Less than 3 weeks following the publication of the Garnaut Climate Change Final Review, further examination of the report raises questions regarding the recommended 10% emission cut (by 2020 relative to 2000) and the upper target limit of 550 ppm CO2.

The Review states:

Mitigation for 450 (ppm) costs almost a percentage point more than 550 (ppm) mitigation of the present value of GNP through the 21st century.


The difference in environmental outcome between successful achievement of a 550 ppm objective and of a 450 ppm objective is substantial for Australia.

How “substantial”?

Table 11.1 compares global warming scenarios for 2050in the report of (A) 450 ppm CO2 and 1.6 – 2.1 degrees C rise; (B) 550 ppm CO2 and 1.7 – 2.2 degrees C rise. This implies a rise of only 0.1 degrees C per 100 ppm CO2.

However, even the most conservative estimate of the “climate sensitivity” (the relations between greenhouse gas levels and temperatures, commonly estimated at 3 degrees rise per doubling of CO2), implies a rise by 100 ppm CO2 (from 450 to 550 ppm CO2) will elevate global temperatures by a very minimum of 1 (one) degree Celsius, not 0.1 degree Celsius as in the Report.

Estimates of global temperature rise, which refer to carbon cycle feedbacks, do not appear to take the albedo (reflection) loss effects of ice sheet melt/water-induced remelt, estimated by leading US climate scientists to be responsible for nearly half of global warming.

For a report recommending an upper CO2 limit of 550 ppm, instead of 450 ppm, the neglect of comparing the conditions of the biosphere under the respective scenarios constitutes a major omission. Studies of the recent history of the atmosphere suggest CO2 levels above 350 ppm are dangerous (Hansen et al., 2008). It is once CO2 levels declined below 400 ppm (after 3 million years ago) and the northern hemisphere ice sheets formed, allowing the Earth to cool toward conditions allowing the flourishing of large mammals and humans.

The difference between 450 and 550 ppm is therefore a qualitative difference.

The Report refers in a number of places to “stabilisation” of CO2 levels and the climate. For example:

In the 550 (ppm) scenario, concentrations of greenhouse gases stop rising by around 2060, and after slight overshooting, stabilise around 550 ppm CO2-e, one-third of the level reached in the no-mitigation scenario, by the end of the century.

There is little evidence in the recent history of the atmosphere for “stabilisation”. By contrast, the rise of greenhouse gas levels and advanced ice sheet melt have commonly culminated at tipping points involving several degrees C rise followed by tens of metres of sea level rise.

In his essay “Politics trumps science in Garnaut report” Clive Hamilton juxtaposes the minor economic advantages assumed to arise by allowing CO2 levels to rise to 550 ppm with the calamitous consequences of such a rise.

Nor can confident economic projections be made in isolation from the effects of market crushes such as the current credit collapse, the effects of oil peak, the effects of ongoing warming on agricultural production and water supply and, by distinction, the environmental and economic promise of potentially flourishing alternative energy industries and transport systems.

Peter Fray

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