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.
3 Comments
i am trying to understand this. my one question is that isn’t your time period of 8 years (1999 thru present) far too small of a sample to make any kind of long term assessment? I am by no means an expert but from my readings climate trends take much longer than 8 years to define a trend. plus i was just reading that the West in DEC 07 and JAN 08 is having above average snowfall and below average temperatures….which again is part of the natural sinusoidal cycle of weather? Again just trying to understand the science here. thanks
Hi fred1,
The shift in global winds has actually been taking place over a period of several decades, not eight years. Note that I wrote: “The tropical belt has widened by several degrees latitude since 1979.” I simply pointed to the western drought from 1999 to the present as an illustration of the possible consequences of a shift in climatic zones. I also stated that the current western drought is complex, with other factors contributing to it, including changes in the hydrological cycle that I discussed in an earlier post.
There will still be short-term fluctuations in the climate as the Earth goes through an overall warming trend. So there will still be cold spells, and wet spells in desertifying regions, such as the recent two months of relative wetness in the western U.S. that you pointed to.
As we discuss global climatic shift, I understand the need to discuss the greater contributing factors of this. That’s a given. We also know that given the complexity of something of this magnitude, to me there seems to be a glaring omission in something that should be included in discussion. Please correct me if I’m wrong so I don’t perpetuate incorrect thinking.
One of the factors of conditions of place is degree of impervious surface, and how that contributes to heating of cities. The contribution is not only direct heat bouncing off pavement and rooftops, but also in the impact this has on water tables, and surface/groundwater quantities. Increased impervious surface increases runoff, decreases the amount of water that is allowed to infiltrate, and therefore percolate into groundwater aquifers, or travel as interflow to feed rivers and streams. And its not just paving of open space that contributes to this, but also the filling of wetlands for construction.
There has to be an impact in regard to local heating, but also to the cooling affect of the earth with a supply of water beneath it, or in a waterbody. Perhaps in one small area it wouldn’t seem like much of an impact, (or that the impact is only felt locally) but if you consider the US as a whole, per se, doesn’t this contribute, quite possibly, to a significant degree?
Given that there is soooooo much unnecessary construction of pharmacies, strip malls and the like, that don’t really improve the livability of a place, why is it that developers are given such carte blanc to do whatever they want where ever they want? Environmental regs and protections/impact rarely seem to make a difference. They just sue to get their way. Community groups that attempt to keep development to a minimum only occasionally keep things at bay.
Over development and increase in impervious surface is having a devistating impact on watersheds everywhere, and wehre the storm water program is rather successful in a handful of random locations and actually quite well designed, were it implented as it was intended, it is largely ignored in many more. Why is there not more outcry about this? The health of our watersheds is linked dramatically to global climatic health, or so it seems to be in my thinking. Its not just the weather, but everything, globally, that is being impacted. There needs to be a much increased degree to the outreach and education of these issues nationwide. Its not nearly enough to be available to those who are already concerned and whose support comes naturally. In order for there to be a paradigm shift, which is what we need, the education against the rhetoric needs to be significantly stronger. It needs to reach those who otherwise would not seek it out. And the discussion needs to be inclusive of some of the more random parameters affecting the shift. kinda like the butterfly effect. yes?