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Nasa’s latest press release on climate change says,

“Nasa researcher finds last decade was warmest on record, 2009 was one of warmest years.”

The statement is worth looking at in detail not only because of the scientific data it uses but also because of the way it portrays it. It also reveals a major difference of opinion amongst the most prominent climate researchers. The Nasa researcher referred to is Jim Hansen of the Goddard Spaceflight Centre. The press release was based on a report he wrote a little earlier.

It continues: “A new analysis of global surface temperatures by NASA scientists finds the past year was tied for the second warmest since 1880. In the Southern Hemisphere, 2009 was the warmest year on record.”

As is often the case, one has to take claims like this with reservation. It is not a new analysis and everyone knows already that the last decade has been the warmest. The press release then proceeds to dilute its headline message with some more facts by adding, “The past year was a small fraction of a degree cooler than 2005, the warmest on record, putting 2009 in a virtual tie with a cluster of other years –1998, 2002, 2003, 2006, and 2007 — for the second warmest on record.” In reality this makes the claim that 2009 was the second warmest year specious.

“There’s always interest in the annual temperature numbers and a given year’s ranking, but the ranking often misses the point,” Jim Hansen is quoted as saying, “There’s substantial year-to-year variability of global temperature caused by the tropical El Nino-La Nina cycle. When we average temperature over five or ten years to minimize that variability, we find global warming is continuing unabated.”

But it’s not like that. Ranking of years is the very point and when done reveals that there is no upward trend in the temperature data. If anything the GISS global temperature data set, and the HadCRUT3 one as well, shows that there is not a substantial year-to-year variability. A look at the figures shows that when the errors are taken into consideration there is not much variability as the scatter of means lies within well those errors. Also, despite La Nina – El Nino activity, the data since the two cooler years following the very strong 1998 El Nino shows am impeccable straight line

Gavin Schmidt is also quoted in the press release saying, “The difference between the second and sixth warmest years is trivial because the known uncertainty in the temperature measurement is larger than some of the differences between the warmest years.” This is a statement of the obvious.

Nasa adds, “January 2000 to December 2009 was the warmest decade on record. Looking back to 1880, when modern scientific instrumentation became available to monitor temperatures precisely, a clear warming trend is present, although there was a leveling off between the 1940s and 1970s. In the past three decades, the GISS surface temperature record shows an upward trend of about 0.36 degrees F (0.2 degrees C) per decade. In total, average global temperatures have increased by about 1.5 degrees F (0.8 degrees C) since 1880.”

It is, in my view, misleading to mix the overall warming seen since the Victorian period with the warming seen since 1980 without any qualification. They are highly likely to be due to different causes and one does not support or confirm the other.

Then there is the question about the perceived lack of warming seen in the past ten years.

Jim Hansen writes: Frequently heard fallacies are that “global warming stopped in 1998” or “the world has been getting cooler over the past decade”. These statements appear to be wishful thinking – it would be nice if true, but that is not what the data show. True, the 1998 global temperature jumped far above the previous warmest year in the instrumental record, largely because 1998 was affected by the strongest El Nino of the century. Thus for the following several years the global temperature was lower than in 1998, as expected.

However, the 5-year and 11-year running mean global temperatures  have continued to increase at nearly the same rate as in the past three decades. There is a slight downward tick at the end of the record, but even that may disappear if 2010 is a warm year. Indeed, given the continued growth of greenhouse gases and the underlying global warming trend there is a high likelihood, I would say greater than 50 percent, that 2010 will be the warmest year in the period of instrumental data.

This is an example of how simple averaging can obscure something that is obvious in the data. Most scientists see this. Mojib Latif of the Leibniz Institute of Marine Science says, “There can be no argument about that. We have to face the fact.” Jochem Marotzke, director of the Max Plank Institute for Meteorology adds, “We really don’t know why this stagnation is taking place at the moment.” “I hardly know a colleague who would deny that it hasn’t got warmer in recent years.”

There are some that agree with Jim Hansen. Phil Jones in the notorious leaked emails from the University of East Anglia Climatic Research Unit says, “those idiots saying global warming has stopped.” Although elsewhere in the leaked emails Kevin Trenberth says, “The fact is, we can’t account for the lack of warming at the moment and it’s a travesty that we can’t.”

In summary, it would be fair to say that we live in a warm decade as a result of warming in the 1980’s and 1990’s but it is now incontrovertible that it hasn’t become any warmer in the past decade. I’m surprised there is still debate about this and about the lack of clarity in the Nasa press release.