As the Globe Warms, the Tropics Are Expanding

by Bill Chameides | May 23rd, 2012
posted by Erica Rowell (Editor)

Overweight Americans are loosening their belts; in Europe it’s been all about fiscal belt tightening. But what about the tropical belt?

What Is the Tropical Belt, Anyway?

Think of the line that circles the globe along the equator. Now thicken that line so that it extends as far north as the Tropic of Cancer (about 23 degrees North latitude) and as far south as the Tropic of Capricorn (about 23 degrees South latitude). The resulting band is what is known as the tropical belt. (In the figure below, the tropical belt is indicated by areas colored green and brown.)

From a climatic point of view the extent of the tropical belt is important because it demarcates the zone dominated by a circulation pattern known as the Hadley cell (see graphic). This cell plays a major role in determining global patterns of rainfall. Near the equator the Hadley cell causes upward air currents that lead to lots of rainfall. At the poleward edges, the cell causes downward vertical motion and suppresses rain. It is no accident therefore that much of the world’s deserts are located in subtropics along the poleward edges of the tropical belt. Move those edges a bit to the north or south and you will likely see those deserts move as well.

What’s Been Happening to the Belt: A Short History

We all know that changes in the climate have been afoot. These include those that are most often talked about in the media such as rising global temperatures (see here, here and here) and melting ice (see here and here). Other trends may also be underway, such as more severe storms (see here and here) and weather extremes, but they have proven harder to establish in a statistically robust manner.

But here’s a trend I bet most people didn’t know about: the tropical belt has been expanding. The northern and southern edges of the belt have both moved on average about 100 miles further from the equator and closer to the poles over the past 30+ years.

A 100-mile expansion to the north and south in the tropical belt may not seem like a lot, but in fact it can have profound effects. For example, it could lead to a further drying or desertification of the subtropics. That could be especially problematic for already dry areas like the American Southwest.

Why the Expansion?

It has long been theorized [pdf] that global warming from greenhouse gases would cause the tropical belt to expand. But the rate of the observed expansion has been considerably larger than what one would expect from greenhouse gases alone. So what gives?

Some have pinned the blame for this unaccounted widening on the depletion of stratospheric ozone. (See here and here.) But while that thinning of the ozone seems to work for the Southern Hemisphere, the amount of depletion in the Northern Hemisphere is just not large enough to make it work for the northern regions.

Now a new paper published in the journal Nature by Robert Allen of the University of California at Riverside and colleagues offers another theory. This one positing that expansion is being mainly driven by increasing concentrations of two pollutants — tropospheric (or lower atmospheric) ozone and black carbon. Why these pollutants? One, they are on the scene — they’re concentrated in the atmosphere above the northern mid-latitudes. And two, both pollutants warm the atmosphere differently from greenhouse gases.

While greenhouse gases warm by absorbing and trapping infrared radiation emitted by the Earth’s surface, ozone and black carbon warm by absorbing visible light from the sun. By their location, tropospheric ozone and black carbon concentrate the warming in the region of the atmosphere that is critical for determining the edge of the tropical belt. And given that they both trap visible light, they cause the largest amount of warming in the summer months (when the largest amount of the sun’s radiation reaches the northern hemisphere); this corresponds to the time of year when the northern hemisphere’s part of the tropical belt expands the most.

Does It Work?

Great idea, but does the explanation hold water? To answer that question, the authors ran a series of model simulations with and without increasing concentrations of tropospheric ozone and black carbon, and compared them to observed trends in the tropical belt.

The bottom line is that it kind of works. Including tropospheric ozone and black carbon warming in the models does lead to a northward expansion of the tropical belt and does get the summer peak expansion right. But the amount of expansion predicted by the model for the two pollutants, while larger than that for greenhouse gases, is still on the small side when compared to observations. On an annual basis, the model-predicted displacement is a factor of two to three smaller than observed displacements. The authors speculate that the discrepancy could be explained by an underestimate in the amount of black carbon that is being emitted into the atmosphere, especially from Southeast Asia. Maybe, but I’m not totally convinced.

The authors conclude by arguing for emission controls of black carbon and the pollutants that lead to production of ozone in the troposphere (e.g., nitrogen oxides, carbon monoxide and volatile organic compounds). Such controls, they write, “would thus not only help mitigate global warming and improve human health, but could lessen the regional impacts of changes in large-scale Northern Hemisphere atmospheric circulation.” Yeah, I’m down with that.

filed under: climate change, faculty, global warming, rainfall, weather
and: aerosols, air pollution, black carbon, desertification, equator, Hadley cell, ozone, ozone layer, pollutants, subtropics, tropical belt, tropics