One of the side-effects of global warming is that warmer air can hold more water vapor. In particular, storm clouds can hold more water so they can dump more water on the land. Sometimes it’s true that “when it rains, it pours.”
Not everyplace is going to get more rainfall, however. Also, some places that get the same amount of rain will nonetheless get drier because more heat means more evaporation. That’s one of the chief reasons global warming is such a big factor in the ongoing California drought. Their precipitation has been low lately (2013 was an especially bad year), but they’ve seen just as bad spells in the past. What they haven’t seen is a drought as bad as this one, and that’s because drought brought on by reduced rainfall has been made worse by increased evaporation due to the heat.
But rather than talk about California, let’s consider Maine, and how much rain can fall in a single day.
There are 12 USHCN (U.S. Historical Climate Network) stations in Maine. Looking at daily rainfall, we can see that at some locations patterns have changed over the years. One of the first questions which springs to mind is, does it rain more or less often? I’m not talking about the amount of rain, just whether or not it happens at all. The answer for Portland is yes:
With a p-value of 0.000376, the increase is statistically significant. There’s been a statistically significant increase in the number of days per year with at least some rain for 8 of the 12 USHCN stations in Maine, while none show a statistically significant decrease.
Another question is, when it does rain, does it rain more or less? I compared the data before 1990 to that since 1990, and found that for Portland, the answer is yes: the mean amount when it does rain has increased from 0.33 in to 0.37 in.
Portland has seen an increase in the number of days with relatively heavy rain as well. I counted the number of days per year with at least 1 inch but not as much as 2, with at least 2 inches but not as much as 3, and with 3 inches or more of rain in a single day. It turns out, all three classes of day have gotten more frequent:
The solid lines are trend estimates using Poisson regression. The number of days per year in Portland with 3 inches or more of rain in one day, has increased from about 0.25 (only once per 4 years) to about 1.24 (more than once per year on average).
Portland isn’t the only Maine town to show such increase. Again, 8 of 12 USHCN stations show a significant increase in at least one of these size classes of rain. Particularly striking is the increases in Eastport:
I also wondered, is the increase of 1-inch rain days and 2-inch rain days and 3-inch rain days due only to the increase in the average rainfall per rainy day? To investigate, I took all the days on which some rain fell in Portland, no matter how much, then split it into the time spans before and after 1990. For each, I divided the daily rainfall by the mean daily rainfall for that time span (0.33 in. pre-1990, 0.37 in. post-1990) to define normalized rainfall. Then I computed the survival function (the probability of being that high or higher) for each time span (pre-1990 in black, post-1990 in red):
There seems to be a divergence, with daily rainfall above the 5-times-average value being more common after 1990 than before.
We’ve known for some time about the changes in California because its drought has been so much in the news. But I was a bit surprised how plain and clear are some of the changes in precipitation patterns on the opposite side of the country, in Maine. But they’re there. It’s a clear sign that the warming of Earth, its increase of water vapor in the atmosphere, and hence the change of rainfall patterns, is already upon us. Climate change is here already.
As for the impact on human life, changes in precipitation can bring some of the most profound. Increases, especially of heavy-rain days, can make flood more likely, sometimes far more likely. Decreases make drought more likely. Unfortunatly, it seems right now that the worst combination is happening, that the dry places are getting dryer and the wet places wetter. We need to be ready to deal with it, and we should do all we can to prevent its getting as bad as it will if we just keep burning fossil fuels as usual. Maybe we need a new saying: “When it rains it pours, when it dries it crumbles to dust.”
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