- - Tuesday, September 12, 2017

Climate cooling, as opposed to warming, presents serious problems for humanity. As cooling causes agriculture to fail, most of the world’s population will starve and we will be reduced from its present level to about a million, hunting animals and collecting nuts and seeds for sustenance. This has happened before during the ice ages, when nomadic bands of prehistoric humans had to shelter in caves for protection from the cold, and had to rely on uncertain supplies of food.

Geoengineering to combat global warming is controversial. It is expensive and presents risks to the environment. However, when geoengineering is applied against climate cooling, both expense and risk become minor items.

We need to distinguish between two kinds of climate cooling events. The first kind is “astronomical” — as studied by the Serbian astronomer Milutin Milankovitch — and determined by the orbit of the Earth in the solar system and the obliquity and precession of the spin axis. There’s very little we can do about that. In the past 2-3 million years, we have experienced about 20 of these glaciations, typically lasting 100,000 years, interrupted by interglacial warm periods of about 10,000 years. We’ve been in our present interglacial, which is called the Holocene epoch, for about 10,000 years, and many think we’re due for another glaciation within a few decades or centuries. Some disagree and think that the Holocene may last much longer, about 45,000 years.

The accepted way in which a glaciation begins is when a snow-ice field at high latitude survives the summer and then grows during the winter months, getting larger and larger all the time. The remedy is quite simple, at least in concept. We need to identify the surviving snow-ice fields, which can be done easily by means of weather satellites. Once we identify them, we can remove them by dumping black soot and allow the summer sun to melt the snow and ice. However, these concepts need to be tested, so experiments are in order.

The second kind of climate cooling is controlled by solar activity and has a short period of 1,000-1,500 years. Our civilization experienced what we call the Little Ice Age (LIA) from about 1400 to 1800 A.D. The cooling was severe enough to destroy agriculture and the budding civilization in southern Greenland. We observed serious effects of the cooling in Europe when harvests failed and people starved; epidemics caused additional deaths. Since about 1850, the climate has been recovering from the LIA, showing some warming.

Even though the cooling of a Little Ice Age is not as severe as an astronomical glaciation, we need to move urgently to counteract a future LIA. This is not simple, but greenhouse effects can help to warm the climate and overcome the cooling. For various reasons, release of carbon dioxide is not the best remedy; carbon dioxide is saturated and doesn’t have much additional climate impact. Furthermore, as far as we can tell, the recent LIA was patchy and individual cooling episodes lasted only years or decades.

In my view, the best way to overcome a Little Ice Age is to release water vapor at the tropopause, the boundary between the troposphere and stratosphere, at an altitude of about 12 kilometers. The water vapor will form a cirrus cloud of ice particles, just like a contrail from an aircraft. Theory predicts that this ice cloud will have a strong greenhouse effect that is localized to the dimension of the cloud — just what we want. However, tests are essential to demonstrate how much water is needed, to measure climate effects on the ground and verify that the cloud produces a strong local warming of sufficient duration.

While there is much current discussion about geoengineering, the expense and the risk have been forbidding. I strongly believe that the time is right for conducting experiments to test the concepts described above to offset a sure-to-occur catastrophic climate cooling.

In a nutshell, as opposed to global warming, global cooling is a very real problem for a number of reasons. Based on the historic past, we can be sure that cooling will occur again, and maybe very soon. When it does occur, it will have serious effects on agriculture and lead to mass starvation. Unlike for warming, geoengineering against cooling seems physically possible, relatively inexpensive and environmentally benign.

• S. Fred Singer is professor emeritus of environmental sciences at the University of Virginia. He served as the founding director of the U.S. Weather Satellite Service and as chief scientist of the U.S. Department of Transportation.

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