The world has always had droughts, wild storms and all sorts of weather-related natural disasters. There's also a very real phenomenon in the world right now called human-caused climate change.
So what, if anything, is the link between climate change and Hurricane Harvey? Broadly speaking, there are three ways you might look at that.
The first way is to dismiss the link entirely in provocative terms -- as the conservative pundit Ann Coulter did this week.
I don't believe Hurricane Harvey is God's punishment for Houston electing a lesbian mayor. But that is more credible than "climate change." https://t.co/K7d7mopY5Q— Ann Coulter (@AnnCoulter) 29 de agosto de 2017
Another -- arguably no more helpful -- response is to blame climate change for all weather-related disasters around the world, without direct supporting evidence.
The third way? It's not quite as as sexy as the other two. It doesn't give you a simple three-second news grab that says "yeah, climate change is responsible" or "don't be silly, no it's not".
But it's the sensible middle ground which essentially argues that a warming climate creates conditions which make severe weather events much more likely.
Climate scientist Michael Mann, best known for his so-called "hockey stick" graph which first definitively showed the link between northern hemisphere warming and CO2 emissions, made the link really clear in a Facebook post this week.
"What can we say about the role of climate change in the unprecedented disaster that is unfolding in Houston with Hurricane Harvey?
There are certain climate change-related factors that we can, with great confidence, say worsened the flooding."
Mann went on to detail those factors. They included warmer-than-normal air temperatures, and warmer sea temperatures at both the surface and deeper levels. These made the atmosphere in the region of the storm much more moist than usual.
Throw in weaker prevailing winds (again, caused by a shift in weather patterns thought to be climate change-related), and you had yourself a great big, almost-stalled storm which was like a massive airborne super-saturated sponge.
All that water had to go somewhere -- and go somewhere it did, in the form of unprecedented heavy rain.
As Mann concluded:
"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."
Here in Australia, the Climate Council came out firing.
"The fingerprints of climate change are all over Harvey," it said.
"This is a window into our future. If we don't rapidly reduce our pollution levels now, we can expect worsening extreme weather events."
The Climate Council illustrated the phenomenon of warmer sea temperatures leading to increased rainfall in this diagram in its 2016 report Cranking Up The Intensity: Climate Change And Extreme Weather Events.
As ever with questions of climate science, the website skepticalscience.com is also a good reference point.
In the section entitled Is Extreme Weather Caused By Global Warming?, Skeptical Science has this to say:
Rising temperatures can have several effects on the factors involved in weather. For example:
- They increase the rate of evapotranspiration, which is the total evaporation of water from soil, plants and water bodies. This can have a direct effect on the fequency and intensity of droughts.
- A warmer atmosphere can hold more water vapour. The atmosphere now holds 4% more water vapour than it did 40 years ago as a result of increasing temperatures. This increases the risk of extreme rainfall events.
- Changes in sea-surface temperatures (SSTs) also have an effect by bringing about associated changes in atmospheric circulation and precipitation. This has been implicated in some droughts, particularly in the tropics.
In other words, a wide range of weather-related natural disasters have a super strong link to human-caused global warming. Warming does not automatically make them happen. It just changes the odds to make them more likely.
As Skeptical Science concludes:
"It is equivalent to the loading of dice, leading to one side being heavier, so that a certain outcome becomes more likely. In the context of global warming, this means that rising temperatures increase the odds of extreme events occurring."
Harvey was a disaster. We've seen plenty of disasters like it in the past.
But according to the world's most authoritative climate scientists, a warming world means we'll statistically see them more often in the future, and they'll likely be more intense.