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All the extra shipping traffic passing through the Arctic might have an unexpected upside: slightly slowing the rapid warming around the North Pole, helping the sea ice linger a little longer. But it would also have other, less pleasant consequences.

The Arctic is warming faster than anywhere else on the planet, and as a result the sea ice has retreated for decades. The Arctic may see its first ice-free summer for millennia in the next few decades 鈥� although the ice does grow back in winter.

The shrinking ice means shipping routes that were聽once impassable听补谤别听opening up. Shipping firm Maersk聽said 聽that one of its vessels had travelled聽from Vladivostok on Russia鈥檚 Pacific coast聽to St Petersburg, via the Northern Sea Route. Such routes can shave聽thousands of kilometres聽off journeys.

鈥淲e鈥檝e known for some time that shipping in the Arctic is likely to have climate impacts of one form or another,鈥� says聽Scott Stephenson聽of the University of Connecticut in Storrs.

Not quite as hot

Ships release black carbon, which both traps more of the Sun鈥檚 energy and darkens the ice 鈥� causing it to absorb more heat. On the flip side, aerosols like sulphur dioxide cause more clouds to form, which reflect heat and therefore cool the area. Previous studies focused on one but not the other of these processes, or used simple climate models in which certain variables like the extent of the sea ice were fixed.

鈥淣o previous study had attempted to look at this question comprehensively using a fully coupled earth system model,鈥� says Stephenson. So he and his colleagues did so.

They simulated Arctic temperatures from 2006 to 2099, calculating shipping emissions by estimating which routes would be passable at different times and then inferring how much the ships would emit.

鈥淭here was a very clear signal and that signal was cooling,鈥� says Stephenson. If greenhouse gas emissions continued as normal but Arctic shipping did not increase, the Arctic warmed by 10掳C, but when shipping was included the warming was reduced to 9掳C. The sea ice also retreated slightly less.

The many other downsides

Nevertheless, Stephenson says shipping is not a solution to climate change, in the Arctic or elsewhere. The benefit he found is small. 鈥淭he effect of shipping itself is cooling, but the Arctic is still warming at twice the global average and will be much warmer by the end of the century,鈥� he says.

Besides, in order to obtain this reduction in warming, ships would need to keep polluting the air, says聽Dominick Spracklen聽of the University of Leeds, UK. That鈥檚 a bad idea, because such pollution is deadly. The World Health Organization says聽, and shipping contributes to that. 鈥淲e need to act as rapidly as possible to reduce air pollution,鈥� says Spracklen.

Climatologists know that some forms of air pollution cool the planet, and have聽partially masked the warming from our greenhouse gas emissions. This leads to a nasty choice. 鈥淎s we try to clean up the atmosphere in Europe and the US, which we have to do to reduce air pollution and the problems it causes for human health, we could be exposing ourselves to more rapid warming,鈥� says Spracklen. 鈥淏ut we can鈥檛 not do it, because we know what a problem it鈥檚 causing us.鈥�

Stephenson points out that the shipping industry does have plans to begin cut its sulphur emissions from 2020. He and his colleagues didn鈥檛 include this commitment in their modelling, but if they had it could have changed the outcome. He also adds that sending shipping through the Arctic carries other risks, like oil spills. 鈥淭here鈥檚 very little infrastructure in the Arctic to deal with any disasters and mishaps that might occur,鈥� he says.

Finally, might we instead cool the Arctic on purpose? Some proponents of climate-cooling 鈥済eoengineering鈥� propose聽spraying droplets of sea water into the sky, causing more clouds to form and聽cooling the area. But Spracklen points out that in the new study, the extra clouds only stopped one-tenth of the Arctic warming.

There are also side effects. 鈥淵ou can cause quite crazy things to happen,鈥� he says. 鈥淵ou鈥檇 have cooling in some places and much stronger warming in others,鈥� which could radically change Earth鈥檚 wind patterns.