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Smoking dawn over Wiersbaden, Germany. Photo: Martin Fisch via Flickr (CC BY-SA).
Smoking dawn over Wiersbaden, Germany. Photo: Martin Fisch via Flickr (CC BY-SA).
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Climate negotiators, please cut soot and methane - but not at the expense of CO2!

Myles Allen

6th June 2015

Climatologists have spotted an 'easy hit' to reduce global warming by cutting emissions of short lived forcing agents like soot from dirty power plants, 2-stroke engines and cooking fires, writes Miles Allen. But while we should take action on these pollutants, it must not come at the expense of holding back on cutting emissions of the big long term climate forcer: carbon dioxide.

This issue arouses strong passions: colleagues and I were recently accused by fellow scientists of putting "tens of millions of lives" at risk by calling for a delay in including short-lived climate pollutants in the UN climate process.

If you ask the climate negotiators gathering in Bonn this week for their last get-together before the Paris conference in December who they are doing all this for, the reply would probably mention future generations.

You can be as cynical as you like about what actually drives them but most people, including the negotiators themselves, no doubt think that the point of the exercise is to protect the interests of the planet as a whole.

But is it? And whose planet are they trying to protect? Worryingly, the basic negotiating framework of the UN climate process increasingly favours the interests of people alive today over future generations, in ways that perhaps even the negotiators themselves do not understand.

At the heart of the problem is the challenge of setting priorities between two very different kinds of climate pollutant. On the one hand, we have cumulative pollutants such as carbon dioxide, which build up in the climate system like heavy metals in the food chain.

On the other, we have short-lived pollutants like methane and soot, which are 'washed out' naturally within a few weeks or years.

Action on short-lived pollutants has become very fashionable. It has an entire movement devoted to it, the Climate and Clear Air Coalition, enthusiastically supported by the United Nations Environment Programme.

The reasons are clear: unlike carbon dioxide, many of these emissions can be reduced cheaply, with massive immediate benefits to human health and agriculture in precisely the countries where these emissions come from.

Who could possibly object to measures that would save the lungs and lives of women in developing countries and at the same time could help improve our prospects of keeping global temperatures below two degrees?

Look to the long-term

But there is a problem. As I explain in a new report released by the Oxford Martin School, as long as carbon dioxide emissions are still rising - and last year's blip notwithstanding, they are - emissions of short-lived climate pollutants are almost entirely irrelevant to peak warming.

The reason is simple: because carbon dioxide accumulates in the system, stabilising temperatures at any level means we have to get net global carbon dioxide emissions to zero. Even on the most heroic assumptions about future reductions, until carbon dioxide emissions are falling, and falling fast, net zero is still many decades off - by which time today's methane and soot emissions will have long since washed out of the climate system.

So why are countries so enthusiastic about them? Part of the reason is obscurely technical. For reasons long since forgotten, the whole UN process is based on the notion of 'carbon dioxide equivalent' emissions, with equivalence measured in terms of a kind of 'exchange rate' called the '100-year global warming potential'.

Given the name, you might be forgiven for thinking this was something to do with global warming over 100 years, but it isn't. It turns out GWP100 actually measures the relative impact of different emissions on warming rates over the next 30-40 years. And on this timescale, cutting short-lived climate pollutants could indeed have some impact.

Halving global methane emissions immediately, for example, could cut expected global temperatures by a couple of tenths of a degree by 2050 - which would be useful, but only comparable to natural fluctuations on these timescales. Halving global CO2 emissions would have a much bigger impact (and an immeasurably bigger impact after 2050), but would also be far, far more difficult and expensive.

So much of the UN process is constructed around a measure of the impact of different emissions that explicitly focuses on the interests of the generation of decision-makers alive today.

To be fair, some countries, like Brazil, have been calling for years for GWP100 to be replaced. But just replacing it with another exchange rate won't help, because any rate that works on one timescale will fail on another.

Time to stop pretending

The solution is extremely simple. Countries need to acknowledge the need to get net carbon dioxide emissions to zero and limit the total amount we dump in the atmosphere in the meantime.

So until CO2 emissions are falling, and falling fast enough that there is a realistic prospect of getting them to zero in the foreseeable future, we avoid the temptation of pretending that action on short-lived climate pollutants is helping to limit peak warming.

This issue arouses strong passions: colleagues and I were recently accused by fellow scientists of putting "tens of millions of lives" at risk by calling for a delay in including short-lived climate pollutants in the UN climate process.

Let me be clear: I am all in favour of cutting soot emissions in developing countries. My first job, back in the 1980s, was developing clean and efficient wood-stoves in East Africa. I well recall the infernal conditions experienced by women cooking over open fires in 40 degree heat: we don't need a global climate treaty to need a reason to do something about this kind of thing.

But until carbon dioxide emissions are falling, we shouldn't pretend cutting soot emissions is helping to stabilise global temperatures, because it isn't.

So a crucial test of whether the UN climate process is actually working in the interests of future generations is whether the negotiators in Bonn, and in Paris in December, acknowledge the need for net zero carbon dioxide emissions. Watch this space.

 


 

Myles Allen is Professor of Geosystem Science, Leader of ECI Climate Research Programme at University of Oxford.

This article was originally published on The Conversation. Read the original article.

The Conversation

 

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