The Alarmist Rains in Phoenix

The floods in Phoenix from the severe rainstorm earlier this week, on cue, brought out the climate alarmists.

At Mashable, Andrew Freedman writes that "so far today, 3.29 inches of rain has fallen in the desert city [Phoenix], which makes it the wettest calendar day on record there. This smashed the previous record of 2.91 inches, which was set in 1931." Actually, according to the NOAA National Weather Service database, the previous daily record was 2.91 inches in 1939 (September 1939, coincidentally), not 1931.

The possible link to climate change come with this statement:

"Climate studies have shown that all across the U.S., precipitation has been falling in heavier bursts in recent years as air temperatures have increased, since milder air holds more moisture. The biggest increase in the amount of precipitation falling in very heavy precipitation events has been in the Northeast, but the Southwest has also a greater share of its rains come in heavy bursts like this during the past few decades. (However, the trend in the Southwest is small enough that this could be due to natural climate variability.)

An increase in heavy precipitation events is one of the more robust findings of how global warming is already affecting the U.S. and other countries."

Eric Holthaus at Slate goes farther in his linkage of the Phoenix rainstorm to climate change:

"As with other mega rainstorms so far this year in Seattle; Detroit; Baltimore; Long Island, New York; and Pensacola, Florida, it's very likely that global warming played a role in today's desert deluge. According to this year's National Climate Assessment, heavy rainfall events have been increasing all across the United States since the 1950s, boosted by a quirk of atmospheric physics: Warmer air can hold more water vapor. That means thunderstorms in climate-change-altered 2014 can now pack a heftier punch."

Of course, neither author appears to have looked at the historical climate trends for the Phoenix region. When we do so, it becomes very clear climate change is not likely playing a role. Remember that in order for us to have causality, we must meet the prerequisite of correlation.

Here are the maximum one-day rainfall totals on an annual basis and for the month of September since records began in 1896 for the Phoenix area.

Your eyes probably see a slight negative correlation towards less extreme one-day precipitation events in this region -- and they would be correct. On both an annual basis and during September, there are non-significant negative correlations towards smaller maximum one-day rainfall totals in Phoenix since 1896, as well as since 1970 and over the past three decades.

If anything, climate change is making Phoenix less susceptible to extreme one-day precipitation events, not more.

The other point that was suggested is that it is somehow unusual for Phoenix to get a substantial portion of its annual rainfall on a single day. This, too, is an incorrect assumption.

The following chart shows the percentage of the annual rainfall total coming on the day with the highest one-day precipitation of each year for Phoenix.

Over the historical record, there is no significant trend in how much of Phoenix's annual rainfall comes from the maximum single-day rain event of the year. Actually, the overall correlation is negative, not positive.

Finally, if we consider the number of days each year having more than one or two inches of rain, there are non-significant negative correlations for both of these as well since records began. In fact, the last time Phoenix saw a day with over two inches of rain was back in 1988, and the time before that was 1970, and the time before that was 1951. During 1911, there was a record two days each with over two inches of rain, and they both occurred in July. Oh, the horrors of that extreme climate of 1911. Good thing those days are long gone.

Other than this week's event, there have been only 11 days with greater than two inches of rain in Phoenix since 1896, and 9 of those 11 days occurred between 1911 and 1951.

If the historical record is any guide -- which it should be the only guide -- the odds that this week's rain event in Phoenix was due to anthropogenic climate change appears to be approaching zero.