by Judith Curry
In today’s Hearing on “Climate Science and EPA’s Greenhouse Gas Regulations,” John Christy and Francis Zwiers both presented testimony that focused on extreme events, climate sensitivity and warming trends.
Their respective testimonies provide starkly different perspectives on these subjects, with Zwiers representing the “convinced” perspective and Christy representing the “skeptical” perspective. By all accounts that I’ve seen in the blogosphere, Christy and Zwiers each did a good job presenting their oral testimony. Lets take a look at their respective arguments and how they were presented in their written testimony.
Francis Zwiers’ written testimony can be found here. Excerpts from Zwiers’ arguments related to extreme events:
Recently we have seen a spate of extreme climate and weather events that have drawn intense media interest, including this winter’s intense storms affecting the US and Canadian eastern seaboard, similarly extreme winter storms last year, the Russian heat wave and Pakistani flooding of summer 2010, the extraordinary Australian flooding event of this past January. These events have certainly tested our ability to cope with weather and climate variations, have had significant negative impacts, and pose the question as to whether human influence on the climate system has played a role. While the research required to answer this question specifically in the context of recent events is yet to be completed, two new papers in Nature (Min et al., 2011; Pall et al., 2011) have presented evidence that changes in the intensity of extreme precipitation since the middle of the 20th century may be linked to human induced global warming, and that in at least in one instance, that human influence on climate had likely substantially increased the risk of flooding.
Changes in extreme temperature and the intensification of extreme precipitation events are natural consequences of a warming climate. A warmer climate would inevitably have more intense warm temperature extremes than the present climate, including longer and more intense heat waves, and less intense cold temperature extremes.
The clear answer to this question, and one that is underscored by the Meehl et al. (2009) study of the occurrence of record breaking temperatures, is that individual extreme events cannot be ascribed to human influence on the climate system in the sense that the event could not have occurred if it were not for human influence. It is, however, possible to assess how human influence on climate may be “loading the weather dice”, making some events more likely, and others less likely.
The weather events that do most damage are very often those that are most difficult to predict: we can, however, assess the impact of an external factor like human influence on climate on the odds of a weather event occurring, even if we cannot predict when it will occur (if you load a dice to double the odds of a six, you still cannot predict precisely the result of any particular roll). Hence the fact that seasonal forecasting of extreme weather is clearly very difficult does not prevent us from assessing the role of long-term drivers in extreme weather risk or attempting to predict seasonal variations in risk.
Any human influence on extreme weather risk combines with these episodic variations and the chance fluctuations that are inevitable when dealing with rare events: hence we should not assume that, if human influence is making a particular event more likely on average, it will necessarily do so every year.
However, the “smoking gun” evidence from these studies suggests that human influence is now affecting the frequency and intensity of high impact events that put people and their livelihoods at risk. While assessments of the abilities of climate models to simulate temperature and precipitation extremes (e.g., Kharin et al., 2007) are sobering, there is a firm physical basis for the expectation that increasing greenhouse gases will intensify warm temperature extremes, moderate cold temperature extremes, and intensify extreme damaging precipitation events.
John Christy’s written testimony can be found here. Some relevant excerpts regarding his comments on extreme events:
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, 184.108.40.206.2.4). 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.
JC’s comments: Apart from the issue of the relevance of Christy’s and Zwiers’ testimony to the issue at hand, which is the EPA CO2 Endangerment ruling. My take on the attribution of extreme events is laid out on several previous threads (see here and here). I don’t agree fully with the statements by either Christy or Zwiers, although my own take on this is much closer to Christy’s.
So what are the Congressmen and their staffers to make of these two different statements on the same topic? The statement from Congressman Inslee is apt: “folks in the press report this like a divorce trial. He said she said.” Apart from the divorce trial issue, the two testimonies were very different in style. Zwiers’ statement was in the style of the IPCC assessment, focusing on conclusions and citations of peer reviewed papers to back them up. Christy focused on making common sense-style arguments (with fewer journal article citations). In the context of Congressional testimony, I suspect that Christy’s style is more effective (independent of the relative strengths of the actual science on each side).
Your take on this?
Moderation note: this is a technical thread, and comments will be moderated for relevance.