April 10, 2009
Spinning Probabilities in GRL
Cross posted from Prometheus: The Science Policy Blog
Spinning Probabilities in GRL
April 7th, 2009
Posted by: Roger Pielke, Jr.
Earlier this week Andy Revkin pointed to a new article in press with Geophysical Research Letters by David Easterling and Gerald Wehner, which Revkin summarizes as follows:
The paper shows, both in recent records and projections using computer simulations, how utterly normal it is to have decade-long vagaries in temperature, up and down, on the way to a warmer world.
The goal of the GRL paper is to show that the current period of no rise in temperature is, in Revkin's words, "utterly normal," and in the words of Easterling and Wehner, "entirely possible" and "likely." Revkin was duped by the paper, and I suspect many people will be. What the paper is arguing is not that the current period of no-warming is "utterly normal" or "likely" but that such periods of no warming are "likely." The difference is subtle but critical to understand. That such a paper would pass peer-review with such basic confusion and spin is remarkable in some sense, but probably is to be entirely expected.
The confusion can be illustrated as follows. Imagine that you are playing a game of poker, in which you are dealt 5 cards. You've never played poker before so you don't know the odds for a particular hand. You look at your 5 cards and see that you've been dealt two pairs. You then ask your companion, a poker expert, whether or not your hand is "likely" so that you can evaluate it rigorously. Which of the two responses that follow would you consider to be a more straightforward response to your question?
Response #1 You can see over many hands that being dealt two pairs can and likely will occur. In fact, Joe had two pairs in a hand dealt 20 minutes ago and Tim had one 10 minutes before that. And if you simulate a game of poker you'll find a hand dealt 25 minutes from now has 2 pairs and one 7 minutes later also has two pairs. So in conclusion, both observations and simulations show that two pairs can and are even likely to occur. Your hand, therefore is utterly normal and entirely possible.
Response #2 The odds of you being dealt two pair in any given hand in about 1 in 21, so it is a statistically rare event.
To be more explicit, Easterling and Wehner have (purposely?) confused/conflated two questions, both of which are fair to ask. But they are not the same question.
One is, what are the odds of seeing a decadal cooling trend in a long period of warming?
They answer this by saying that:
. . . it is reasonable to expect that the natural variability of the real climate system can and likely will produce multi-year periods of sustained "cooling" or at least periods with no real trend even in the presence of long-term anthropogenic forced warming.
They support this argument by pointing to historical periods where a decadal lack of warming occurred and also to model runs that show similar decadal periods. This is interesting but unremarkable, and certainly not a novel claim (e.g., you can find similar claims on any number of blogs). This observation certainly wouldn't justify publication of this paper.
The second question is, how unusual is it to see the current period of lack of warming?
Easterling and Wehner focus our attention on the current period of warming by introducing the paper as follows:
Anthropogenic climate change is one of the most contentious scientific issues of our time. Not surprisingly the issue has generated numerous blogs and websites with a wide range of views on the subject. According to a number of these sources the climate is no longer warming, in fact, some claim the planet has been "cooling" since 1998.
Although Easterling and Wehner never answer the question explicitly about how rare the current period of observed warming is, they imply throughout that this is the question that they are addressing. The answer to this question is that the current period with a lack of warming is a pretty rare event. How rare? Easterling and Wehner allow us to answer this question by providing a distribution of 10-year trends from a set of climate model realizations:
. . . for the simulations of the entire 21st century there is still about a 5% chance of a negative decadal trend, even in the absence of any simulated volcanic eruptions. If we restrict the period to the first half of the 21st century the probability increases to about 10% revealing that the trend in surface air temperature has its own positive trend in the A2 emissions scenario.
So a negative decadal trend (though not statistically significant, so perhaps better called a period with a lack of warming) is according to the distribution from these models is a 1 in 10 event. In other words, if Easterling and Wehner were asked ten years ago what the odds of seeing a decade of no warming, they would have answered 10%. They further report that a statistically significant (>95%) negative decadal trend is, according to their analysis, a 1 in 100 event based on 20th century observations, and an impossibility in the 21st century, since it is not found in the realizations (Table 1 of their paper). Of course, all of this and much, much more has been done far more rigorously by Lucia Liljegren, but I digress.
So while it is fair to say that the current period of an extended lack of warming certainly does not disprove global warming over the longer term, it is not appropriate to say that such a period is "utterly normal" or, misleadingly, to imply that this specific occurrence is "likely." Given that we are in the midst of a rare event, it is strange to see a peer reviewed paper claim that "misleading" to raise questions about model predictions or to question established theory in such a context. Are such politicized editorial comments the norm now in climate science?
If temperatures cool further or remain without warming for a few years, it could very well be the case that we do see a statistically significant cooling trend over a decade or longer. Then we would get to see the Easterling and Wehner paper cited again, but in that case by skeptics as evidence that global warming has indeed stopped. That argument would be misleading as well.
In the current decade, climate modelers may have gotten unlucky or there may be real issues with predictions from climate models. We don't know the answer to this question. But the "analysis" of Easterling and Wehner gets us no closer to an answer. They do provide some ammunition for the political debate, but little insight to the science. If one wants to perform rigorous comparisons of climate forecasts and observations, there are far more robust approaches than found in GRL this week.