[An updated version of this article now appears as
Global Cooling Over, Hottest Year to Date So Far, July 15 2010.]
One of the most frequently and loudly repeated claims of climate sceptics is that the Earth has cooled since 1998. The data to not show that.
There are two main groups estimating global mean temperature on a year-by-year basis NASA and the Hadley Centre of the British Meteorological Office. Recent versions of their graphs are shown below.
What do the data really show?
The Hadley estimate does show 1998 as the warmest year, but the NASA estimate shows 2005 as the warmest. For both, the five-year moving average has continued to increase.
When scientists talk about climate change, they are not talking about year-by-year fluctuations, because many factors cause short-term fluctuations. Rather, they are talking about longer-term trends. The five-year moving average smooths out the annual fluctuations and reveals the longer-term trend. That trend is still going up.
One important cause of annual fluctuations is the El Niño effect. 1998 was an El Niño year, the strongest of the twentieth century, so it was expected to be hotter than the trend, as it is seen to have been in both graphs. However there was no El Niño in 1999 and 2000, and they were cooler than the trend, something the sceptics fail to note. Furthermore 2007 was a La Niña year, meaning it was expected to be cooler than average, as indeed it was.
[Note, 23 Jan 2010: the NASA graph has been updated to include 2008 and 2009. The data show the expected effect of the La Niña 0f 2007-8, with 2008 cooler than the trend. The 2009 temperature is back among the warmest ever. The next couple of years may show whether the general warming of the past 30 years or so returns in full strength.]
[Note, 14 Mar 2010: NASA has posted an animated version of this plot that identifies some volcanic eruptions and El Niño events, so you can see their effect on global temperature.]
Why does the claim persist?
The claim that the world has cooled since 1998 can only persist because either the data have not been properly examined and understood, or because some are willfully selecting only the parts of the data that suit their prejudice.
Whereas many sceptics are probably genuine, if ill-informed, it is well known that ExxonMobil has been funding some groups that deliberately cherry-pick and misrepresent the both data and the legitimate scientific debates. They have employed people who formerly worked for tobacco companies to obscure the link between smoking and lung cancer. Their slogan? “Doubt is our product.” They are not interested in the truth, only in obscuring the truth.
They trumpet the one side of the debate and neglect the other side. Over-enthusiastic and under-qualified sceptics (people like Andrew Bolt) pick up their claims and propagate them. More genuine sceptics are misled by unbalanced reporting and claims.
Sceptics who would like to live up to the term need to be careful to ensure they get a reasonably balanced view of the data and debates. Thats not easy. They contradict the great majority of professional scientists at their peril.
A great conspiracy?
One of the most laughable claims is that global warming is a conspiracy concocted by scientists to pad their research grants. Aside from being a slur on a great many honest and hard-working scientists, and reflecting ignorance of the competitiveness of scientists, the amount of money involved in climate research grants is tiny in comparison to other ineterests.
What about the motives of Exxon-Mobil, that has hundreds of billions of dollars annually riding on the global warming question? Are their motives pure as the driven snow?
Addendum, 25 Nov 2009 – Senator Fielding
Senator Stephen Fielding recently appeared in front of Parliament House showing the following diagram (which is on his website).
It purports to show that global temperature isn’t rising. However by including only data from 1995 a quite misleading impression is created. The misperception is exacerbated by the use of monthly data, which show greater fluctuations, rather than the annual data shown at the beginning of this post. Here is the full data set from the Hadley Centre
When the earlier data are visible it is evident that temperatures from 1995 on are clearly higher than earlier temperatures. As I noted above, 1998 was an El Nino year, and expected to be hotter, whereas 2007-8 was a La Nina period and expected to be cooler.
Senator Fielding is not a scientist, so his folly is to have been misled in an elementary way. Reading a graph is not that hard. Nor is it so hard to understand the difference between short-term fluctuations and long-term trends.
Senator Fielding cites several scientists as his sources, the first-named being Professor Bob Carter of James Cook University. Carter is a scientist and should know better. The best one can say is that he has made elementary errors in the interpretation of a straightforward data set. Any competent undergraduate should be able to do better.
For a quite good rundown on popular global warming myths, see New Scientist: Climate Change: A guide for the perplexed.