30 September 2014

The Devil In The Climate Change Details

Posted by Dan Satterfield


California gets most of it's rain in the winter, but a strong high pressure ridge aloft forced the rain storms well to the north. This was in January 2014, but the ridge persisted for over a year. Is this kind of blocking weather pattern made more likely by rising greenhouse gases?

California gets most of its rain in the winter, but a strong high pressure ridge aloft forced the rain storms well to the north. This was in January 2014, but the ridge persisted for over a year. Is this kind of blocking weather pattern made more likely by rising greenhouse gases?

Greenland is melting, the oceans are warming, the sea is rising (and becoming more acidic), and the Arctic sea ice is in a serious decline (that seems to be faster than predicted). These are all things that those who work in climate science understand and accept. They also accept that they are almost certainly being caused by rising greenhouse gases in our atmosphere. Yes, large portions of the public may not accept it (for various mainly political reasons), but among those who publish in the peer-reviewed journals, there is little to no argument about it.  I find it sadly amusing that the public in general has no real idea about what the real questions are regarding climate science.

I still hear silliness like, global cooling is underway, or it’s the sun! (We’d better hope it isn’t the sun, because no matter what the sun does, carbon dioxide still warms the planet, and if it’s both, we are in double trouble!) Fortunately, the solar output has been very stable, but there is tremendous uncertainty in how all of those changes I mentioned at the beginning of this post are affecting our present weather.

Dr. Jennifer Francis has published work showing a possible connection between the loss of Arctic ice and blocking patterns in the upper atmosphere, and a new paper makes a related claim that the warming planet is making the atmospheric block responsible for the worst drought on record in California, more likely by a factor of three. There are others who doubt the connection has been made and the discussions have been lively. This is the back and forth of science, that will eventually lead to some important answers.

Screen Shot 2014-09-30 at 2.29.58 AMThe American Meteorological Society has published a special supplement this month containing several papers that all ask the same question: was this unusual weather event related to our changing climate. The answer seems to be mixed, with Australias extremes seeming to be likely due to anthropogenic changes, while other events do not. The take-away here is that attributing extreme weather to climate changes is very difficult, and in a way this is the real overlooked news story.

We are pulling the tiger tail of our climate, and we really do not know how it will respond.

The paper on California’s drought is by Noah Diffenbaugh (and others) and the press release by Stanford University about the paper is below:

80059_webPRESS RELEASE: The atmospheric conditions associated with the unprecedented drought currently afflicting California are “very likely” linked to human-caused climate change, Stanford scientists say.

In a new study, a team led by Stanford climate scientist Noah Diffenbaugh used a novel combination of computer simulations and statistical techniques to show that a persistent region of high atmospheric pressure hovering over the Pacific Ocean that diverted storms away from California was much more likely to form in the presence of modern greenhouse gas concentrations.

“Our research finds that extreme atmospheric high pressure in this region—which is strongly linked to unusually low precipitation in California—is much more likely to occur today than prior to the human emission of greenhouse gases that began during the Industrial Revolution in the 1800s,” said Diffenbaugh, associate professor of Environmental Earth System Science at Stanford and a senior fellow at the Stanford Woods Institute for the Environment.

Scientists agree that the immediate cause of the drought is a particularly stubborn “blocking ridge” over the northeastern Pacific –popularly known as the Ridiculously Resilient Ridge, or “Triple R”–that prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.

Blocking ridges occur periodically at temperature latitudes, but the Triple R was exceptional for both its size and longevity. While it dissipated briefly during the summer months of 2013, it returned even stronger by fall 2013 and persisted through much of the winter, which is normally California’s “wet season.” “At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska,” said Swain, who coined the term “ridiculously resilient ridge” last fall to highlight the unusually persistent nature of the offshore blocking ridge.

An important question for scientists and decision-makers has been whether human-caused climate change has influenced the conditions responsible for California’s drought. Given the important role of the Triple R, Diffenbaugh’s team set out to measure the probability of such extreme ridging events. The team first assessed the rarity of the Triple R in the context of the 20th century historical record. They found that the combined persistence and intensity of the Triple R in 2013 was unrivaled by any event since 1948, which is when comprehensive information about the circulation of the atmosphere is first available.

To more directly address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with Bala Rajaratnam, assistant professor of Statistics and of Environmental Earth System Science, and an affiliated faculty of the Woods Institute for the Environment. Rajaratnam and his graduate students Michael Tsiang and Matz Haugen applied advanced statistical techniques to a large suite of climate model simulations. Using the Triple R as a benchmark, the group compared geopotential heights–an atmospheric property related to pressure– between two sets of climate model experiments. One set mirrored the present climate, in which the atmosphere is growing increasingly warm due to human emissions of carbon dioxide and other greenhouse gases. In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution.

The interdisciplinary research team found that the extreme geopotential heights associated with the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate. They also found that such extreme values are consistently tied to unusually low precipitation in California, and the formation of atmospheric ridges over the northeastern Pacific.

 Diffenbaugh added. “This isn’t a projection of 100 years in the future. This is an event that is more extreme than any in the observed record, and our research suggests that global warming is playing a role right now.”

There is growing evidence that if the planet warms another 2 degrees  (Fahrenheit not Celsius!) we will cross a threshold where the uncertainty of the result is much higher. The Devil really is in the details, and the search for them is really fascinating. Have you ever noticed how those who really do not understand much about a subject tend to have strong opinions, and talk a lot while those (like scientists) who know a great deal speak with almost too much caution?

I know why.

The first sign that you really are starting to accumulate a lot of knowledge on any science, is the sudden realization of how much you DO NOT know.