Short-term view reveals the error of warm-earth deniers

• Tim Colebatch
• The Age, July 25, 2008
• http://www.theage.com.au/national/shortterm-view-reveals-the-error-of-warmearth-deniers-20080724-3kls.html

AS THE Liberal Party turns into a battleground between those who believe Australia should do its share to tackle global warming and those who deny that global warming exists, the graph at right is worth seeing.

Will you start reducing your power consumption to prepare for emissions trading?Click here to vote.

It is the raw data on which deniers base their claim that since 1998, the world has stopped warming, and begun to get colder.

The data comes from the UK Meteorological Office and the Climatic Research Unit at the University of East Anglia. It shows how temperatures each year differed from a base period (1961-90). It's one of just three data sources on global temperatures, but the one deniers like to use. Look at it, and you might wonder why. The warming trend during the 20th century is unmistakeable, as is the series of skyscraper readings in recent years.

But critics such as geologist Bob Carter (BusinessDay, 16/7) want us to focus on those recent years. Note that the highest reading was back in 1998. Note that temperatures in 2006 and 2007 were lower than the years before them (and 2008, so far, is sharply lower). What's that telling us?

Carter's readout is blunt: "Earth has not warmed since 1998, and has been cooling since 2002," he argues. "And this, despite an increase in atmospheric CO2 (carbon dioxide) of almost 5%."

But that assumes that annual shifts in temperature or rainfall are a guide to long-term trends. It's like saying that a hot year or two means the earth is warming, or that since 2007 was wetter than 2006, then Australia is getting wetter.

You don't need to be a climatologist to know what's wrong with that logic.

Yes, the weather varies, from month to month, from year to year. It always will. But that tells us nothing about long-term trends.

So climatologists use an 11-year rolling average  shown here as the black line  to define the trend. The line here appears to end at 2002, but only because that's the midpoint of the 11 years from 1997 to 2007. The trend is clearly up.

On its website, Climate Research Unit director Phil Jones sums up what they think the data tells us. "The year 2007 was the eighth warmest on record," he says. "Twelve of the 13 warmest years in the series have now occurred in the past 13 years. The period 2001-07 is 0.21 degrees warmer than the 1991-2000 decade ... the warmest complete decade.

"Analysis of over 400 proxy climate series (from trees, corals, ice cores and historical records) shows that the 1990s is the warmest decade of the millennium, and the 20th century was the warmest century."

And the likely cause? "Increased concentrations of greenhouse gases in the atmosphere due to human activities."