This year may be the worst for West Nile virus in the United States since it emerged 13 years ago, and there’s reason to think that global warming may be to blame.

One of the most frustrating things about discussing climate change is the difficulty to pin down cause and effect. Anyone with a basic understanding of the subject knows that an October snowstorm doesn’t disprove global warming. But it can be hard to convince anyone to worry about greenhouse gasses if you can’t point to real problems caused by a changing climate.

Scientific American blogger Christie Wilcox does an important service by breaking down the issues around this year’s West Nile outbreak. She notes that the Centers for Disease control counts 1,100 reported cases of the disease, and 42 related deaths, in the U.S. so far this year. That’s the most since West Nile was first detected in the country in 1999, and the numbers will almost certainly rise dramatically before the end of the summer.

The CDC notes that “unusually warm weather” may be to blame for the outbreak. So far, 2012 is the hottest year on record in this country. And more than half the West Nile cases have been in Texas, which was hit especially hard by the hot weather.

Many researchers see a connection between unusually hot weather — heat waves in Europe in 2003 and Russia in 2010, for example — and long-term climate change. While weather fluctuates from year to year, higher average temperatures mean higher extremes. And the notion of “global weirding” suggests that the greenhouse effect can cause unanticipated, extreme weather, not just incremental temperature increases.

West Nile offers a striking example of the kinds of problems that could come from warmer, weirder weather. Mosquitoes pick up the virus more easily in hot weather, and the particular type of mosquito that usually transmits the virus actually does very well in drought conditions like the ones much of the country has faced this year.

The good news is that West Nile is usually not terribly serious in humans. Around 80 percent of those infected don’t exhibit any symptoms at all, and less than 1 percent end up with severe problems like meningitis or encephalitis.

The bad news is, with climate change becoming more of an issue every year, West Nile probably won’t be the last disease to break out all over the place during an American summer.

Main photo credit: JJ Harrison