Sunday, December 2, 2007

Climate changes San Diego wildfires?

Do the recent San Diego fires have any relation to global warming? The San Diego Union, in Many blame threat on global warming, touches on most of the significant issues. Wikipedia also provides information on both the 2003 fires and the 2007 fires, two of the largest in California history.

Consier this quote from the Union article in particular:

Five years ago, Neilson and other Oregon State University researchers predicted that periodic increases in rain and snowfall, combined with higher temperatures and rising levels of carbon dioxide in the atmosphere, would spur vegetation growth. That would add to already extensive quantities of fuel caused by decades of fire suppression, in which blazes are not allowed to burn out of control and thereby eliminate dead or dying vegetation. One expectation outcome from global warming: more extreme weather patterns.
This made me wonder: what has been the pattern of San Diego rainfall in recent history. To evaluate this, I found data for rainfall at Lindberg Field (San Diego's Airport), covering the years 1964 through 2005. I then added the values for 2006 and 2007. Here's what we get (click on graph for larger version):

What does it mean? Well, let's start with several big caveats: (1) This is not a lot of data, historically speaking. Climates shift-and-drift to-and-fro, like the tides or the seasons. (2) "Eyeing the data," in other words just looking at it, is often misleading.

With those caveats in place, I still argue that one should be able to "eyeball" the data to find patterns. If no pattern is visible, then there seems little need to quantify it statistically. But if a pattern does exist, perhaps we're onto something.

So the question we should ask ourselves: do we see a pattern? Here's my response:

1. The extremes seem to be increasing. In 1966, 14.76 inches of rain fell on Lindberg field. In 1978 and 1983, the maxima rise to 17.3 and 18.49 inches respectively. In 1998 and 2005, we see further rises in the maxima, to 20.89 and 22.81 inches respectively. The trend may not continue; and it might not even be statistically significant. But it does appear, to the eye, that the the local climate is experiencing more extremes.

2. Consider what has happened after the 1998 and 2005 rainfall peaks: abnormally dry periods. Both 2002 and 2007 were the driest years since 1964. In other words, heavy rains followed by drought.

3. Locate the Cypress and Witch/Harris fires on the graph: Both are located in the abnormally dry years following historically heavy rainfall: 2003 and 2007. In other words, heavy rain produced excessive growth followed by dry years that turned that growth into dry kindling. With all that fuel in splace, the slightest spark will do the rest.

This is all speculation. We'll have to monitor rainfal in the years to come. And look for data further back in time. But it is not unreasonable to speculate that this data fits within the framework of the global warming story.

Is this analysis pertinent? Or an example of Global Warming Ate My Homework?