Judith Curry reports here
judithcurry.com/2011/03/08/extreme-testimony/#more-2637 about John Christy's testimony to the hearings on "Climate Science and EPA’s Greenhouse Gas Regulations" re the incidence of Extreme Weather.
His full written testimony can be found in the link at the end. Here is the part concerning the incidence of extreme weather
......What this means today should be considered a warning – that the climate system has always had within itself the capability of causing devastating events and these will certainly continue with or without human influence. Thus, societies should plan for their infrastructure projects to be able to withstand the worst that we already know has occurred, and to recognize, in such a dynamical system, that even worse events should be expected. In other words, the set of the measured extreme events of the small climate history we have, since about 1880, does not represent the full range of extreme events that the climate system can actually generate.
The most recent 130 years is simply our current era’s small sample of the long history of climate. There will certainly be events in this coming century that exceed the magnitude of extremes measured in the past 130 years in many locations. To put it another way, a large percentage of the worst extremes over the period 1880 to 2100 will occur after 2011 simply by statistical probability without any appeal to human forcing at all. Going further, one would assume that about 10 percent of the record extremes that occur over a thousand-year period ending in 2100 should occur in the 21st century. Are we prepared to deal with events even worse than we’ve seen so far? Spending resources on creating resiliency to these sure-to-come extremes, particularly drought/flood extremes, seems rather prudent to me.
A sample study of why extreme events are poor metrics for global changes. In the examples above, we don’t see alarming increases in extreme events, but we must certainly be ready for more to come as part of nature’s variability. I want to illustrate how one might use extreme events to conclude (improperly I believe) that the weather in the USA is becoming less extreme and/or colder.
For each of the 50 states, there are records kept for the extreme high and low temperatures back to the late 19th century. In examining the years in which these extremes occurred (and depending on how one deals with “repeats” of events) we find about 80 percent of the states recorded their hottest temperature prior to 1955. And, about 60 percent of the states experienced their record cold temperatures prior to that date too. One could conclude, if they were so inclined, that the climate of the US is becoming less extreme because the occurrence of state extremes of hot and cold has diminished dramatically since 1955. Since 100 of anything is a fairly large sample (2 values for each of 50 states), this on the surface seems a reasonable conclusion.
Then, one might look at the more recent record of extremes and learn that no state has achieved a record high temperature in the last 15 years (though one state has tied theirs.) However, five states have observed their all-time record low temperature in these past 15 years (plus one tie.) This includes last month’s record low of 31°F below zero in Oklahoma, breaking their previous record by a rather remarkable 4°F. If one were so inclined, one could conclude that the weather that people worry about (extreme cold) is getting worse in the US. (Note: this lowering of absolute cold temperature records is nowhere forecast in climate model projections, nor is a significant drop in the occurrence of extreme high temperature records.)
I am not using these statistics to prove the weather in the US is becoming less extreme and/or colder. My point is that extreme events are poor metrics to use for detecting climate change. Indeed, because of their rarity (by definition) using extreme events to bolster a claim about any type of climate change (warming or cooling) runs the risk of setting up the classic “non-falsifiable hypothesis.” For example, we were told by the IPCC that “milder winter temperatures will decrease heavy snowstorms” (TAR WG2, 15.2.4.1.2.4). After the winters of 2009-10 and 2010-11, we are told the opposite by advocates of the IPCC position, “Climate Change Makes Major Snowstorms More Likely” (http://www.ucsusa.org/news/press_release/climate-change-makes-snowstorms- more-likely-0506.html).
The non-falsifiable hypotheses works this way, “whatever happens is consistent with my hypothesis.” In other words, there is no event that would “falsify” the hypothesis. As such, these assertions cannot be considered science or in anyway informative since the hypothesis’ fundamental prediction is “anything may happen.” In the example above if winters become milder or they become snowier, the hypothesis stands. This is not science.
As noted above, there are innumerable types of events that can be defined as extreme events – so for the enterprising individual (unencumbered by the scientific method), weather statistics can supply an almost unlimited set of targets in which to discover a “useful” extreme event. Thus, when such an individual observes an unusual event, it may be tempting to define it as a once-for-all extreme metric to “prove” a point about climate change. This works both ways with extremes. If one were prescient enough to have predicted in 1996 that over the next 15 years, five states would break record cold temperatures while zero states would break record high temperatures as evidence for cooling, would that prove CO2 emissions have no impact on climate? No.
Extreme events happen, and their causes are intricately tied to semi-unstable dynamical situations that can occur out of an environment of natural, unforced variability.
Science checks hypotheses (assertions) by testing specific, falsifiable predictions implied by those hypotheses. The predictions are to be made in a manner that, as much as possible, is blind to the data against which the prediction is evaluated. It is the testable predictions from hypotheses, derived from climate model output, that run into trouble. Before going on, the main point here is that extreme events do not lend themselves as being rigorous metrics for convicting human emissions of being guilty of causing them.
Written Statement of John R. Christy
The University of Alabama in Huntsville
Subcommittee on Energy and Power
Committee on Energy and Commerce
8 March 2011
Republicans.EnergyCommerce.house.gov/Media/file/Hearings/Energy/030811/Christy.pdf