How many storms do humans cause? This study has the number

Tuesday, April 28, 2015, 2:38 PM -

Climate change is responsible for a growing amount of extreme weather, according to new research that shows 75 per cent of all extreme heat days and almost 20 per cent of all extreme precipitation events can be traced to global warming. And governments should prepare for this to be the new normal, the researchers say.

"We saw that the number of hot days was increasing, the number of heavy precipitation events was increasing," Prof. Reto Knutti of the Institute for Atmospheric and Climate Science at ETH Zurich told The Weather Network. "By doing that, we could basically define a fraction, in a statistical sense, that was caused by human influence, meaning the burning of fossil fuel that causes warming."

As for extreme precipitation events, Knutti and his colleague E.M. Fischer have a ratio for that too: Around 18 per cent of extreme precipitation events have been due to human-caused warming, based on a temperature rise of 0.85^oC.

That means, in theory, one in five extreme summer weather events such as the Calgary flood, Hurricane Arthur on the East Coast, and the tornado in Barrie, Ont., would have been due to human-caused climate change. Knutti is quick to point out, however, that the findings are an average, rather than identifying which storm was due to climate change and which was not.

To came up with the numbers, the researchers counted the number of record warm days and instances of heavy precipitation and ran the data through a climate model. Then they ran the numbers for a world that was 2^oC warmer.

The result should set off alarm bells. Once we reach that stage, about 40 per cent of extreme weather events will be attributable to human activity.

The findings were published in the latest issue of Nature Climate Change.

"If we reduce CO² emissions quickly, then the 2^oC ... may actually be at the end of the 21st century," Knutti says. "If we continue with high CO² emissions, we may reach 2^oC in maybe 2040 or even earlier."

And Knutti does not mince his words about the human influence on warming, noting numerous studies that solidified the link, along with the stand of the United Nation's Intergovernmental Panel on Climate Change.

"They explicitly say human influence on the climate system is clear, and they explicitly say its extremely likely that most of the observed warming since the mid 20th Century ... is caused by humans," he says. "That has been made clear many times before."

Time to prepare

Knutti's research isn't an idle exercise: He wants to link its results to extreme weather impacts on the ground.

"We can adapt to some of these things -- not all, but some of these things -- by building better infrastructure, by better planning, by better disaster-response management and so on," Knutti says.

For example, he contrasts Hurricane Sandy's hit on New York in 2012 with Hurricane Katrina's devastation of New Orleans in 2005.

While Sandy had a widespread impact on the U.S. northeast, infrastructure was relatively sound and disaster-management measures were well organized. Although costly, the death toll was only in the dozens.

Hurricane Katrina, by contrast, was marked by widespread infrastructure failure and a death toll of more than 1,800.

"Both storms were predicted well by climate scientists, but in the case of Katrina, they didn't evacuate, they weren't prepared, the infrastructure was bad, and ... the response was poor," he says. "By making better decisions, we can improve and be less vulnerable."

SOURCE: Nature Climate Change