Comment: Reports the devastating storm was made worse by humanity’s carbon emissions fail to grasp climate change is not just about warming

By Friederike Otto

The unprecedented amount of rainfall accompanying Hurricane Harvey immediately raised the question whether and to what extent climate change is to blame.

In a warming world the vapour capacity of the atmosphere increases, and more extreme rainfall, like Texas is witnessing right now, is to be expected as a result. This leads many to conclude that climate change exacerbated the impacts of hurricane Harvey.

It is very appropriate to highlight that this is the kind of event we expect to see more of in a warming world. However, to apply this argument directly and attribute (and quantify) the impacts from Harvey itself to human-induced climate change, neglects that climate change is not just about warming.

In a changing climate, two effects come together: not only does the atmosphere warm up (thermodynamic effect) but the atmospheric circulation, which determine where, when, and how weather systems develop, can change as well (dynamic effect).

Changes in the atmospheric circulation can increase the thermodynamic effect (as we saw during floods Louisiana in 2016) or act in the opposite direction to the thermodynamic effect, leading to locally decreasing the risk of extreme rainfall or canceling the effect of the warming alone (examples here).

Dynamical factors and thermodynamic aspects of climate change can interact in complex ways and there are many examples where the circulation is as important as the thermodynamics.

Hence, while it is very likely that climate changes played a role in the intensity of the rainfall, it is far from straightforward in practice to quantify this role. As such, determining the role of climate change in increasing or decreasing the present and future likelihood of a rain storm like Harvey presents a challenge.

It is possible that while the magnitude of Harvey’s rainfall could have been increased due to climate change, the overall risk of extreme rainfall like this occurring in a warming world could have decreased or not changed.

Did climate change intensify Hurricane Harvey? @yayitsrob reports with input from Kevin Trenberth: https://t.co/JeyJHBwbU2 — Dana Nuccitelli (@dana1981) August 28, 2017

For individual extreme weather events there are three possible ways climate change can affect the likelihood of the event:

It could increase the likelihood

It could decrease the likelihood

It could have no effect on the likelihood of the event occurring

The latter is not equivalent to climate change not playing a role, because such results are obtained when the effect from a warmer atmosphere is in the opposite direction to the effect on the atmospheric circulation.

To quantify the impact of human-induced climate change on Harvey and to estimate whether it indeed exacerbated the rainfall thus requires taking into account the atmospheric circulation as well as the overall warming.

This is possible and the emerging science of extreme event attribution is doing exactly that. However, while such studies are now routinely conducted for heatwaves and large scale rainfall in mid-latitudes (see Carbon Brief for an overview), it is still the cutting edge for hurricanes.

Attribution science sounds relatively easy in theory: “simulate what is possible weather in a world with climate change and compare it to simulations of possible weather in the world that might have been in a world without anthropogenic climate change”. But in practice this requires climate models that are able to reliably simulate the weather systems in questions over and over again to assess the likelihood of its occurrence. Many simulations are necessary to sample the statistics of extremes events. And to simulate hurricanes very high-resolution models are required that are expensive to run.

If we thus want to know whether Harvey is a “harbinger” for the future of Houston, the attribution question addressing the overall likelihood of a hurricane like Harvey to occur, which includes many variables other than temperature and sea level rise that interact, needs to be answered by carefully estimating the likelihood of such hurricanes developing in a warming world as well as how much rain they bring. It is a question scientists now can answer, but it requires a dedicated study.

Furthermore, attributing the flooding and damages to climate change add more complexity. The answer does not depend on the weather alone but on the land surface, local hydrology, management, … and ultimately who and what is in harms way.

Dr Friederike Otto is a senior researcher at Oxford University’s ECI Global Climate Science Programme and leads and coordinates the distributed computing climate modelling project climateprediction.net.