Heat, Flood, Fire

Heat, Flood, and Fire

The Atlanta Journal Constitution, today, reported that much of Georgia was now in exceptional drought. "Exceptional", in this case, means once in one hundred years.



Figure 1: Drought Index, just to be different, from the U.S. Forest Service.

Much of the South in the U.S. is in the midst of a heat wave. This year we have seen heat waves of note in India, Europe and the United States. We have seen severe floods in Texas-Oklahoma-Kansas, England, southern and eastern Asia, and now the upper Midwest. This has been another year of severe wild fires in the U.S. West, and earlier in the U.S. South.

What do all of these things mean? How do we put it into perspective?

To be sure, we have more access to information from around the world. Therefore, we hear about these things. Also, there are more people impacted by the weather. We are all sensitized about climate. As scientists and observers of the atmosphere, we know that there have been droughts and floods and heat waves before. It's the nature of the beast; is the beast changing?

First, heat waves: Public health experts will tell you that heat waves are responsible for more deaths than any other weather-climate hazard. Heat waves are complex events; often it is not the absolute temperature that is the most important variable for predicting a dangerous heat wave. People acclimate to heat. The same temperature late in the spring and early in the summer is much more dangerous than in August. A dangerous heat wave in Montreal is a normal summer day in Houston. This interaction of the environment and human physiology blends physical and biological science. Among other things we know about heat waves, when it stays warm at night for several nights in a row, the heat wave gets much more dangerous. Heat waves, or at least the direct impact on human health, are often viewed as an urban problem. All of those issues of the urban heat island come up.

Meehl and Tebaldi (Science, 2004) have projected that heat waves will become longer, more intense, and more frequent in the next 100 years. Given that urban heat waves are already a significant public health problem, a warming planet would amplify this problem.

Second, drought and fires: We do know that drought and floods, heat waves and cold snaps are all part of nature. Like the problem of urban heat waves, we have an event that already exists, and there should be a change associated with global warming. I have already mentioned that some studies have attributed the pinyon pine die off in the U.S. Southwest to the fact that the temperature in the recent drought years is higher than in previous droughts. Therefore, ground water is reduced; there is more stress on the plants. (And perhaps it is really the warmer nighttime temperatures that matter?)

There have also been papers which make a compelling argument that wild fires in the western U.S. are increasing in intensity and duration. In the paper of Westerling et al. (Science, 2006), the conclusion is drawn that this is directly related to snow melt occurring earlier in the year, a hotter and drier forest, and hence, a longer burning season. Plus they isolate the impact to be at mid-elevations in the Rockies, and hence, relatively free of land-use changes. While many newspapers reported that this work showed an increase of wild fires due to climate change, I quote directly from their paper: "Whether the changes observed in western hydroclimate and wildfire are the result of greenhouse gas-induced global warming or only an unusual natural fluctuation is beyond the scope of this work".

In the examples discussed above, we have existing events, heat waves, droughts, floods, and wildfires. Casually, one can look at our history and conclude that all of these things have been around forever, and hence, they are not evidence of climate change. Alternatively, with our current sensitivity to climate change, the casual observer can see each of these events as another brick in the wall. The scientists approach to this work is to look how the characteristics of these existing events are changing, not whether or not they exist. Then those changes have to be considered in light of other sources of change--land-use, population, climate, etc. The question, the argument, then focuses on how do we determine cause and effect?