martes, 14 de agosto de 2012

Study fingers climate change as cause of recent heat waves

By Laura Shin | August 13, 2012, 3:03 AM PDT

It’s long been a tenet of climate science that specific weather events cannot be attributed to climate change.
But a prominent NASA climate scientist and advocate for policies to combat climate change, James E. Hansen, has now come out with a paper that upends that scientific conventional wisdom.
Using a statistical analysis, Dr. Hansen, the head of NASA’s Godard Institute for Space Studies in Manhattan, and his colleagues found that specific heat waves of recent years – the Texas heat wave in 2011, the Russian heat wave in 2010 and the European heat wave of 2003 – were so out of line with what had traditionally been considered natural variability that they must have been directly caused by climate change.
Additionally, he published an op-ed in the Washington Post asserting that once this summer is over, the U.S. heat wave and drought will also likely be found to be a direct result of climate change.
“I don’t want people to be confused by natural variability — the natural changes in weather from day to day and year to year,” Dr. Hansen said in a press release. “We now know that the chances these extreme weather events would have happened naturally — without climate change — is negligible.”
The paper has divided climate scientists for two reasons: One is simply Dr. Hansen’s visible role as an advocate for action to mitigate climate change. The other is the fact that the paper did not rely on climate science to pinpoint climate change as the cause of these heat waves but simply on math and statistics.
Some felt he had come up with a new way to understand climate extremes and others asserted that he had simply put a new spin on old data.

The study

The paper, published in the Proceedings of the National Academy of Sciences, looked at how temperature varies within a season, and how that variability is changing.
Dr. Hansen and his co-authors, who included Makiko Sato, also a NASA scientist at the Columbia University’s Earth Institute, and Reto Ruedy, of Trinnovim LLC, which provides scientific support for NASA, looked at the climate variability of the years from 1951 to 1980, and contrasted that against the variability of the years since.
They compared how much of the earth’s land surface was under what would have been considered extreme heat from June through August of each year.

From 1951 to 1980, only 0.2% of land surface experienced these heat waves.
But in the years 2006 to 2011, 4% to 13% of land surfaces experienced extreme heat.

They concluded that the heat waves during those years would not have occurred were it not for the greenhouse gases warming our planet.
Therefore, the paper did not show, using climate science, how climate change caused each of those events, but instead looked at the likelihood of these events happening without the presence of climate change and decided that climate change was the only plausible cause for them.

Good science? Or spin?

Scientists were split on whether they agreed with the way Dr. Hansen and his co-authors reached his conclusions.
Andrew Weaver, a climate scientist at the University of Victoria in British Columbia told the Associated Press that the study reframes the way we think about extreme weather in relation to climate change:

“Rather than say, ‘Is this because of climate change?’ That’s the wrong question. What you can say is, ‘How likely is this to have occurred with the absence of global warming?’ It’s so extraordinarily unlikely that it has to be due to global warming.”

But not all scientists were so convinced. Peter Stott of the U.K. Met Office, who co-authored a landmark study on the 2003 European heat wave, had previously found that global warming made it much more likely this type of heat wave would occur, but still attributed some of the cause to natural variability.
Other scientists not involved in the study, pointed out that that the increase in heat extremes demonstrates an overall shift toward warmer global average temperatures, not a change in climate variability.
“The one stretch in the paper is in the linking of the increase in areal extremes to an increase in climate variability,” Gavin Schmidt, a climate scientist who works with Dr. Hansen at GISS, told Climate Central.
What do you think? Did Dr. Hansen come up with a new way to understand climate variability or did he just tell a new tale using old data?