June 6, 2017, by Tim Radford
Cloudburst, North Vancouver: Wet or dry, extremes will become more intense.
Image: Terence Thomas via Wikimedia Commons
Wet and dry extremes across the world will become more marked as the planet heats up, evidence from past climates shows.
LONDON, 6 June, 2017 – Two US scientists have once again confirmed one of the oldest predictions of climate change: that those regions already wet will become wetter, while the arid zones will become drier.
This time the reasoning comes not just from computer models of future climate, but also from the evidence of the past.
Because the northern hemisphere will warm faster than the southern, the temperature difference will drive the planet’s rainbelts northwards, at least during the winter months. The tropics will become wetter, while the subtropics and the mid-latitudes will become drier, and this will be most noticeable in June, July and August.
The predictions – made in the journal Science Advances – come from two researchers. Aaron Putnam is a glaciologist who studies ancient climates at the University of Maine. Wallace Broecker is an oceanographer at the Lamont-Doherty Earth Observatory of Columbia University, and one of the pioneers of climate research.
They looked at the evidence from ancient lake beds, cave stalagmites, ice corings and other examples of what scientists call “proxy data” to reconstruct the pattern of change 15,000 years ago, near the close of the last Ice Age, when the climate of Greenland is known to have warmed abruptly by around 10°C.
So the hemisphere difference shifted the thermal equator – the hottest parts of the tropics – northwards and, with them, the mid-latitude jet streams. There was a dramatic change in precipitation patterns, and the extra rain turned into extra run-off that would enlarge the ancient lakes.
A 10% increase in rainfall led to a 30% increase in run-off. So, effectively, a twofold increase in rainfall left its traces in a six-fold expansion of a lake basin, which would leave fossil evidence that would be detectable a thousand or more years later.
“The northern hemisphere monsoon rainfall will intensify. The southern hemisphere monsoon system will weaken”
Armed with this model from the past, the two scientists could start to make predictions about change in a rapidly warming world.
They say that the northern hemisphere monsoon rainfall will intensify. The southern hemisphere monsoon system will weaken, probably because the thermal equator will have shifted northwards.
The drylands of the western US, inner Asia and the Middle East will become even drier. The climate region known as Amazonia will shift northwards, so that Venezuela will become wetter, while eastern Brazil and the Bolivian Altiplano will become more arid.
The authors proceed with caution, because the present is very different from the past: in the last 15,000 years humans have added not just carbon dioxide to the atmosphere but also dust and polluting aerosols that must change the radiation levels.
But the two scientists then applied the same techniques to precipitation behaviour during the so-called Little Ice Age to find a predictable pattern of reversal as temperatures seemingly fell: between 1200AD and 1850, the thermal equator shifted southward, the south Asian monsoons weakened, and Peru became much rainier, which suggests at least a tentative confirmation of their thinking. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.