Base greenhouse gas targets on science not expediency, says leading climate scientist

28 May 2015

© Oxford Martin School

A policy paper from the Oxford Martin School urges policy-makers to base greenhouse gas emissions priorities on science and not expediency. Short-Lived Promise? The Science and Policy of Cumulative and Short-Lived Climate Pollutants has been released by the Oxford Martin School at the University of Oxford ahead of the UNFCCC meeting next week in Bonn.

Author Myles Allen, Professor of Geosystem Science, spells out the difference between short-lived climate pollutants (such as methane, which has an atmospheric lifetime of about a decade) and CO2 emissions (which have an indefinite lifetime). In the paper he clarifies why comparisons between the two are misleading, and explains that short-lived climate pollutant (SLCP) reductions will have little impact on peak warming unless CO2 emissions are substantially reduced first or at the same time.

“It is no surprise that many leaders want to focus on SLCP targets,” Allen explains. “It is easier and cheaper to reduce methane (and other short-lived) emissions, and cuts could bring significant health and welfare benefits. But to stabilise temperatures, net CO2 emissions must be reduced to zero; so we must avoid offsetting SLCP reductions against CO2 reductions, if this delays tackling the cumulative CO2 budget.”

Short-Lived Promise? has been designed for governments preparing for the upcoming climate negotiations and draws on the latest research by climate scientists to highlight the importance of peak warming and the need to reduce net CO2 emissions to zero. The paper concludes that the ‘short-lived’ versus ‘long-lived’ discussion is not a technical issue at all, but an expression of inter-generational priorities.

Short-Lived Promise? recommends a clear ‘peak CO2 first’ strategy:

  • Policies should recognise that near-term SLCP reductions will only affect peak warming if CO2 emissions are reduced at the same time
  • Policies must recognise that no single metric can represent both the short-term impact of SLCP emissions and the cumulative impact of CO2 emissions
  • New policies are needed to contain CO2 within the cumulative budget, which must be independent of, and in addition to, any multi-gas emission goals
  • Until CO2 emissions are falling, the need to reduce net CO2 emissions to zero overrides opportunities to offset CO2 reductions against SLCP reductions
  • As soon as CO2 emissions are falling fast enough to give a realistic prospect of meeting the cumulative budget, SLCP emission reductions should become a crucial priority to limit peak warming.

Allen concludes: “Prioritising SLCPs ahead of CO2 puts the welfare of this generation above that of the next. Reductions in both are vital, but there is no equivalent to addressing CO2, no matter how you compare different pollutants.”

Facts at a glance

  • To limit the warming they cause to 2°C, CO2 emissions must be limited to a cumulative budget of one trillion tonnes of carbon
  • Almost sixty per cent this cumulative budget has already been released
  • Net global CO2 emissions must reach zero to stabilize global temperatures: any delay in starting policies aimed at this reduction reduces the chances of limiting warming to 2oC.
  • Both CO2 and SLCP emission reductions could affect the expected warming over the next few decades, and many measures to reduce SLCPs have very important benefits for public health.
  • CO2 has an atmospheric lifetime of many thousands of years, compared to methane’s atmospheric lifetime of about a decade.
  • On current trends, a 20-year delay in beginning CO2 reductions will add almost 50% to eventual peak warming