There is a circulation pattern in the tropics called the Hadley cell, and it is this circulation that this study is referring to specifically. The Hadley cell refers to the tendency of air to rise at the equator (the warmest part of the earth) to roughly 10 miles high, and then move towards the poles to about 30 degrees latitude, where it sinks and then moves back to the equator. (This low-level movement towards the equator gets twisted to the west by the spin of the Earth, and thus we have the trade winds in the tropics.)
As this ABC article says, computer modelling conducted several years ago predicted that if climate change were to occur, and the Earth were to get several degrees warmer, the area covered by the Hadley circulation would grow by about 200 miles (or 2.5 degrees of latitude) over the course of the 21st century.
The new study that just came out says that the Hadley circulation grew by almost 200 miles just in the last 25 years. Put another way, the predicted change for the next 100 years has already come to pass.
Well, why does some obscure circulatory pattern really matter? What does this mean? Drought, for one. This article talks about the effect on Australia, but I think there will be effects much closer to home.
As the air rises near the equator, it gets dried out as it rises and loses pressure. The air that moves towards the poles at altitude is thin and dry, and it remains dry as it descends at roughly 30 degrees latitude. This is why so many of the world’s deserts and semi-arid regions are around this area. Think of the southwest US, central Australia, the Sahara, the Gobi, the entire Middle East…
If the Hadley circulation expands, it could bring desertification to new areas that are currently productive agricultural areas. For example, the Gulf Coast from Texas to the Florida panhandle is pretty much exactly 30 degrees north latitude. Any increase in the Hadley circulation could have profound impacts on these states. And that’s just one example. Other regions which could be affected include the Mediterranean, southern Australia, parts of Central America and South Africa.
Basically, what we’re looking at is one more piece of the puzzle that is global climate change. And the picture on this puzzle is not looking pretty at all. Fortunately, this study came out just before the next UN climate change conference in Bali this week. Hopefully there it will get the attention it deserves, because it is surely not getting that attention in the media.