Global temperatures: magnitudes and trends

One thing I often see in media reporting on climate change is confusion regarding the difference between the magnitude of temperatures for individual years over the past decade (expressed as anomalies) and the trend in temperatures over the past decade.

It is not uncommon, for example, for proponents of global warming to berate climate skeptics for stating that the world has experienced a modest cooling over the last 10 years.  After all, how can that be true when so many years of the last decade have been among the warmest this century? Doesn’t that imply that significant global warming is still occurring? Well, some people certainly think so.

Consider the following excerpt from this article.

Lord Bernard Donoghue will tell the Institute of Travel and Tourism conference in Benidorm: “The travel industry is in danger of being a victim of excessive policy decisions. A lot of the case for global warming has been propaganda by zealots.”

He will say there has been “an absence of global warming in the last decade” and argue: “We need to question the policy [on global warming] because wild policy reactions could devastate our economy.”

In fact, the US National Aeronautics and Space Administration (Nasa) reports that 2000-09 was the warmest decade on record – with 2005 the warmest year yet, and 1998, 2002, 2003, 2006, 2007 and 2009 “in a virtual tie for the second- warmest year on record”.

Notice how that last paragraph, describing the warm temperatures of the last decade, is intended to refute the claims of Lord Bernard Donoghue that there has essentially been no warming over the past 10 years. Later in the article we have this:

Forum for the Future head of futures James Goodman, who launched the industry vision document Tourism 2023 at last year’s Abta Travel Convention, said: “We shouldn’t be nervous of discussing climate change, but it’s a tragedy we’re still having a debate about whether it’s happening.

“The proposition that the world is in a cooling period is reckless. This will be a test of the industry.”

So, is it reckless to suggest that, during the last 10 years, there has been an absence of global warming? Not really. The proposition “5 of the last 10 years have been the warmest this century” is perfectly consistent with the proposition “there has been an absence of global warming over the last 10 years.”  The magnitude of the recent temperatures is simply not relevant when considering the trend.

Let’s look at the HADCRUT3 data for global land and sea surface temperatures.  This is the temperature product produced by the CRU folks at the University of East Anglia (the ones embroiled in the Climategate scandal) and is one of the primary surface temperature data sets used by climate researchers (the others being produced by NCDC and GISS).

Here is a plot of HADCRUT3 temperature anomalies over the last 30 years using the wonderful interactive tools at

I applied some minor smoothing to the curve to make it a little easier to analyze and also added a trend line (also computed by the tools) that shows the trend from 2001 to 2010.   As can bee seen, it does appear that quite a few of the recent years have been the hottest in quite some time (based on their temperature anomalies), but it’s also true that the trend in the temperature anomalies over the last 10 years is actually negative.    Mind you, the slope is not strongly negative, but it certainly doesn’t show significant warming, either, even though the temperatures themselves are warm.

So can we conclude from this that global warming is a hoax, a delusion? No. In fact, if I plot the trend line starting in 2000 instead of 2001, the slope is slightly positive, though not significantly so. Thus it really depends on which year you choose to start. Still, there has not been significant warming in quite some time.

Bottom line: it is perfectly correct to say that there has been no significant warming over the past 10 years.

Stating facts is not “reckless.”

– Dave


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