10 August 2012

NASA Climate Expert James Hansen & An Amazing Forecast

Posted by Dan Satterfield

From NASA and Hansen, Sato, Ruedy in PNAS here.

In 1988 James Hansen published a paper with a prediction that many of his fellow scientists thought was a bit over the top. I want to explain the prediction and then give a summary of the paper he published this week showing it was (almost amazingly) correct.

Imagine that the odds of a cold winter, or hot summer, are represented by a 6 sided dice. Two sides are white for an average (winter)summer, two sides are red for a hot (winter) summer and 2 sides blue for a cold (winter) summer. From 1951-1980, the climate most of us grew up in, the odds were just right.  Hansen predicted, in the 1988 paper, that if greenhouse gases continued to rise, that the climate would warm enough by the first decade of the 21st century to be noticeable. He further predicted that under scenario B of rising greenhouse gases (which turned out to be most correct), that by the first decade of the 21st century, the climate dice would become “loaded” so that there was one blue side, one white, and 4 red sides.

His paper from this week has the following:

“Loading” of the “climate dice” describes the systematic shift of the frequency distribution of temperature anomalies. Hansen et al. (2) represented the climate of 1951-1980 by colored dice with two sides colored red for “hot”, two sides blue for “cold”, and two sides white for near average temperatures. With a normal distribution of temperatures the dividing point would be at 0.43σ to achieve equal (one third) chances of being in each of these three categories in the period of climatology (1951-1980).

A climate model was used (2) to project how the odds would change due to global warming for alternative greenhouse gas scenarios. Scenario B, which had climate forcing that turned out to be very close to reality, led to four of the six dice sides being red early in the 21st century based on global climate model simulations.

Fig. 5  (Below) confirms that the global occurrence of “hot” anomalies (seasonal mean temperature anomaly exceeding +0.43σ) has approximately reached the level of 67% required to make four sides of the dice red, with the odds of either an unusually “cool” season or an “average” season now each approximately corresponding to one side of the six-sided dice. However, the loading of the dice over land area in summer is even stronger

 The August 2012 Paper

Remember when your teacher graded on a curve? He/she would take all the test scores, and the top 10% would get an A and so on. If you plot data like test scores you get a curve that is shaped like a bell. Most of the scores will be B’s and C’s, but there will be a few high scores on the right (from nerds like those you saw on TV this week when Curiosity landed on Mars ;) ), and some low scores on the left from the class flunkies.

From Hansen et.al June–July–August surface temperature anomalies in 1955, 1965, 1975, and in 2006–2011 relative to 1951–1980 mean temperature in units of the local detrended 1981–2010 standard deviation of temperature. Numbers above each map are percent of the area with data covered by each category in the color bar.

Imagine now that you do the same thing for the planet’s temperatures from 1951-1980, and then compare it with the temperatures from the last couple of decades.

What James Hansen and is co-authors found when they did this is the graph at the top of this post. Notice how there are now many more really hot days than there are cold days. Cold summer days still happen but not nearly as many as the warm ones.

Even more, there are now some incredibly hot days that almost never occurred in the 1951-1980 period. In short, if you think of a brutally hot summer as an “A”, then more of Mother Nature’s students are now getting a lot more “A”s ever year, and now a few of them are somehow scoring higher than ever before! (while the number of class dunces has dropped dramatically).

Portion of Fig. 5 from Hansen et. al "Fig. 5. Area covered by temperature anomalies in the categories defined as hot ð> 0.43σ), very hot (>2σ), and extremely hot (>3σ), with analogous divisions for cold anomalies. Anomalies are relative to 1951–1980 base period, with σ also from 1951–1980 data."

Most surprising to me, is that the area of the globe seeing extremely high temperatures has gone from around 1% in the period from 1951-1980 to nearly 10% in the last few years. This is why you keep seeing stories on TV about brutal heat-waves. The image on the right is from Hansen’s paper and this was published coincidentally two days before the NOAA/NCDC made it official that July 2012 was the hottest month on record in the contiguous United States (To save myself from having to answer emails from the conspiracy folks, the paper was mostly complete last winter).

For those that are not used to standard deviations, all data outside the 2 sigma level is 5% of the total data and 3 sigma or 3 standard deviations will cover 99.7% of the data. So a temperature anomaly over 3 sigma is less than 0.3% of the data. That’s three tenths of one percent! Notice how many more high temperatures outside of 3 sigma (of the 1951-1980 bell curve) have occurred in the last decade.

Dr. Hansen writes papers in a very easy to read fashion, so I will let it speak for itself: Probably the most important change is the emergence of a new category of “extremely hot” summers, more than 3σ warmer than climatology. For practical purposes it is important to look at the changes over land areas, where most people live, rather than the global mean for which anomalies are more constrained by the ocean’s thermal inertia. Fig. 6 illustrates that +3σ anomalies practically did not exist in the period of climatology (1951-1980), but in the past several years these extreme anomalies have covered of the order of 10% of the land area…

Seasonal-mean temperatures have changed dramatically in the past three decades. The global shift of the probability distribution for seasonal mean temperature anomalies is more than one standard deviation and the shift is even larger for land areas (Fig. 4). In addition, there is a broadening of the probability distribution, the warming shift being greater at the high temperature tail of the distribution than at the low temperature tail (Fig. 4). Seasonal-mean temperatures in the category defined as “cold” in 1951-1980 climatology (mean temperature below -0.43σ), which occurred about one-third of the time in 1951-1980, still occur with a probability about 10% over land areas.

 

Thus an occasional unusually cool winter is not evidence against global warming. Temperature is less “noisy” in the summer than winter. The chance of summer falling in the “hot” category of 1951-1980 is now about 80% (Fig. 7). The climate dice are now loaded to a degree that the perceptive person (old enough to remember the climate of 1951-1980) should recognize the existence of climate change. The most important change of the climate dice is the appearance of a new category of extremely hot summer anomalies, with mean temperature at least three standard deviations greater than climatology.

 

These extreme temperatures were practically absent in the period of climatology, covering only a few tenths of one percent of the land area, but they have occurred over about 10% of land area in recent years. The increased frequency of these extreme anomalies, by more than an order of magnitude, implies that we can say with a high degree of confidence that events such as the extreme summer heat in the Moscow region in 2010 and Texas in 2011 were a consequence of global warming. Rahmstorf and Coumou (23), using a more elegant mathematical analysis, reached a similar conclusion for the Moscow anomaly.

 While it seems there is no doubt from the data that Hansen’s prediction for the first decade of this century has verified, there will likely be disagreement in the science community over his assertion that the heat waves in Moscow and Texas were a result of climate change. Don’t misunderstand me here, we are not talking about the silliness you see online about volcanoes, sunspots and an untrustworthy temperature record. That’s stuff is political poppy-cock, and no one who knows anything about the real science thinks otherwise. 

Instead, the very difficult question about how much climate change can be attributed to theses individual events is certain to be highly debated in the peer-reviewed literature (which is the only place it really counts). This is how science marches forward and it will be fascinating to watch. Dr. Michael Mann, (who has been the recipient of more death threats from the anti-science extremists than James Hansen I suspect) has supported Hansen’s conclusions. Keith Kloor has a post on The Crux that is quite good regarding this as well.

Dr. Ben Santer at Lawrence Livermore Labs. (you should see the crazy letters he gets!) is an expert on attribution, and it will be interesting to hear his thoughts on it as well. That said, from a guy who spends everyday trying to get the forecast for the next 7 days right, I give Dr. Hansen a deep bow for a steely-eyed rocket man forecast!