Storm Harvey: impacts likely worsened due to global warming
Tropical storm "Harvey" is causing severe flooding in Texas. Because many people are asking if there is a link to climate change, the Potsdam Institute for Climate Impact Research issued a statement by its Earth System Analysis Co-Chair Stefan Rahmstorf. "Harvey" was not caused by climate change, he says. Yet its impacts – the storm surge, and especially the extreme rainfall – very likely worsened due to human-caused global warming.
"First: climate change is not the cause of tropical storm Harvey," . Asking for a single cause of a specific weather event usually doesn’t make sense since there’s always a number of factors at play. What can be said is whether and how climate change affects the character of some extreme events. Scientific findings from the past decades offer some insights here.
A logical consequence of global warming is a global increase of extreme rainfall events – in the case of Harvey, it is in fact the heavy rain and the resulting flooding which is probably the greatest threat. The increase of extreme precipitation is due to the fact that warmer air can hold more water vapour. There’s a physical law for this, the Clausius-Clapeyron equation. A global increase of daily rainfall records is indeed seen in the rainfall observations, as has been shown in a PIK study (Lehmann et al, 2015). This trend will continue as long as we keep pushing up global temperatures by emitting greenhouse gases.
More tentative, yet quite possibly also relevant, is a general slowdown of atmospheric summer circulation in the mid-latitudes (Coumou et al, 2015). This is a consequence of the disproportionally strong warming in the Arctic; it can make weather systems move less and stay longer in a given location – which can significantly enhance the impacts of rainfall extremes, just like we’re sadly witnessing in Houston.
We do not expect a change of the overall frequency of tropical storms, and so far we do not observe a significant change in this regard.
In contrast, we expect from theory and models a change in intensity – the strongest tropical storms could become even stronger due to increasing sea surface temperatures, because this is where these storms get their energy from. That’s the reason they develop only above water being at least 26 degrees Celsius warm. It is under debate whether observational data support the theoretical expectation of an increase in the strongest storms, since these are rare and the observational data series therefore need to be very long and homogeneous to be conclusive.
Rising seas are a clear result of global warming, causing ocean levels to increase by roughly 20 cm since the late 19th century. Thermal expansion of sea water, melting mountain glaciers and mass loss in Greenland and Antarctica are the main contributors. Higher sea levels worsen the effects of storm surges. Until the end of our century, we expect sea levels to rise by up to one meter, possibly even more."