In 2013, after some controversy, South Carolina’s Department of Natural Resources released a report on risks the state could face due to climate change. One of those risks? “A predicted result of climate change is the increase in intense storm events causing greater water inputs in shorter periods of time, affecting flood frequency and duration,” the report noted.
Now, with an unfathomable amount of flooding hitting the state, it’s easy to wonder if this is the sort of event that South Carolina’s scientists had in mind. This isn’t merely a 1 in 1,000 year event for rainfall totals. Some have suggested a “probable” climate change connection.
That said, scientists debate about how and when to link extreme events to climate change, and the questions are anything but simple. An essay in the journal Science warned about such connections, noting that “even if it is certain that anthropogenic climate change has caused the frequency of European heat waves to double … the odds that this summer’s European heat wave was caused by anthropogenic climate change are only even.”
In the long term, climate scientists perform statistical studies to calculate whether they can say that a given event was made more likely to occur in a warming climate than in a climate that was not influenced by greenhouse gas emissions. This takes time to perform and requires large numbers of computer model simulations. So we can’t consult such a source yet.
In the absence of such studies, then, what can we say about the South Carolina floods in a climate context? At least three things:
1. In general, more extreme rainfall events are a predicted consequence of a warming climate.
A warmer atmosphere is capable of holding more water vapor - and thus, more rain (or snow, for that matter) is expected in the most extreme precipitation events. And that’s just what has been happening in the United States, according to the 2014 National Climate Assessment.
Thus, you can certainly say for the South Carolina floods - as you can for the 2013 Boulder, Colo., floods, and the Texas and Oklahoma floods this year - that they are consistent with what we would expect in a warming world.
“As the world warms, more water evaporates from the ocean, as well as lakes and rivers,” says Katharine Hayhoe, a climate scientist from Texas Tech University. “That means that, when a hurricane or a storm system comes along, there is on average more water vapor available for it to pick up and dump on us than there would have been 50 or 100 years ago.”
Some scientists are cautious about going beyond this relatively basic point and engaging in actual, causal attribution of the event.
2. The complicated connection to Hurricane Joaquin.
The reason things get complicated is that the rains over South Carolina are a complex meteorological event with multiple causes, including Hurricane Joaquin (the rains tapped some of its tropical moisture) but also numerous other factors.
Still, some scientists think the tropical moisture - partly linked to Joaquin - was key, and moreover, that its presence is tied to warm sea temperatures that, in turn, may have a climate connection.
Climate scientist Michael Mann of Penn State University said: This is yet another example, like Sandy, of another case where climate change worsened the effects of an already extreme meteorological event.
3. In the end, it’s about how much you stress the thermodynamics.
The much-cited climate researcher Kevin Trenberth of the National Center for Atmospheric Research tried to change the paradigm for how we think about the link between climate change and individual weather events - in effect, shifting the burden of proof more onto those who deny such a connection (and away from those who assert one).
Writing with his colleague John Fasullo and Theodore Shepherd, Trenberth argued that while it’s hard to blame a changing climate for particular atmospheric dynamics - the atmospheric “steering” currents that first held Joaquin in in the Bahamas for an extended period, and then sent it out to sea, say - it is clear that the thermodynamic environment has changed for all storm events, because there is more available heat and moisture.
The climate is changing: we have a new normal.”
Trenberth therefore argued that events like Superstorm Sandy, 2013’s Hurricane Haiyan and the devastating Boulder floods all had a climate change component to them. It seems reasonable to assume that the same argument would apply here.
So in sum: The floods were not “caused” by climate change, and the exact meteorological circumstances that caused them to occur are complex. However, the idea that extreme rains are worsening due to climate change is well established - and the rain in this particular event was likely worsened by thermodynamic factors that are tough to separate from a changing climate.
And several scientists are willing to say that.