Countering a climate of scepticism

Since the Garnaut review released its findings, climate change deniers have mounted a rearguard action.

Their cry is that Ross Garnaut is no scientist and that he uncritically uses discredited science from the Intergovernmental Panel on Climate Change.

In fact, the Garnaut review relied on the Australian climate science community to make its scientific case. And while Garnaut stands by his economics, the science community stands by its science, particularly research following on from the IPCC's fourth assessment report, released in 2007.

This research was a key input to the Garnaut review, not least the finding that emissions, greenhouse gases, temperature and sea level rise are tracking the upper limits of projections assessed by the IPCC.

Furthermore, projected emissions over the next few decades as well as the risks of severe impacts are higher than previously thought.

Some arguments from the denial community qualify as fatal errors. I will detail two, one relating to statistics and the other to models.

The first argument is that the Earth has not warmed since 1998 and the second is that the models used to project future climate are fatally flawed. My view is that anyone with a higher degree in science who maintains that the Earth stopped warming in 1998 should hand their degree back.

The only way to maintain such a position violates the basic principles of statistics: significance, use of the appropriate test, causality and independence. Free speech it may be, but scientific speak it is not.

Three surface temperature records are widely used by climate scientists. The first two show that 2005 was the warmest year on record, while the third shows that it was not quite as hot as 1998.

According to all three the past decade was the warmest on record, while 12 of the 13 warmest years on record have occurred since 1995. If we take the 31 10-year periods since 1977 - that is, 1977-87, 1978-88, and so on - the 1998-2007 trend that best fits the data is not even the lowest trend in that sequence.

Ten-year trends range from virtually no change to +0.35C a decade, averaging about 0.2C a decade over the whole sequence.

The choice of 1998 as the origin of a recent trend is cherry-picking, especially as the ElNino event that year transferred a record amount of heat from the ocean to the atmosphere.