As the world warms, weather patterns will change. The already arid Middle East and American West will get even drier, and so will the well-watered Amazon region. Monsoon Asia and equatorial Africa, wet already, will get even rainier, according to new research from Columbia University in the US.
The oceanographer Wallace Broecker and his colleague Aaron Putnam of the university’s Lamont-Doherty Earth Observatory warn in the Proceedings of the National Academy of Sciences that such things happened in the past as climate changed, and will happen again.
In effect, they have extended the famous dictum of the 18th century father of geology James Hutton, who described the present as the key to the past. For Broecker and Putnam, the past is also the key to the future.
The two scientists looked at the pattern of events that followed when the Earth warmed 15,000 years ago, at the end of the Ice Age. The North Atlantic ocean became more vigorous and the Arctic ocean began to crack, break up and melt.
But in the southern hemisphere the sea ice around the Antarctic continent expanded. So a temperature contrast between the two hemispheres appears to have pushed the tropical rain belt and the mid-latitude jet stream north.
Right now, the Arctic sea ice is in retreat and the northern hemisphere is once more heating faster than the south. History could be about to repeat itself. The so-called “thermal equator” – the string of the hottest spots around the planet – could be about to move even further north.
The thermal equator does not march along the geographical equator: the hottest regions of the world are, mostly, in the northern hemisphere because that is where the continents are concentrated. If climate patterns move with global average temperatures, then the thermal equator should shift, too, along with rhythms of wind and rain.
History repeating itself?
So the two scientists began looking again at evidence from the past: tree rings, polar ice cores, stalagmites and other cave formations, the pollens in lake sediments and detritus in ocean ooze are all silent, enduring testimony to past climates, and the duo used these to build up a picture of things that happened around the planet between 14,600 and 12,700 years ago.
Lake Tauca in the Bolivian Andes all but dried up; rivers in eastern Brazil slowed and stalagmites stopped growing. A lake in Jordan’s rift valley began to shrink, and so did prehistoric bodies of water in the western US.
The tropical rains shifted north and began to flood the Cariaco Basin in Venezuela, and water poured into Lake Victoria and Lake Tanganyika in Africa. Stalagmites in a cave in China got bigger, and the Asian monsoon became stronger.
And then between 1300 and 1850, the so-called Little Ice Age, the whole process went into reverse. Sea ice expanded, the Asian monsoon weakened and a series of droughts blighted and perhaps helped along the collapse of empires in Asia - among them the Khmer civilisation in Cambodia, the Ming dynasty in China and so on.
Weather patterns changed: the evidence of ice cores, muds and cave formations is clear enough. The problem is in making a direct connection between such changes and global average temperatures, and the authors concede that there is room for argument.
But that is how science works: researchers frame a hypothesis, make a prediction, publish it and invite other researchers to try to prove the prediction wrong and kick the hypothesis to death.
Broecker and Putnam have done just that, and they conclude their paper by saying: “We predict that Earth is indeed capable of undergoing rapid adjustments in response to future differential heating between hemispheres.
“In particular, we anticipate that with current and future global warming, Earth’s rain and desert belts will respond by shifting northwards, giving rise to changes in water availability around the globe.”
