The great trump card of those uninterested in addressing the warming climate is to note that extreme storms, like the hurricane that overwhelmed Houston over the weekend, have been happening for centuries. Droughts have been happening since before recorded history; hot days have, as well. Scientists sing a variation of that tune, noting that it’s often impossible to blame a severe weather event specifically on climate change, instead pointing out the ways in which climate change may have made a bad thing worse. That’s the trick here: Hurricanes didn’t suddenly appear as people dumped greenhouse gases into the atmosphere. But it’s almost certainly not a coincidence that Hurricane Harvey set a record for the amount of precipitation from a storm during an era in which global temperatures have never been higher.
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.
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.
Already today 75% of the moderate hot extremes and about 18% of the moderate precipitation extremes occurring worldwide are attributable to warming, of which the dominant part is extremely likely to be anthropogenic. [Ed. — That is, caused by humans.] The fraction increases non-linearly with further warming such that the probability of hot extremes at 2◦ C, for example, is double that at 1.5◦ C global warming. With every degree of warming it is the rarest and the most extreme events — and thereby the ones with typically the highest socio-economic impacts — for which the largest fraction is due to human-induced greenhouse gas emissions.
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.
We can also say that scientific analysis suggests that future hurricanes will be similarly strengthened — and, further, that research suggests that the warmer the climate gets, the stronger those storms might be.
