Friday, 23 May 2014

Reducing methane emissions - timing matters

By Luke Harrington

Recent climate modelling research has found that countries with high emissions of short-lived climate pollutants (SLCPs) should keep mitigation of carbon dioxide (CO2) a top priority, and working to reduce methane emissions in isolation will not be any more effective than doing so in several decades time. In essence, “action on short-lived climate pollutants will not ‘buy time’ to delay action on carbon dioxide”, says co-author Professor David Frame, director of the New Zealand Climate Change Research Institute.

The research, led by Niel Bowerman at the University of Oxford, was published in November 2013 in the leading scientific journal Nature Climate Change, and argues that the impact of reductions in SLCPs depends strongly on their timing.  Using a simple coupled climate-carbon cycle model, the authors look at how a reduction in SLCPs will influence the rate of temperature rise when implemented at different time horizons – this approach was applied to four representative concentration pathways (RCPs), each corresponding to a different scenario of future emissions and severity of climate change.

Results show that, if SLCP mitigation coincides with continuing emissions of long-lived climate pollutants (LLCPs) such as CO­2 and N2O, the impact of such reductions on lowering overall peak warming is only several hundredths of a degree Celsius. However, if these SLCP reductions coincide with corresponding drops in LLCPs, the relative impact on peak warming becomes several orders of magnitude larger.

So why is this a significant result? Well, it effectively means methane emissions today are of little consequence to peak warming that will be experienced under climate change, unless CO2 emissions begin to drop rapidly in the next few decades.  For all scenarios (other than the 2-degree restrictive profile … which effectively requires slowing CO2 emission rates now), there is very little difference in the outcome of reducing methane emissions now, compared with doing so in several decades time.

The paper notes that there are several benefits of reducing SLCP emissions (which also include black carbon and tropospheric ozone), including to agriculture and human health. But here is the key message for an agrarian economy like New Zealand, where methane accounts for an estimated 35% of total emissions: we must keep carbon dioxide at the top of the list for mitigation policy measures. Attempting to first tackle methane, which could be seen as an easier target, will otherwise be a flawed approach.

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