[Updated 21 July 10.]  One of the most frequently and loudly repeated claims of climate sceptics is that the Earth has cooled since 1998.  What the data have actually been showing is a pause similar to pauses in the 1980s and 1990s.  The likely cause of the recent pause has been the El-Niño-La-Niña cycle.  So far 2010 has been the hottest year on record, so the pause is over.  Let’s see if false claims of global cooling also cease.

The most visited post on this site has been Global Cooling Since 1998? It is time to take an updated look at the question.

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.

James Hansen of NASA argues that their estimate is more realistic because it takes more account of polar regions.  Because the Arctic has been warming rapidly and fairly steadily over recent decades, their data show steadier warming, with 2005 as the hottest rather than 1998 as in the Hadley record.

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 many sceptics fail to note. Furthermore there was a La Niña condition 2007-8, meaning it was expected to be cooler than average, as indeed it was.  The 2009 temperature is back among the warmest ever.

The Recent data

The latest word is the the first part of 2010 has been the warmest ever. The average for January to June was the warmest ever, according to NOAA, 0.7°C above the twentieth century average.  Each of April, May and June 2010 were the warmest April, May and June ever, according to NASA and NOAA. The northern spring (March-April-May) was the warmest ever, 0.73°C above the 1951-1980 mean (NASA).

These numbers may seem small, but if you compare them with the graphs above you will see they are well above previous temperatures.  They indicate that the global mean temperature is jumping to new highs, as it did after the pauses in the early 1980s and early 1990s.

Here is an image that compares the average temperature over the Earth in early 2010 with 2005 and 1998.  The lower right panel also shows NASA’s monthly global averages for these years compared with the all-time record for each month (some of which occurred in other years).

Why has the global cooling claim persisted?

The claim that the world has cooled since 1998 only persisted because either the data were not properly examined and understood, or because some willfully selected only the parts of the data to 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. That’s 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.

The conspiracy theory received a great boost when emails hacked from East Anglia University were released late in 2009.  It was widely claimed the emails revealed conspiracies to select, alter and withhold data.  Three enquiries have now investigated, and all have reported that the scientists involved maintained high scientific integrity.  There are questions remaining concerning freedom of information, but that is a separate question that needs more thorough examination for all scientists, not just those climate scientists.  Also the climate scientists were legitimately concerned that data were being misrepresented by climate sceptics, and that proper peer-review processes had not been followed for some published papers by sceptics.

On the other hand, 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?

Thanks to New Matilda and Fiona Katauskas.

Senator Fielding’s misrepresentations

Late in 2009 Senator Stephen Fielding appeared in front of Australia’s 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.