From Mensagenda - May 2007

The Future: The Greenhouse Effect

by Mary Abbey

A feedback mechanism, as concerns global warming, is an effect of a change that continues that change or negates that change. Although the labels seem contrary, a positive feedback mechanism increases the warming, and a negative feedback mechanism is one that decreases the warming. There are so many variables to this process of the enhanced greenhouse effect that it is extremely difficult to make good predictions, even with our high-powered computers and experienced scientists. The following feedback mechanisms should be included when considering the likely prospects for the future.

Last month I mentioned that the greatest changes are occurring in the Arctic. This is due mainly to a positive feedback mechanism associated with albedo, the reflectivity of a surface. Ice and snow tend to have a high albedo, reflecting most of the sunlight back to space. All other things being equal, snow on the ground lowers our air temperature by approximately 10 degrees. When warming temperatures melt the snow and eat away at the edges of the Arctic ice cap, more open land and open water are exposed. These darker surfaces absorb most of the incoming solar radiation and give off infrared energy. The greenhouse gases capture this heat and the atmosphere gets warmer still. In addition, as the permafrost in the higher latitudes melts, stored CO2 and methane are released, again, increasing the warming. As the boreal forests expand into the tundra, they reduce the surface albedo, more sunlight is absorbed, more heat is given off, and the result is even more warming. Thus, the Arctic is experiencing greater changes than we are here.

Plants require CO2 for photosynthesis. Many, but not all, plants respond to a greater supply of CO2 by growing faster. They also tend to use water more efficiently. The more plants, the more CO2 is taken from the atmosphere, and another negative feedback mechanism slows the warming. If, on the other hand, weather patterns shift and we lose the majority of rainforests, less CO2 will be taken from the air, and more will be released.

The oceans that cover our planet have a great influence on atmospheric conditions. Water has a higher specific heat than air. Specific heat is a measure of how much energy must be absorbed before a temperature change takes place. Oceans modify global warming by absorbing massive quantities of heat and slowly changing temperature. However, feedback mechanisms are at work here also. Warmer waters can’t hold as much CO2, so less is being absorbed from the air, leaving more to add to the warming. Along our continental margins, in the bottom sediments of our oceans, water-methane ices exist called clathrates. Warmer waters may destabilize these clathrates, releasing large quantities of this greenhouse gas into our atmosphere, for a major positive feedback. We hope we won’t get to that point. A negative feedback mechanism in the oceans comes into play with more CO2, and that is an increased growth of phytoplankton, the tiny plants floating in our surface waters. These plankton give off dimethylsulfide, a trace gas whose molecules can act as condensation nuclei. In order for water droplets to form, water vapor must have a "seed" to condense onto. The presence of dimethylsulfide enables more droplets to form, forming more clouds. These clouds may have a high albedo, reflecting much light, thus enabling the atmosphere to be cooler.

Warmer air will mean more evaporation. Water is also a greenhouse gas, and an increased amount of water vapor will mean more warming — that is, unless more clouds are generated. If they are low clouds, they will act as a negative feedback, but, if they are high clouds, they may act as a positive feedback. Where and when these clouds form can also be important.

As I said, there are many variables and we don’t have all of the answers. Don’t be surprised to be confused at this point. If it was an absolutely clear picture, we wouldn’t be conflicted on the need for action. Have a few extra minutes? Do a little Web surfing, or browse the bookstores for the latest thinking on global warming. Become informed! Next time I’ll discuss some of the changes we are already seeing on our planet, and then we can delve into what can be done.