Recently a friend and I were talking about precipitation and he suggested that global warming was contributing to changing precipitation patterns around the world. This reminded me of something I had looked into a few years ago, so I thought I would share it.
In 2007-2008 there was some discussion in the news about the low rainfall in the San Francisco Bay Area. As is often the case, some commentators ascribed the low rainfall to global warming. I was curious about what the precipitation data actually showed and did a bit of searching online for the historical data, finally finding data back to 1875 for San Jose.
I’ll do some more on this in a later post, but here is the 1875-2007 graph I made at the time, showing monthly average precipitation. You can clearly see the dry-wet seasonal curve typical of Mediterranean-type climates like San Jose. You can also see why it makes sense to gauge the water year from July 1 through June 30.
Interesting enough. But what about the annual rainfall? Did the precipitation go down in any meaningful way that could be linked to global warming? I’ll let the data speak for itself.
Not much of a signal here; if you slap a linear trend on the chart it comes out essentially level. In fact it is very slightly positive (meaning more precipitation, not less), but I don’t think anything can be made of this slightly positive trend, due to startpoint-endpoint selection. In any event, the trend is not significant.
What is clear, however, is that San Jose experiences a wide range of annual precipitation, often swinging radically from year to year. San Jose also often experiences a few dry years in a row, followed by a couple of wet years in a row (or one particularly wet year). The relatively dry 2007 year does not jump out as being at all unusual in the broader context.
Of course this doesn’t cover the entire Bay Area, but was an instructive first look at some data. Given the recent conversation with my friend I’ve looked at a bit more data this past week, which I’ll post soon.
In the meantime, one thing that has definitely changed in the Bay Area is population. According to http://www.bayareacensus.ca.gov, in 1880, just after the above graph started, the total Bay Area population was 422,128. In contrast, in 2010 the total population was 7,150,739.
I’ll leave it as an exercise to the reader to determine whether any water shortages experienced in the Bay Area are more likely due to diminishing precipitation from global warming or due to increased demand.