I'm not a scientist, man, which is why quoting this guy — climatologist and geophysicist Michael Mann — at length is so important.
There are certain climate change-related factors that we can, with great confidence, say worsened the flooding.
Sea level rise attributable to climate change (some is due to coastal subsidence due to human disturbance e.g. oil drilling) is more than half a foot over the past few decades…. That means that the storm surge was a half foot higher than it would have been just decades ago, meaning far more flooding and destruction.
In addition to that, sea surface temperatures in the region have risen about 0.5C (close to 1F) over the past few decades, from roughly 30C (86F) to 30.5C (87F), which contributed to the very warm sea surface temperatures…. That means 3-5% more moisture in the atmosphere.
So Harvey was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage, and a larger storm surge.
Finally, the more tenuous but potentially relevant climate factors: part of what has made Harvey such a devastating storm is the way it has stalled right near the coast…. The stalling is due to very weak prevailing winds which are failing to steer the storm off to sea, allowing it to spin around and wobble back and forth like a top with no direction. This pattern, in turn, is associated with a greatly expanded subtropical high pressure system over much of the U.S. right now, with the jet stream pushed well to the north. This pattern of subtropical expansion is predicted in model simulations of human-caused climate change.
In conclusion, while we cannot say climate change "caused" hurricane Harvey (that is an ill-posed question), we can say that it exacerbate several characteristics of the storm in a way that greatly increased the risk of damage and loss of life.
Climate change worsened the impact of Hurricane Harvey.
As will climate-change deniers.