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Taking stock of climate pollutants


13 Nov 2017

Michelle Cain

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As our planet’s temperature increases, so too, it seems, does people’s level of interest in minimising global warming. For the past week, governments and other groups interested in climate change have come to Bonn, Germany, for the UN Climate Change Conference, COP23. The UN says that COP23 has the “aim of launching nations towards the next level of ambition needed to tackle global warming and put the world on a safer and more prosperous development path.”

This is something that most people would agree with, but what does it really mean? That is what much of the gathering discussed, as underneath its simplicity lies many complex and unresolved issues. We are tackling some of these issues in the Oxford Martin Programme on Climate Pollutants, which was highlighted at an event at COP23 this week: Measuring progress towards the Paris Agreement: Aligning policy and science in global stocktakes.

A key element of the Paris Agreement is for parties to take stock of progress towards the goal set in the Paris agreement of limiting warming to well below 2°C and to work towards 1.5°C. This is becoming ever-more urgent as human-induced warming has already reached 1°C and is rising faster than ever, according to a study published today, and which Myles Allen discussed at the COP23 side event.

To carry out such a global stocktake, and to decide on future policies, we need a way of relating emissions of climate pollutants with warming. Otherwise there is no way of evaluating the policies. Yet there is no perfect method for doing so, especially if it is impractical to run climate models to answer every policy question.

How do we compare an emission of a short-lived climate pollutant (for example methane) with an emission of a long-lived gas like carbon dioxide? The usual way is to convert other gases to “CO2-equivalent” using a metric called Global Warming Potential (GWP). This metric assigns an exchange rate to non-CO2 greenhouse gases, so that multiplying the amount of methane (or other pollutant) by its GWP gives you its CO2-equivalent emission.

This is convenient and simple to calculate, but also flawed. Its key flaw is starkly exposed when greenhouse gases are in decline. As methane has a short (about 10 year) lifetime, if you stop emitting it, the strong warming effect it has on the surface temperature will soon diminish. If methane emissions reduce year-on-year, the warming from methane will reduce year-on-year – in other words, there is a cooling effect. However, when you convert these methane emissions to CO2-equivalent using GWP, it results in positive (but diminishing) CO2-equivalent emissions year-on-year. The problem with this is that any emission of CO2, however small, generates a warming.

In short, declining methane emissions have a cooling effect. When converting this to CO2-equivalent using GWP, we get a warming effect. This becomes important when evaluating the effect of different ambitious mitigation scenarios, which will reduce different climate pollutants by different amounts.

At the side event, and in an accompanying briefing document, Climate metrics under ambitious mitigation, we outlined a new application of GWP, which takes into account this difference. We equate a one-off increase in emission rate of a short-lived climate pollutant with a one-off pulse of CO2. This is a better proxy for temperature change, which is, after all, what the Paris Agreement aims to limit. Using this method, total cumulative emissions predict temperature, and peak warming coincides with net zero total emissions.

The new Oxford Martin Programme on Climate Pollutants will investigate how we can best link emissions with warming. How countries choose to do this will profoundly affect their assessments of progress towards Paris Agreement goals, and could influence which path they choose to take towards achieving net zero emissions (which is essential to limiting warming to 1.5 or 2°C).

 

More information about climate metrics and our new briefing Climate metrics under ambitious mitigation.

More information about the new application of Global Warming Potential:

Allen, M. R., Fuglestvedt, J. S., Shine, K. P., Reisinger, A., Pierrehumbert, R. T., & Forster, P. M. (2016). New use of global warming potentials to compare cumulative and short-lived climate pollutants. Nature Climate Change, 6(8), 773–776. http://doi.org/10.1038/nclimate2998


This opinion piece reflects the views of the author, and does not necessarily reflect the position of the Oxford Martin School or the University of Oxford. Any errors or omissions are those of the author.