Smoke from boreal wildfires could cool the Arctic

There are two things which climate scientists hate about “positive feedbacks”. One is that they are bad news. A positive feedback, in the science of complex systems, is an amplification; in climate change, this comes about when a consequence of rising temperatures drives a further rise in temperature. Such feedbacks are the sorts of things that drive tipping points.

Much to reflect on(Photograph: Getty Images)

The other problem is that they sound like good news. Negative feedbacks face the opposite problem. In a negative feedback, which need not be harmful, a change in the system produces a response that pushes the system back towards where it was. Think of an air conditioner’s temperature setting or a radiator’s thermostat.

A recent analysis by Edward Blanchard-Wrigglesworth of the University of Washington and colleagues suggests that a much more important negative feedback may now be operating in the Arctic, one which could curb the region’s rapid temperature increase and markedly slow the decline in its sea ice. Indeed, it looks strong enough to have an effect on overall global average temperature.

This particular negative feedback is driven by the increasing frequency, size and intensity of wildfires in boreal forests. The climate models that scientists use to simulate warming over the coming century run on scenarios that assume these fires will continue more or less as they did in the 2000s and early 2010s. Since then, though, they have become considerably larger.

Where there is fire, there is smoke. Some is sooty and dark; some is lighter. Dr Blanchard-Wrigglesworth and his colleagues think that the brighter, more reflective smoke wins out, cooling the ground below. Taking the fire-trend into account, they reckon that, in the 2030s, the extent of sea-ice cover in the Arctic ocean will be at least 3m square kilometres more than it would be in a fire-trend-free model. Without the fire trend, an ice-free Arctic September would be expected in 2050. Fires delay its onset by over a decade.

None of this says that the fires are a good thing, or that they will avert catastrophes elsewhere. Fires are a massive shock to ecosystems, and smoke which reflects sunlight also harms humans and other animals. Moreover, the carbon that fires release will warm the entire planet for some time to come. That is clearly bad news.

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