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Scientists use a variety of systems to capture global temperatures. The use of simple thermometers to gauge temperature around the world began in 1850. Weather balloons increased accuracy in the 1950s and today climate scientists rely on a vast number of weather stations around the globe as well as a number of satellites to monitor temperatures. For information before 1850, scientists have had to rely on local reports and on the classic tree ring analysis as well as sophisticated analysis of ice cores from the arctic and antarctic regions.
If you take the daily high and the daily low from all of these measuring devices around the world (both on land and at sea) for an entire century and average them out, the world’s mean temperature is about 14 degrees Celsius.
This little number may help us to understand what an increase of two or three degrees might mean. If you live in the tropics, an increase or decrease of two or three degrees in the daily maximum is barely noticed. At 30 or 32 degrees it’s still damn hot. But if you live somewhere where the average daily maximum is zero, an increase of two or three degrees has impact. For a start, the ice will start to melt!
In percentage terms, the change is also quite different. At 30 degrees, for example, a two degree increase amounts to a six percent change. But two degrees of top of 14 degrees is a relatively large 14.3 percent change.
The Australian Bureau of Meteorology records temperatures around the country all the time. It produced a graph for the final Garnaut Review on climate change that displayed temperature anomalies from 1910 through to the end of 2010. They took as their base the three decades from 1961 to 1990 (a base common with the World Meteorological Organisation), marking the average over that time at zero. Then it logged every year from 1910 against that marker.
Mean temperatures around Australia are remarkably stable given that we are used to daily temperature fluctuations of 10 degrees or more. In fact, only two years over that one hundred year span were more than one degree above or below the zero base – 1918 (below) and 2005 (above). The vast majority have varied by less than 0.5 degrees.
The graph shows that the years from 1910 to 1980 were predominantly below the average. In fact, only 11 years in that 70-year period recorded an average temperature above the zero base. Since 1980 only three years have been below the zero base; 2005 was the hottest at a little more than one degree above the zero base, with 2009 not far behind.
The graph shows the annual anomaly and also averages the anomalies by decade. Not surprisingly, every decade up to 1980 is below the zero base and every one beyond then is above. But the inexorable shift warmer can be seen to begin in the 1970s.
2010 is a curiously anomalous year, demonstrating that focus on one year or one region in one year is likely to be deeply misleading. The WMO shows 2010 to be the hottest on record (remember that accurate records for world temperatures begin in 1850), with 2005 and 1998 close behind. In Australia, 2010 was a mere 0.2 degrees above the zero base. It was one of the cooler years of the 21st century in Australia. This is a strong example of why we should never rely on temperatures in any one region, no matter how large, to make judgements about climate. In this case, it seems, Australia was an anomaly. Other countries will have their turn.
A lot has been said in the debate about 1998. The mean temperature, both in Australia and around the globe, spiked in that year well above the trend. In the years immediately after, the mean returned generally to the trend line. Some people have used that spike year to claim that the world has been cooling since 1998. Well, relative to 1998, the years 1999 through to 2004 were cooler. But one swallow does not make a spring (or summer). Scientists studying a subject quite so broad as climate should not choose to focus on a small group of years. To understand trends, a century is a better idea. A decade is likely to be misleading, let alone four or five or six.
An update on global temperature in 2011 was issued by NASA's Goddard Institute for Space Studies in January 2012.
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