You may not have consciously thought about it, but I imagine that to some of you out there who have an environmentally conscientious brain, the fact that the Northern Hemisphere has more landmass and the Southern Hemisphere has more ocean would have triggered some interesting questions.
For example, if there is such regional variation in a country as boring as the United States of America, surely there would be some difference across the whole planet.
New research from climatologists at the University of California, Berkeley, and the University of Washington, Seattle, have determined that, yes, in fact, the Northern Hemisphere is becoming warmer than the Southern Hemisphere, a factor which could significantly alter tropical precipitation patterns.
The scientists believe that such a shift in hemispheric similarity could increase or decrease seasonal rainfall in areas such as the Amazon, sub-Saharan Africa, or East Asia. Such a shift could see these areas wetter or drier than they are today.
“A key finding is a tendency to shift tropical rainfall northward, which could mean increases in monsoon weather systems in Asia or shifts of the wet season from south to north in Africa and South America,” said UC Berkeley graduate student Andrew R. Friedman, who led the analysis.
“Tropical rainfall likes the warmer hemisphere,” added John Chiang, UC Berkeley associate professor of geography and a member of the Berkeley Atmospheric Sciences Center. “As a result, tropical rainfall cares a lot about the temperature difference between the two hemispheres.”
The research shows that a warmer Northern Hemisphere is going to cause atmospheric overturning in the north to weaken and strength in the south, pushing rainbands northwards. University of Washington’s Dargan M. W. Frierson, who was part of the team to publish their results in the Journal of Climate, believes that the regions most affected by this shift are likely to be on the bands’ north and south edges.
“It really is these borderline regions that will be most affected, which, not coincidentally, are some of the most vulnerable places: areas like the Sahel where rainfall is variable from year to year and the people tend to be dependent on subsistence agriculture,” said Frierson, associate professor of atmospheric sciences. “We are making major climate changes to the planet and to expect that rainfall patterns would stay the same is very naïve.”
The study was reliant upon over 100 years of data and model simulations, which allowed the authors to compare the yearly average temperature difference between the Northern and Southern hemispheres with rainfall throughout the 20th century.
Their results showed abrupt changes in the difference coincided with rainfall disruptions in the equatorial tropics. The largest drop in temperature difference was a one-quarter degree drop in the late 1960s which coincided with a 30-year drought in the African Sahel which caused famine throughout the region, increased deforestation in North Africa, and decreased monsoons in East Asia and India.
“If what we see in the last century is true, even small changes in the temperature difference between the Northern and Southern hemispheres could cause measureable changes in tropical rainfall,” Chiang said.
Is there a need to worry though? Are these correlations minimal and unlikely to ever increase in volume?
According to most computer models which are simulating past and future climate, the team found a steady rise in hemispheric temperature difference through to the end of the century. Even if we humans manage to get our act together enough to lower our greenhouse gas emissions, models predict a 1 degree Celsius increase in difference by 2099.
Chiang makes an interesting point in discussing the future of climate sciences. With a greater percentage of land mass in the Northern Hemisphere more susceptible to warming than the ocean-laden Southern Hemisphere, he and his colleagues believe that climate scientists need to not only look at the rise of global mean temperature but specific hemispheric variability as well.
“Global mean temperature is great for detecting climate change, but it is not terribly useful if you want to know what is happening to rainfall over California, for example,” Chiang said. “We think this simple index, interhemispheric temperature, is very relevant on a hemispheric and perhaps regional level. It provides a different perspective on climate change and also highlights the effect of aerosols on weather patterns.”