This post is by James Wang, Ph.D., a climate scientist at Environmental Defense.
You may have heard about the persistent droughts in the western U.S., Australia, and other regions. The Upper Colorado River Basin is experiencing a protracted, multi-year drought that started in 1999. Australia's record drought is threatening the livelihood of traditional farmers and ranchers.
At what point does a passing drought become a permanent shift to desert conditions, and why would such a thing happen?
It can happen because of global warming. Climate change can alter global winds, the strength and location of high and low pressure systems, and other climate factors.
In my post about the California wildfires, I discussed how global warming can lead to more frequent and intense droughts through intensification of the hydrological cycle and earlier snow melts. In this article I'll focus on shifts in global winds.
Global winds shape the Earth's climate, determining – in broad strokes – which areas are tropical, desert, or temperate. Here's a simplified overview of how it works.
The Sun heats the Earth most intensely in the tropical zone around the equator. The heated air rises, cools, and then dumps its moisture as rain. That's why there are rain forests in the tropics.
The now drier air is forced by the continuously rising equatorial air to move towards the temperate latitudes on either side of the equator. At roughly 30° N and S – called the "horse latitudes" – it can move no further due to the Earth’s rotation, and settles to the surface. As the air sinks, it compresses and warms, creating hot, rain-free conditions. This circulation pattern, called a Hadley cell, is why the deserts of the world are located just poleward of the tropics, to the north and south.
Poleward of the desert belt, strong, high-altitude winds known as the jet streams flow from west to east, carrying large storms with them. These mid-latitude, temperate-region storms are an important source of rain and snow, especially during the winter season. Much of the world's population lives in the temperate region. It includes most of the U.S. and southern Canada, most of Europe, East Asia, southern South America, southern Africa, and southern Australia and New Zealand.
But climate regions aren't fixed. Several independent studies have found that global winds are shifting due to global warming, and the shifts are faster than predicted by climate models. Most recently is this new study in Nature Geoscience. The tropical belt has widened by several degrees latitude since 1979. This is consistent with other observations suggesting that the jet streams and storm tracks have moved poleward.
The drought-stricken Upper Colorado River Basin, which includes Lake Powell, is located just poleward of the horse latitudes at around 37° N. This has historically been in the temperate zone, but the desert zone may be gradually encroaching upon it. (Since nothing is simple, there are other factors contributing to this particular drought, as well.) Similarly, water-starved Sydney, Australia at 34° S is just poleward of the southern horse latitude.
What we may be seeing here is not so much drought as desertification – a shift in global climate patterns due to global warming. Areas that used to be in temperate zones may be shifting into desert, while areas that had been arid receive more precipitation.