Hurricane Harvey: The link to climate change

When it comes to the causes of Hurricane Harvey, climate change is not a smoking gun.

However, there are a few spent cartridge cases marked global warming in the immediate vicinity.

Hurricanes are complex, naturally occurring beasts - extremely difficult to predict, with or without the backdrop of rising global temperatures.

The scientific reality of attributing a role to climate change in worsening the impact of hurricanes is also hard to tease out simply because these are fairly rare events and there is not a huge amount of historical data.

But there are some things that we can say with a good deal of certainty.

There's a well-established physical law, the Clausius-Clapeyron equation, that says that a hotter atmosphere holds more moisture.

For every extra degree Celsius in warming, the atmosphere can hold 7% more water. This tends to make rainfall events even more extreme when they occur.

Another element that we can mention with some confidence is the temperature of the seas.

"The waters of the Gulf of Mexico are about 1.5 degrees warmer above what they were from 1980-2010," Sir Brian Hoskins from the Grantham Institute for Climate Change told BBC Radio 4's Today programme.

"That is very significant because it means the potential for a stronger storm is there, and the contribution of global warming to the warmer waters in the Gulf, it's almost inevitable that there was a contribution to that."

Researchers are also quite confident in linking the intensity of the rainfall that is still falling in the Houston area to climate change.

"This is the type of event, in terms of the extreme rainfall, that we would expect to see more of in a warming climate," Dr Friederike Otto from the University of Oxford told BBC News.

Analysis - Roger Harrabin, Environment analyst

Environmental lawyers are questioning whether events like Harvey should still be referred to as "Acts of God" or "Natural Disasters" as they are made worse by emissions from fossil fuels.

In a comment paper in the journal Nature Geoscience, they say legal action may be taken against countries that don't contribute to the global effort to cut emissions.

Lawsuits seeking to apportion responsibility for climatic events have generally failed in the past.

But lawyers from the firms Client Earth in London and Earth and Water Law in Washington say that's likely to change.

They believe a new branch of knowledge called attribution science will allow the courts to decide with reasonable confidence that individual events have been exacerbated by manmade climate change.

They believe in future governments and firms risk being successfully sued if they don't cut their emissions.

"For the intensity of the rainfall (over Houston), it is very reasonable to assume there is a signal from climate change in that intensity."

One big question, though, is the persistence of the storm over the Texas area. This has been key to the scale of the downpour and the amount of flooding that has been seen so far.

Some researchers believe that climate is playing a role here too.

Prof Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research says that a general slowdown in atmospheric circulation in mid-latitudes is a possible follow-on from a changing climate elsewhere in the world.

"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."

However, slow-moving storms over Texas have appeared before. Tropical storms Claudette in 1979 and Allison in 2001 had huge rainfall impacts as they settled in place over the state for long periods. Other scientists think that attributing the slowly meandering nature of this storm to climate change is a step too far.

"I don't think we should speculate on these more difficult and complex links like melting in the Arctic without looking into these effects in a dedicated study," said Dr Otto.

Experts say that in looking at a storm like Harvey, the impact of climate change is not simply about higher temperatures in the atmosphere and in the seas - it is also linked to changes in atmospheric circulation patterns.

Sometimes, the temperature and circulation changes brought about by warming can cancel each other out. Other times they can make the impacts worse. Understanding the full picture will be difficult and expensive.

"For hurricanes, we would ask the question as to what are the possible hurricane developments in the world we live in and compare that to the possible hurricane developments in a world without climate change," said Dr Otto.

"These high-resolution models are very expensive to run over and over again so that you can simulate possible weather rather than tracks of hurricanes."

Other researchers say that we are looking at the issue entirely the wrong way.

Regardless of the human impact on climate change, indirectly making Harvey worse - they believe the real human contribution to the catastrophe is far more simple and straightforward.

"The hurricane is just a storm, it is not the disaster," said Dr Ilan Kelman, at the Institute for Risk and Disaster Reduction and Institute for Global Health at University College London.

"The disaster is the fact that Houston population has increased by 40% since 1990. The disaster is the fact that many people were too poor to afford insurance or evacuate.

"Climate change did not make people build along a vulnerable coastline so the disaster itself is our choice and is not linked to climate change."

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