Shifting baseline of global temperature anomalies

Seems like the weather watchers are fawning over the latest update of the UAH MSU satellite data:

June 2009 saw another — albeit small — drop in the global average temperature anomaly, from +0.04 deg. C in May to 0.00 deg. C in June, with the coolest anomaly (-0.03 deg. C) in the Southern Hemisphere. The decadal temperature trend for the period December 1978 through June 2009 remains at +0.13 deg. C per decade.

Let’s run through this one more time, using the UAH NSSTC data over at WoodForTrees:


Above is the 1979 – 2009 dataset.


If we ‘pick’ the 5yr period between 1993 – 1998, things look like they are getting much warmer!


If we ‘carefully select’ the 5yr period between 1988 – 1993, then it’s abundantly clear that global warming doesn’t exist at all, right?

For some incredibly elaborate cherry-picking, take a look at this post by mathematician Luboš Motls making the most of the ‘shifting baseline’ effect (which he chooses to call ‘trends over different intervals‘) of selecting which years to run the analysis:

Global warming is supposed to exist and to be bad. Sometimes, we hear that global warming causes cooling. In this case, global warming causes global averageness. In all three cases, it is bad news. The three main enemies of environmentalism are warm weather, cool weather, and average weather.

To quote WoodForTrees: “What you find can depend on where (or when) you look!”:

Temperature trends – pick a timescale, any timescale! Temperature trend-lines (linear least-squares regression). I hope this is useful, but I would also like to point out that it can be fairly dangerous…

Depending on your preconceptions, by picking your start and end times carefully, you can now ‘prove’ that:


To summarise, here is the WoodForTrees analysis of ALL datasets (with trendlines, and adjusted anomaly baselines), with trends of 0.13-0.17°C/decade, which projects to between 1.3 and 1.7°C per century.


Whether this continues to increase at the same rate remains to be seen, but hawkishly watching the latest data month and saying ‘it’s colder!’ or cherry-picking the data to your own means isn’t going to ‘disprove global warming’. As John blogged the other day, short term declines in global temperature (as illustrated above) are actually predicted by Global Climate Models.