It is happening frequently lately. A major weather event occurs---perhaps a hurricane, heat wave, tornado outbreak, drought or snowstorm-- and a chorus of activist groups or media folks either imply or explicitly suggest that the event is the result of human-caused (anthropogenic) global warming. Perhaps the worst offender is the organization www.350.org and their spokesman Bill McKibben. Close behind is Climate Central, which even has an extreme weather/climate blog. The media has noted many times that the U.S. in 2011 experienced a record 14 billion-dollar weather disasters--and many of the articles imply or suggest a connection with human-forced global warming. Even the NY Times has jumped into the fray recently, giving front-page coverage of an unscientific survey that found that a large majority of Americans believe recent extreme weather events are the result of anthropogenic global warming. One does not have to wonder very hard about where Americans are getting their opinions--and it is not from the scientific community.
But what is so disturbing about all this is that there is very little evidence that these claims are true....that the extreme events of late are the result of greenhouse gas increases caused by humans.
Take the recent amazing heat wave in the eastern and central U.S.: canary in the coal mine for global warming? No evidence of this. In fact, an in-depth analysis by Dr. Martin Hoerling of NOAA Earth Systems Research Lab (ESRL), found here, suggest that the heat wave was the result of natural variability and an unusual, but not unprecedented, change in the upper level flow pattern that pushed tropical air northward over the eastern U.S.. A recent discussion of the March warming by UW Professor Michael Wallace, one of the nation's leading climate scientists and a member of the U.S. National Academy of Sciences, found here, reaches a similar conclusion.
Well, what about the extensive tornado outbreaks of 2011 over the southeast U.S. and the early tornadoes of 2012. Unusual extreme weather connected with global warming? There is no reason to believe this is true. Backing for this statement comes from a comprehensive report of the Intergovernmental Panel on Climate Change (IPCC) on extreme weather events, found here. To quote the IPCC report: "There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail."
What about hurricanes? Have we seen an upward trend in those? The IPCC conclusions:
"There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities."
What about extreme temperatures, heavy precipitation, and drought? To aid us in evaluating the trends in these extreme climate events, the National Weather Service has developed a Climate Extremes Index that you can access and plot online.
I tried it out and here is the information for these parameters for 1910-2010:
An important issue that is rarely discussed is that changes in extremes...either natural or from human-forced global warming will not be spatially uniform. So even a strong global warming signal will result in some places getting more extreme weather, while others will get less extreme weather. An obvious example is temperature....if temperatures warm there will be a tendency for more extreme highs and more heat waves. But that also implies that the cold waves will be weaker and less extreme. Ever wonder what is the biggest weather killer in the Northwest U.S.? Not hurricanes or tornadoes, not heat waves or droughts, not windstorms and floods. I am convinced from that statistics I have collected that roadway icing kills and injures more people around here than anything else. And warming should help reduce those deaths and injuries.
And consider that most of the climate models suggest the jet stream will move north under global warming. Big storms and floods are associated with the jet stream. So some folks (on the north side of the current jet stream location) may experience more extreme storminess, but those on the south side could well experience less. There will certainly be losers due to changed extremes under global warming, but there will be a lot of winners as well. Never seem to hear about that.
It is somewhat embarrassing for me to admit this, but part of the problem is that a small minority of my colleagues--people who should know better-- are feeding the extreme-weather/climate hype in the mistaken belief that by doing so they can encourage people to do the right thing--lessen their carbon footprint. Here is an example.
Three final points:
(1) Even if there are changes in the frequency of extremes, that does not necessarily mean human influences are behind them. For example, the earth has been warming for roughly 100-150 years as the planet exited the "Little Ice Age". Much of this warming has undoubtedly been natural, with human-forced warming only really significant during the past 30 years or so. Glaciers have been melting back over the past century and thus some of this loss is undoubtedly due to natural causes.
(2) If we haven't seen trends in extremes that does not mean that we won't see them in the future when the impact of anthropogenic greenhouse warming increases substantially. The earth is only starting to warm up due to mankind's influence on greenhouse gases. The big action...including changes in extremes...is AHEAD of us. Activist types have made a huge mistake in thinking they need to point to observed changes in extremes to make their case for dealing with GW. They are particularly making a mistake when they make claims that have no scientific basis. Global warming skeptics and deniers have made the huge mistake of assuming that a lack of clear changes in the atmosphere during the past decades says something about what will happen in the future, since most of the GW impacts have not yet occurred . Ironically, the activist types are providing the deniers with a potent weapon, since it is pretty easy to disprove many of the activist claims of human-induced global warming enhancing past and current extreme weather.
(3) The media has to do more homework on the claims of GW/extreme weather connections. All too often they simply quote and replay the baseless claims of advocacy groups, or juxtapose stories on extreme weather events and the potential for extremes under global warming...leaving their readers to reach their own, and often incorrect, conclusions. And as a side issue, when is the media going to provide information about some of the nonsense than denier groups are pushing (that global warming is ridiculous because the concentrations of CO2 are so small, that we can't forecast climate if we can't predict weather next week, etc....)
I believe the science is fairly clear...the impacts of global warming due to human-enhanced greenhouse gases will be be very significant, that the effects will increase gradually at first, but then accelerate later in the century. There will be substantial impacts on extremes, but the magnitudes and spatial distributions will be complex, and we don't necessarily have a good handle on it at present.