NASA GISS has released the year-end temperature figures for 2008. The numbers are bound to change in the near future, but only slightly, because more data is forthcoming; there’s always a delay receiving all the data from stations worldwide. But enough data has arrived to estimate the temperature anomalies for the year.

Here’s the annual average temperature from 1880 through 2008:

There’s clearly a warming pattern present, especially over the last several decades. Just as clearly, there’s a lot of up-and-down jitter from year to year. The overall pattern is the trend, the jitter is the noise. We can reduce the noise level while preserving the main features of the trend, by computing averages over time spans longer than a single year. These data are already 1-year averages; here are 10-year averages (the last of which has only 9 years, not 10, because 2009 isn’t over yet):

Although taking averages over longer time spans is the simplest way to reduce the noise level and reveal the trend, we can get a better picture by applying a good smoothing method to the data (in this case, a lowess smooth):

Although the smoothed version has more detail, it tells essentially the same story as the 10-year averages. In spite of up-and-down jitter from year to year, over the long haul it’s getting hotter. Clearly the trend is upward. But the year 2008 was below the trend, and cooler than 2007, so last year is only the 9th hottest on record.

We can compute the difference between each yearly value and the smoothed value for the same year. These are the residuals:

It’s evident that the residual for 2008 is negative, but it’s equally evident that the 2008 residual is not at all extraordinary.

Pretty simple, right? Apparently, not to everybody. Realclimate has a post about an episode of “Lou Dobbs Tonight” on CNN, in which Dobbs features an extremely one-sided group of so-called “leading experts.”

Even before the live discussion begins, some comments are reported by videotape. First up to bat is Dennis “Unstoppable Global Warming Every 1500 Years” Avery of the ultraconservative Hudson Institute, and the first words we hear from him are these:

The earth’s temperatures have dropped an average of 0.6 Celsius in the last two years.

In the Realclimate post, NASA’s Gavin Schmidt says rather plainly:

As for his great cherry pick (0.6 ºC in two years - we’re doomed!), this appears to simply be made up. Even putting aside the nonsense of concluding anything from a two-year trend, if you take monthly values and start at the peak value at the height of the last El Niño event of January 2007 and do no actual trend analysis, I can find no data set that gives a drop of 0.6 ºC. Even UAH MSU-LT gives only 0.4 ºC. The issue being not that it hasn’t been cooler this year than last, but why make up numbers? This is purely rhetorical of course, they make up numbers because they don’t care about whether what they say is true or not.

I don’t think Gavin gives Avery enough credit. Credit for obfuscation, that is. Why, I can get more than 0.7 ºC cooling in the last two years, with GISS data! Of course, I’d have to use monthly rather than annual data to emphasize the noise over the trend as much as possible, but then we already know that two years doesn’t really tell us anything useful about the trend. And of course, it can’t really be “the last two years” because if you end with December 2008 it doesn’t happen. But if you go from January 2007 to January 2008 — from a big el Nino to a big la Nina — then you’ll get the required 0.7 ºC cooling:

Could this be the real trend? It uses one whole year of data!!!

No.

Whether Avery just “made up” his 0.6 Celsius, or got it from the kind of wouldn’t-know-the-trend-from-the-noise-if-it-bit-you-on-the-ass cherry-picking I’ve just illustrated, I don’t know. But I do know that his “statistic” is misleading — deliberately misleading — it’s exactly the kind of statement that prompted Benjamin Disrael to say, and Mark Twain to popularize, a famous quote about lies and damned lies.

Now to the title of this post: What if … I did the exact same thing? Suppose I said that over the last 20 years, global temperature has risen by 0.96 ºC, and the even more important fact is that the rate of warming is 6.7 ºC/century. We’re sure to be drowning in molten lava by the end of the century!

Seriously: what would be the reaction in the blogosphere?

Link to Tamino's most excellent blog: http://tamino.wordpress.com/2009/01/15/what-if/