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 precisely the sort of event that South Carolina’s scientists had in mind. After all, as our very own Capital Weather Gang has noted, this isn’t merely a 1 in 1,000 year event for rainfall totals — in some locations the amount of rainfall “blows NOAA’s 1,000-year events scale out of the water.” And some have already suggested a “probable” climate change connection.
That said, climate scientists debate constantly about how and when to link extreme events to climate change, and the questions involved are anything but simple. Indeed, a recent essay in the journal Science issued a warning 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:
- 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 indeed, 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 earlier 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.”
However, some scientists are cautious about going beyond this relatively basic point and engaging in actual, causal attribution of the event.
“The peer review literature certainly suggests changes in the top 1 percent intense rain events, but I think that it would be speculative to conclude this event is caused by climate change, though I am open to the possibility there could be a link,” says J. Marshall Shepherd, director of the program in atmospheric sciences at the University of Georgia.
“We do know that storm water management systems may not be engineered for this century’s rainstorms,” Shepherd adds.
- The complicated connection to Hurricane Joaquin.
The reason things get complicated is that the rains over South Carolina are a very complex meteorological event with multiple causes, including Hurricane Joaquin (the rains tapped some of its tropical moisture) but also numerous other factors. “At least eight key elements conspired to create a highly efficient, small-scale rain machine centered on South Carolina,” writes Jeff Halverson at Capital Weather Gang.
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.
Here’s how climate scientist Michael Mann of Penn State University puts it:
This is yet another example, like Sandy, or Irene, of weather on “steroids,” another case where climate change worsened the effects of an already extreme meteorological event. In this case, we’re seeing once-in-a-thousand year flooding along the South Carolina coastline as a consequence of the extreme supply of moisture streaming in from hurricane Joaquin. Joaquin intensified over record warm sea surface temperatures in the tropical Atlantic, which both allowed it to intensify rapidly despite adverse wind shear, and which provided it with unusually high levels of moisture — moisture which is now being turned into record rainfall.
Indeed, Mann adds: “There is an exponential relationship between sea surface temperature and the amount of moisture in the atmosphere above it. So record warm temperatures means record amounts of moisture.”
Adds Jennifer Francis, a climate scientist at Rutgers University who has argued that the melting of the Arctic is changing the nature of the northern hemisphere jet stream, which shapes weather patterns:
Recent heavy rains in the Carolinas over the weekend resulted from a deep, slow-moving front that tapped into a wealth of tropical moisture from the Atlantic Ocean. Sea-surface temperatures across the tropical Atlantic and along the U.S. eastern seaboard have been running well above normal….which provided extra evaporation and energy to fuel the frontal system. The entire weather pattern was slow-moving because of blocking high pressure over the N. Atlantic. Is there a connection to climate change? Very possibly, as heavy precipitation events like this one have increased in frequency, particularly in eastern North America. Warming oceans contribute to globally increasing water vapor content in the atmosphere. There is also evidence that very large waves in the jet stream, like the one that caused the slow frontal motion, are also occurring more frequently, perhaps in response to the rapidly warming Arctic. While these topics are still a focus of active research, this flooding event in the Carolinas bears all the hallmarks of expectations for a warming — and moistening — atmosphere.
- In the end, it’s about how much you stress the thermodynamics.
In a recent paper, 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.
Or as the authors put it:
The climate is changing: we have a new normal. The environment in which all weather events occur is not what it used to be. All storms, without exception, are different. Even if most of them look just like the ones we used to have, they are not the same….We argue that under such conditions it is better for event attribution to focus not on the synoptic event, but rather on the influences of the changed large-scale thermodynamic environment on the extremes and temperatures and moisture associated with the event.
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. But on the other hand, that doesn’t mean Trenberth has won over all of his colleagues yet.
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 precisely that.
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