PBS show asserts greenhouse gases, atmospheric pollutants dimming future

April 22, 2006

This week, the Public Broadcasting Service aired a NOVA program titled “Dimming the Earth”, which presented research by leading scientists on the complex systems of our global climate and human activity's effect on it. One of the largest interactions (or "inputs") humans have with the atmosphere is the ever-increasing use of fossil fuels. Consumption has risen 2% per year for this decade.

Fossil fuels burnt in factories and automobiles send their waste into our atmosphere in two forms. The first is CO2 and other greenhouse gases, which have received substantial attention in the last few years because of the way they trap heat in the atmosphere. The second is the tiny particles of sulfur dioxide, soot and ash, which scientists call aerosols (basically smog). Research into understanding the negative health effects of air pollution has resulted in the development of catalytic converters for cars as well as devices to remove particulate solids from industrial waste before it reaches the air.

More recently, atmospheric scientists have come upon the phenomenon of the reduction of direct sunlight reaching Earth's surface&mdash; observing a nearly a 5% decline between 1960 and 1990, with evidence of a recovery since then. This has been dubbed the "global dimming" effect, and is probably due to the way these aerosols act upon clouds. It is important to realise that this does not represent a net loss of this much sunshine to the climate system - if so, large temperature declines would have been observed. Instead, the sunshine is absorbed elsewhere in the system, with a much smaller net loss.

Clouds form when moisture gathers around airborne particles, such as pollen or dust. Clouds formed by the aerosol particles emitted by fossil fuel consumption are made of many more tiny droplets than "natural" clouds. These smog-created clouds have two notable effects: they shield sunlight from reaching Earth's surface and, due to water's reflective nature, the millions of tiny droplets suspended in them reflect light back into space, allowing even less light to reach Earth.

Many scientists now believe that global dimming caused by these pollutants has mitigated the temperature rises brought about by global warming. Over the last thirty years, Earth's temperature has increased by about 0.5 oC.

In the absence of global dimming, however, the Earth might be 0.3 oC warmer than it currently is, suggesting that a "tug-of-war" exists between greenhouse gases and particulates released by burning fossil fuels. Efforts to mitigate the human health dangers of smog have allowed more heat into our atmosphere and brought about a sharper increase in global warming.

Dr. James E. Hansen, professor at Columbia University and the head of the Goddard Institute for Space Studies, believes that if we continue on our current pattern, this warming could be as much as five degrees in the next thirty years and ten to fourteen degrees over the course of the century. Such a temperature rise would devastate life on Earth, likely bringing on a cascade of self-reinforcing warming effects. Earth's forests drying and burning, a steady thawing of the Greenland and arctic ice sheets, and, most dangerous of all, a release of the methane hydrates that are now frozen at the bottom of the oceans, could remake the planet into something inhospitable to human life. Dr. Hansen warns that, according to his research, man has just 10 years to reduce greenhouse gases before global warming and other responses to human activity by Earth's climate reach a "tipping point", becoming unstoppable.