Climate and Dust

We tend to think of drought and heat as going together, but on the whole a warmer earth is a wetter earth: more heat means more evaporation means more precipitation. Ice ages are generally dry times. (On the whole, that is; of course it's a big planet some places will get wetter.) This means that during ice ages there is lots of dust blowing around. Around the world are immense deposits of wind-blown soil known as loess, and most of these built up during ice ages.

Interestingly, dust is not only a product of a cooler, drier climate, it can itself help make the earth cooler and drier. Of course, it blocks sunlight. But also, according to a new paper in Nature, it can cool the earth by promoting plankton blooms in the ocean:

Dust is all that's needed to plunge the world into an ice age. When blown into the sea, the iron it contains can fertilise plankton growth on a scale large enough to cause global temperatures to drop. . . .

Iron-rich dust falling on the ocean has long been known to spark blooms of plankton, and researchers suspect the process could have intensified the ice ages that have occurred over the past few million years.

The thinking goes that, during warm periods, much of the Southern Ocean is an oceanic desert because it lacks the iron crucial for plankton growth. That changes at the start of ice ages, when a wobble in the planet's orbit causes an initial cooling that dries the continents, generates dust storms - particularly in central Asia - and sends dust onto the surface of the Southern Ocean.

The plankton that then bloom take the carbon they need from the water, causing the oceans to absorb carbon dioxide from the atmosphere to compensate. This cools the atmosphere further, creating yet more dust-producing regions, and the cycle continues, sinking Earth into an ice age.

The new research confirms the association between dust and cooler temperatures going back 4 million years, using cores from the bottom of the Atlantic. Eventually, ice ages end, because of planetary wobbles or volcanoes or changes in the sun or some other mechanism. Then

the feedback goes into reverse: continents warm, dust storms subside, the Southern Ocean is starved of iron, and CO₂ levels in the atmosphere rise again.

Of course, this implies that we could cool the planet by spreading a few billion tons of iron-rich dust in the lifeless parts of the ocean, which I think is an experiment we really ought to try.