Daily Science Dose

You may remember years ago when SARS broke out and with it fears that this disease could spread with human hosts as they traveled across the globe? Well, it is true that diseases can spread through the human hosts in this age of trans-global travel, and if that happens, it can be dangerous. But something else is happening to spread diseases that just don’t exist everywhere.

Tropical diseases don’t usually show up in places like Europe and North America, but they are starting to, and this problem may get worse before it gets better.

At the heart of the problem is that it is the tropics themselves that are on the move, and with them come certain insect-borne diseases such as malaria and dengue fever.

I had written on tropical diseases moving into Europe back in January. And now, I have run across more bad news and this time it is coming from the World Health Organization. The Nobel-prize winning Intergovernmental Panel on Climate Change (IPCC) has already noted that more and more people will be afflicted by tropical diseases, and why? Global warming and climate change. The warmer parts of the world, where many lethal diseases thrive, are expanding their range into formerly temperate regions. And with milder winters, insects and other disease-carrying organisms are not killed off during winter months, and thus expanding their range every year.

A study published last December in Nature Geoscience reported that tropical zones are moving at a much faster rate than computer models had predicted.

Scientists have found that, during the past 25 years the equatorial region classified as climatologically tropical has expanded polewards by about 172 miles which has meant that a further 8.5 million sq miles of the Earth are now experiencing a tropical climate, compared to 1980.

The study was carried out by Dian Seidel of the US National Oceanic and Atmospheric Administration in Washington, her colleagues from the National Centre for Atmospheric Research in Boulder, Colorado, and the universities of Washington in Seattle and Utah in Salt Lake City.

They found that, during the past quarter-century, the area defined as tropical, based on a list of five recognised climatological criteria, has moved further north and south by about 2.5 degrees of latitude, or about 172 miles in total in both directions. That is greater than the predicted shift of 2 degrees by 2100 predicted under the “extreme scenario” envisaged by the Intergovernmental Panel on Climate Change. –The Independent

What is a “tropical” region? Of course, we think of palm trees and mai tais, but that is really not what we are talking about when we discuss tropical diseases. Tropical regions receive more sunlight than anywhere else. If you need a simple definition, you can use a globe and check out the area between the “Tropics” of Cancer and Capricorn. But scientifically-speaking, the tropics are wetter. Warmer air can hold more moisture, and the tropics have the hottest air thus the wettest air. As you move away from the tropics, air cools and thus the “sub-tropics” have less moisture in the air. Meanwhile, that moisture has fallen somewhere before hitting the sub-tropics. So not only is the air in the tropics wet, but there is an awful lot of precipitation in the tropics. That much water can create very inviting environments for such disease-carrying insects like the mosquito.

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