“There is a roughly 3 percent increase in average atmospheric moisture content for each 0.5°C (about 1°F) of warming,” Mann notes, citing a rigid formula known as the Clausius-Clapeyron equation.
“Sea surface temperatures in the area where Harvey intensified were 0.5-1°C warmer than current-day average temperatures, which translates to 1-1.5°C warmer than the 'average' temperatures a few decades ago. That means 3-5 percent more moisture in the atmosphere.”
This is what triggered the record-breaking rainfall in Hurricane Harvey. It may have still been record-breaking without man-made climate change playing a role, but it’s 100 percent certain that our addiction to fossil fuels made it worse.
There are a few other more ambiguous climate change-related points to make here, but for the sake of clarity, we’ll leave it at what we definitely do and do not know for now.
In sum, though, climate change made Harvey worse. How much worse is hard to quantify at this point – the science of attribution (i.e. what percentage of Harvey was a direct result of human activity?) is still a very nascent field.