The two major satellite-based estimates of lower-troposphere temperature, from RSS and UAH, have published their December values to complete the year 2010. In both cases the annual average for 2010 ended up a close 2nd to 1998. Some eagerly anticipate the imminent GISS value for surface temperature for December, to complete 2010, as many expect the GISS annual average for 2010 to set a new record high. But we don’t need December’s value from GISS to continue our comparison between different temperature data sets.
In our previous comparison of 3 land-based temperature estimates (GISS, HadCRUT3v, and NCDC) and 2 satellite estimates (RSS and UAH) we discovered that by and large the agreement between data sets is impressive. We also discovered that the satellite data sets differed from the ground-based data in notable ways — in particular, they show greater response to el Nino and to volcanic eruptions. We even noted that different data sets show a different residual annual cycle. And we noted that all five main global temperature data sources show about the same rate of global warming, except UAH which warms slightly more slowly.
A reader asked whether or not it’s possible to remove the el Nino influence from temperature data, since the el Nino response is one of the key differences between satellite and land-based temperature estimates. Of course we can’t remove the el Nino influence perfectly but we can do so approximately, and the same is true for volcanic eruptions and the residual annual cycle. When we remove the influence of these exogenous factors we hope to eliminate some of the variance which obscures what remains, so to bring that into sharper focus: the global warming signal, and other natural variations. Let’s do just that, for GISS, RSS, and UAH. Compare and contrast. Of course our approximation of the influence of exogenous factors is imperfect, but rather than let the “perfect” be the enemy of the “good,” we’ll remove what influence we can and see what’s left over.
For el Nino we’ll use MEI, the multivariate el Nino index, which looks like this
For volcanic forcing we’ll use Ammann et al. 2003, which is a gridded data set so it must be area-weighted and averaged to give a global estimate. To extend the volcanic data to the present, I assumed zero volcanic forcing after the end of the Ammann et al. data (December 1999). The volcanic signal therefore looks like this:
We can approximate the residual annual cycle with a 2nd-order Fourier series, and the global warming signal (since 1975 for GISS, since the beginning of data for RSS and UAH) as a linear trend.
Hence for each temperature data set, we’ll do a multiple regression of the data since 1975 (or whatever we’ve got) as a function of MEI, volcanic forcing, a 2nd-order Fourier series, and a linear time trend. We’ll allow for a time lag in the influence of MEI and volcanic forcing. Then we’ll take the original data and remove the estimated part due to MEI, volcanic forcing, and annual cycle. Finally we’ll put them all on a common baseline, using 1980.0 to 2010.0. This will give us an “adjusted” data set (a name which may give some people fits), one which is adjusted to compensate for el Nino, volcanoes, and annual cycle residue.
For example, here’s the original data for GISS, together with the model fit:
If we remove the el Nino, volcanic, and annual-cycle signals we have what we’ve called the adjusted GISS data:
With much of the variation due to exogenous variables gone, the warming trend is even more evident.
We can likewise compute a model, and adjusted data, for RSS and UAH temperatures, which produces very similar results. We can see just how similar they are by plotting them all on the same graph (click the graph for a larger, clearer view, it’s worth a close look):
In spite of their differences, the most impressive thing is their agreement. It’s also enlightening to compute annual averages for each of the adjusted data sets (the GISS annual average for 2010 is the average of only 11 months, not 12):
The agreement between the different sources is indeed impressive. Even more so, is the inexorable increasing trend in all three temperature series. This is what global warming really looks like, when we clear our view of as much of the clutter as we can. Any talk of a recent “levelling off” or even “cooling” is nonsense.
It may deserve notice that for 2010, the GISS adjusted temperature is lower than either RSS or UAH — so during 2010 it was the satellites that ran hot. With a month yet to report, it’s unlikely that the GISS December value will change that situation.
The models we used for the exogenous factors confirm that the satellite data respond more strongly to both el Nino and volcanos. The satellite response to el Nino is nearly twice as large as the GISS response, while the response to volcanic forcing is about 50% larger for the satellites than for GISS. Eliminating this difference between them has brought them much closer together.
The remaining global warming trend (+/- 2-sigma) is 0.0166 +/- 0.0026 for GISS, 0.0155 +/- 0.003 for RSS, and 0.0133 +/- 0.003 for UAH.
And for those in love with hottest years, all three adjusted data sets rank 2010 as #1, and both GISS and UAH place 2009 in the #2 slot.