reisinger, Raymond T. Pierrehumbert and Piers M. Forster ; New use of global warming potentials to compare cumulative and short-lived climate pollutants; Nature Climate Change (2016) doi:10.1038/nclimate2998