Desert dust helps lower ocean temps

Sea levels continue to rise, and not just because of melting ice caps, but also because warmer global temperatures cause thermal expansion. The rise in temperature would be even greater if not for dust and aerosols in the atmosphere that deflect some of the sun’s heat. Scientists don’t have a great understanding of exactly how important this dust effect is. A new paper suggests that it’s actually very important — and that’s bad news.

A host of reasons have been offered for the roughly 0.25 °C per decade temperature increase in the North Atlantic. Global warming is one culprit. A broken deep sea “conveyer belt” — that giant convective loop that drives cold, nutrient rich water up certain coasts and warmer water toward the deep basins — is another. Scientists also suspected that dust blown off the Saharan desert and volcanic aerosols push temperatures downwards, but the relative strength of these effects was unknown.

A team of climate scientists at the University of Wisconsin and at NOAA, by feeding satellite data into a model of ocean-atmosphere energy exchange, have found that all those dust and aerosol particles are responsible for up to 67% of the variability in ocean temperatures. In other words, if not for the cooling effect of these particles, ocean temperatures would be much warmer than they are now.

Airborne, these small particles deflect sunlight away from the earth’s surface, and can cause between -0.1 and -2.0 °C of cooling in the Atlantic Ocean. Most of this cooling can be attributed to the stratospheric class of aerosols — the stuff spewed into the upper atmosphere from the likes of Mt. Etna and Mt. Pinatubo. The dust and sand kicked up from the desert also has a measurable effect.

This is where the problem arises. Some have predicted a 40-60% decrease in the amount of dust after a doubling of CO2 in the atmosphere (pdf), creating a positive feedback loop: more warming means less dust, which means more warming.

To give some scale, up to 1.6 million metric tons of dust (the tropospheric class of aerosols) are swept off the Saharan desert every year — enough to fill up to 58 million dump trucks. To geoengineer our way out of this positive feedback loop, we’d have to dump tens of millions of trucks-worth of dust into the atmosphere every year. And even if we did this, all that dust would do nothing to counteract the acidification of the ocean from elevated CO2. Humans may be able to adapt our way into surviving climate change, but ocean-dwelling creatures may not be so lucky.

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