September 24, 2013
Rainfall Redistribution Will Make Some Areas Warmer And Drier
April Flowers for redOrbit.com - Your Universe Online
A new study from Columbia University's Lamont-Doherty Earth Observatory reveals that a northward shift of Earth's wind and rain belts could make a broad swath of regions drier. The findings, published in Proceedings of the National Academy of Sciences, show that these drier regions include the Middle East, American West and Amazonia, while Monsoon Asia and equatorial Africa will become wetter as humans continue to heat the planet.This new prediction is based on the warming that ended the last ice age around 15,000 years ago. During that warming, the North Atlantic Ocean began to churn more vigorously, melting Arctic sea ice, and setting up a temperature contrast with the southern hemisphere where sea ice was expanding around Antarctica. The tropical rain belt and mid-latitude jet stream were pushed north by the temperature gradient between the poles. This redistributed water in two bands around the planet.
Currently, the Arctic sea ice is retreating again and the northern hemisphere is heating up faster than the south. Because of this, history could repeat itself. "If the kinds of changes we saw during the deglaciation were to occur today that would have a very big impact," said Wallace Broecker, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory.
Broecker and his colleague Aaron Putnam, a climate scientist at Lamont-Doherty, combined climate data collected from around the world, from tree-rings, polar ice cores, cave formations, and lake and ocean sediments, which allowed them to create their theory that the wind and rain belts shifted north from about 14,600 years ago to 12,700 years ago as the northern hemisphere was heating up.
At this time, at the southern edge of the tropical rain belt in the Bolivian Andes, the great ancient Lake Tauca nearly dried up while rivers in eastern Brazil slowed to a trickle and rain-fed stalagmites in the same region stopped growing. The northward advance of the jet stream in the middle latitudes may have caused Lake Lisan, a precursor to the Dead Sea in Jordan's Rift Valley, to shrink, along with several prehistoric lakes in the western U.S., including Lake Bonneville in present day Utah.
Changes continued to stack up, with a northward shift of the tropical rains that recharged rivers draining the Cariaco Basin in Venezuela and East Africa's Lake Victoria and Lake Tanganyika while the stalagmites in China's Hulu cave grew bigger. Scientists have found evidence of a strong Asian monsoon show up in the Greenland ice cores.
The study authors hypothesize that between 1300 and 1850, as northern Europe transitioned from the relatively warm medieval era to a colder period known as the Little Ice Age, the process worked in reverse. During this time, ocean circulation slowed and North Atlantic sea ice expanded, according to the climate record. The rainfall declined in Monsoon Asia, which led to a series of droughts that have been linked to the decline of Cambodia's ancient Khmer civilization, China's Ming dynasty and the collapse of kingdoms in present day Vietnam, Myanmar and Thailand.
The reconstruction of glacier extents in New Zealand's Southern Alps in the southern hemisphere suggests that the mid-latitudes may have been colder during medieval times. Such evidence supports the idea of a temperature contrast between the hemispheres that altered rain and wind patterns.
Each year, a similar migration of the wind and rain belts on Earth occurs. The tropical rain belt and mid-latitude jet stream migrate north during the boreal summer. The northern hemisphere heats up disproportionately to the south, with more continents to absorb the sun's energy. In winter, the northern hemisphere cools off as the winds and rains revert south.
The winds and rains have rearranged themselves for longer periods of time, somehow. For example, in the 1970s and 1980s a southward shift of the tropical rain belt is thought to have brought devastating drought to Africa's Sahel region. This was attributed to air pollution cooling the northern hemisphere. The tropical rain belt has since reverted back and might be moving north, suggested by a number of recent droughts, including in Syria, northern China, western US, and northeastern Brazil, the research team says.
At least one climate model demonstrates the tropical rain belt moving north as carbon dioxide levels climb and temperatures warm. This is consistent with the study findings. "It's really important to look at the paleo record," said Dargan Frierson, an atmospheric scientist at University of Washington whose modeling work supports the authors' hypothesis. "Those changes were huge, just like we're expecting with global warming."
The researchers admit that their theory has some challenges. Changes in sea ice cover in the past drove the temperature gradient between the two hemispheres. Today, rapidly rising industrial carbon emissions are responsible. Additionally, no clear evidence has been found so far that ocean circulation is increasing in the North Atlantic or that the monsoon rains over Asia are strengthening. There is, however, speculation that sulfate aerosols produced by burning fossil fuels may be masking this effect.
Temperatures may warm as the air pollution in the northern hemisphere declines. This may create the kind of temperature contrast that could move the winds and rains north again, said Jeff Severinghau, a climate scientist at Scripps Institution of Oceanography. Severinghaus was not involved in the study.
"Sulfate aerosols will probably get cleaned up in the next few decades because of their effects on acid rain and health," he said. "So Broecker and Putnam are probably on solid ground in predicting that northern warming will eventually greatly exceed southern warming."