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Science
| 03/01/17

Mid-century air pollution ‘prevented’ ice loss in warming Arctic

TCE

Arctic sea ice growth around the middle of the last century that until now appeared to be in sharp contrast to the losses observed since the 1970s only occurred due to the effects of air pollution, scientists have found, masking what otherwise would have been even greater sea ice losses.

A recent study based on recovery of Soviet observations had shown that the extent of Arctic sea ice had been on the increase from 1950 to 1975. This expansion had perplexed researchers. But in a separate study out last week in Geophysical Research Letters, a journal of the American Geophysical Union, researchers including climate modelling scientists with Environment and Climate Change Canada (ECCC) based at the University of Victoria found that the expansion was due to higher atmospheric concentrations of sulphate aerosols. Aerosols are very fine solid particles or liquid droplets that are suspended in a gas. Fog and smoke are two common examples of aerosols. Sulphate aerosols are those that contain negatively charged groups of sulphur and oxygen atoms (SO42-). They are produced by volcanoes and a host of industrial processes, and lead to acid rain and lung irritation. Due to these problems, legislation in many Western countries in the 1970s cleaned up a lot of air pollution.

Sulphate aerosols work in reverse to the way that greenhouse gases do: they cool the climate directly by reflecting away incoming solar radiation and indirectly by boosting the reflectivity of clouds. Unlike carbon dioxide (CO2), which lingers in the atmosphere for centuries, the sulphate only hangs around in the atmosphere for a few days or weeks, so by the 1980s, the cooling effect, known as ‘global dimming’, had ceased.

The researchers deployed computer modelling to simulate the impact of sulphate aerosols over the Arctic from 1950-75. They then compared these simulations to the Russian observational data, and found that the cooling effect of the aerosols actually offset the warming that had occurred as a result of greenhouse gas concentrations over the Arctic. In other words, if not for these aerosols, sea ice losses would have been much greater than it is today.

John Fyfe, one of the authors and a senior research scientist with the EEC’s Canadian Centre for Climate Modelling and Analysis, said it appears that humans had an impact on the Arctic much earlier than had been thought.

The global dimming phenomenon has led to suggestions from some quarters of the deliberate delivery of sulphate aerosols into the atmosphere by weather balloons, high-flying jets or even artillery shells to counteract global warming—in essence producing the effect on the climate that the eruption of a very large volcano would have. The technology already exists to do this, and the process would be relatively cheap: costing in the single-digit billions.

Such efforts, a type of ‘solar radiation management’, are controversial for a number of reasons. Potential side effects include a return of acid rain and depletion of the ozone layer, depending on the amount and composition of aerosols released. In addition, due to their short time spent in the atmosphere, delivery of such particles would have to be regularly repeated, perhaps once every four years. Once started, the process would have to continue indefinitely. And the technique would also do nothing to prevent the build-up of CO2 in the atmosphere, only mask its effects on global temperatures. If it were ever to be halted, temperatures might rapidly rebound towards what they would have been otherwise, a phenomenon described as “termination shock” due the difficulty of many organisms being able to adapt to a rapid and large change in temperature. Finally, the technique would do nothing to prevent ocean acidification, the other major Earth system effect of increased atmospheric concentration of CO2.

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Mid 20th century air pollution protected Arctic sea ice from melting, researchers have found

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