During the past 28 years, the tropics have become several hundreds of kilometres wider, according to a team of American climatologists. According to current simulations this should only occur a century from now.
According to astronomers, the tropics are the area around the equator, between the tropics of Capricorn (23.5 degrees South) and Cancer (23.5 degrees North). It isn’t quite that simple for climatologists, however. They use a whole list of climatological criteria to determine where the tropics are.
Nevertheless,
Noorderlicht, a leading scientific web log in The Netherlands, reports that Dian Seidel en colleagues wrote in Nature Geoscience that these criteria all have “something” in common. That “something” is the fact that the tropics have crept southward and northward for several hundreds of kilometres during the past 28 years, according to a number of different types of measurements.
A tropical climate is always hot and virtually always wet. The adjacent regions, the subtropics, are usually quite a bit drier. This is caused by large-scale air movements. These movements have changed a lot, according to the climatologists who did not do any original research for their article, but studied recent literature instead.
The most important of these air currents is the Hadley circulation, named after George Hadley, a British physicist (1685 – 1768) who created a model to explain global atmospheric circulation patterns in 1735. The Hadley circulation consists of air that has been heated up around the equator, releases rain and flows north or south high in the atmosphere. This drier air descends in the subtropics to flow back to the equator.
The area where there is no net wind blowing north or south, is the edge of the Hadley-circulation. Measurements over the past few years have shown that this edge has moved quite a bit. According to wind pattern and cloud analyses, the tropical area has widened by about 180 to 400 kilometres during the period between 1979 and 2005.
Another type of measurement is the ozone concentration. This is relatively low in the tropics. The area with little ozone, i.e. the tropics, has widened by about 200 kilometres, according to the measurements. Numbers are not known for the southern hemisphere.
Another method to find the boundaries of the tropics, is measuring the air temperature by satellite. Measurements made between 1979 and 2005 show a movement of about 90 kilometres north and south, respectively. That is about half of what other analyses find.
Lastly, the boundaries between air strata (layers of air) have been studied by using sounding (weather) balloons. The boundary between the lowest air stratum, the troposphere (older) or the tropopause (newer), and the stratosphere above is at a relatively stable altitude of at least 15 kilometres in the tropics. It is much less stable in the subtropics. According to sounding balloon measurements, the stable area has widened by about 600 to 900 kilometres during the period from 1979 to 2005.
Hence, all analyses seem to indicate that the tropics are advancing, but they do not agree as to how much and how fast this happens. Seidel and colleagues think this may have to do with measuring errors, but possibly also with the fact that the different tropical characteristics may not have simple one-to-one relationships.
The measurements also do not agree with computer models that simulate the climate. According to Seidel, this simulations did not pay much attention to large-scale air movements, but this is changing. Certain recent simulations indicate that the tropics will widen under the influence of human-caused global warming, but not nearly as fast as the facts are now showing.
The most extreme of all IPCC scenarios, the one with the most pronounced warming, the boundaries of the tropics would have moved by about 180 kilometres at the end of this century. This prediction has now been superseded, since it has already happened. According to the climatologists, it is clear that there is still room to dramatically improve the models. However, more knowledge of air circulation is necessary to achieve this.
According to Seidel and her supporters, this advancement of the tropics is a bad thing for many people because it will lead to fundamental changes in eco systems and living conditions. Especially areas that used to have a good climate and will now become part of the dry subtropics, will experience the consequences.
These areas are often densely populated, such as the Mediterranean, the southwestern United States and the south of Australia. Areas closer to the equator will probably receive more rain because of the changes, and that will often be welcomed.
The authors warn that an increase of the tropics can also strengthen the global warming effect because some of the humid air that rises in the tropics gets to the stratosphere. If this leads the stratosphere to hold more water, it will become more efficient at retaining heat because water is a strong greenhouse gas. This way, the process drives itself.
As if that was not enough, ocean currents may be affected as well. The consequences of this cannot be predicted, but they will probably be unpleasant.
Once again, climate research paints us a picture of doom and gloom. However, this research is desperately needed, because all measurements, analyzes and hypotheses are still far from certain. They are somewhat plausible however, because they all seem to point in the same direction.
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