Data from the National Climate Assessment released in 2014 shows how extreme precipitation events have increased over time in most parts of the country.

In most regions east of the Rockies, the amount of precipitation falling in the top 1 percent of all daily events has increased over the past few decades.

Nationally, the percentage of the Lower 48 states that has experienced an extreme precipitation event has increased over the past few decades.

Why? For two reasons. First, in warmer air, water molecules are moving more rapidly, allowing more water to be in the atmosphere without condensing into a liquid. The result is more moisture in the air — and, therefore, more that can then eventually condense and fall to the ground. Second, higher temperatures speed evaporation from the ground — drawing more moisture into the atmosphere.

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Temperatures have increased; precipitation events have increased. But are those things a result of human activity? A study released in 2015 suggests that they are.

That study found that, as global temperatures increased, so did the probability (PR) of heavy precipitation events.

The effects aren’t distributed evenly, but, generally, more warming means an increased likelihood of heavy precipitation events in most parts of the world.

The study’s authors are explicit in connecting the effects of that increase to human activity. They liken attribution of extreme events to the difficulty in blaming any one case of lung cancer on cigarettes — but by looking at long-term global data, the link is made.

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Already today 75% of the moderate ho t extrem es and ab ou t 18% of t he moderate pr ecip i tation extremes occur ring wo rldwide ar e attrib utable to warming, of w hich t he domin an t par t is extremel y likel y to b e an thro pogenic. [Ed. — That is, caused by humans.] The f ract ion increases no n-linearly w ith fur ther war ming such that the pr obabili ty o f hot extremes at 2◦ C, for exam ple , is doub le that at 1.5◦ C global warming. W ith ever y degree of war ming it is the rar est and t he most extreme even ts — and thereb y the on es with typically the highest socio-econo mic im pac ts — fo r which the largest f ract ion is due to h uman-ind uced greenhouse gas emission s.

We come back to our original point: We cannot say that 52 inches of rain fell on parts of Texas solely because of climate change; that, had humans never burned a single piece of coal, there would have been no hurricane at all. We can say, though, that climate change almost certainly made Harvey stronger than it would otherwise have been by warming the air and the Gulf of Mexico.