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May 30, 2013

Carbon-dioxide ‘overshoots’ could weaken Atlantic ocean currents

What is the best way to reduce our carbon-dioxide emissions? Should we wean ourselves off fossil fuels slowly but steadily over the next 100 years, or aim for a sharp exit over the next 50 years? Ultimately both options would release similar amounts of carbon dioxide overall, but the sharp exit would produce a large pulse of carbon dioxide now, whereas the weaning option would be more of a slow exhale of the gas. Which leads to greater climate change: a steady trickle or a big bubble of carbon dioxide? A new model indicates that, in most cases, this timing is irrelevant; what matters is the overall cumulative emissions. However, one scenario was found to affect regional climate change, particularly over Europe.

Daisuke Nohara from the Central Research Institute of Electric Power Industry in Japan and colleagues used a fully coupled climate-carbon cycle model to evaluate the dependency of climate and carbon-cycle change on carbon-dioxide “emission pathways” – the timing of carbon-dioxide release. They studied five different pathways, all of which levelled off to final cumulative emissions of 2000 gigatonnes of carbon. One of these scenarios included an “overshoot” where emissions temporarily peaked at 4000 GtC, but subsequently reduced to 2000 GtC as a result of continuous negative emissions, from carbon-capture technology, for example.

In most cases the model supported the idea that climate change is independent of carbon-dioxide emission pathway. However, Nohara and his colleagues found that temporary “overshoots” of carbon dioxide had a significant impact on regional climate. Despite the eventual cessation in emissions they found that any temporary pulses of carbon dioxide led to a weakened Atlantic meridional overturning circulation (AMOC). “This change in AMOC has a major impact on the northward heat transport in the North Atlantic, leading to regional climate change, in particular, over Europe,” explained Nohara. These findings are published in Environmental Research Letters (ERL).

It seems that the overshoot of carbon dioxide triggers rapid warming in the Arctic region, which enhances a weakened AMOC. And a weakened AMOC induces falling air temperatures and falling rainfall over the northern Atlantic and Europe. The model indicated that this regional variation would persist for at least 150 years after the cessation of carbon-dioxide emissions.

So it seems that in most cases the way in which we emit our carbon dioxide is irrelevant; it is the cumulative amount that matters. But crucially there are exceptions, which could have serious long-term regional implications. Exactly what these implications are, and how we should adapt our future mitigation strategies, is not yet clear. “We need to carry out more comprehensive analysis of multi-climate model ensembles, rather than basing our results on one single climate model,” said Nohara.

About the author

Kate Ravilious is a contributing editor for environmentalresearchweb.

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