The title of section 24 of Ross Garnaut’s final report, released yesterday, is titled “Fateful Decisions”. It states:
Achievement of a comprehensive agreement around a 550 ppm objective would be a step forward of historic dimension. It would bring the next step to 450 closer to reach.
However, the Report also states:
The difference in environmental outcome between successful achievement of a 550 ppm objective and of a 450 ppm objective is substantial for Australia, as demonstrated in chapters 6 and 11 in particular. But it is small compared with the difference between 550 ppm and the complete failure of mitigation.
There is little evidence in the Final Report as to what a world of 550 ppm CO2 with temperatures of 2 to 3 degrees C higher than pre-industrial levels would be like to live in.
The Report appears to assume as if, once 550 ppm CO2 concentration is reached, by manipulating some magic levers global GHG concentrations can be reversed to 450 ppm or any other level.
This is not the case. Unless CO2 draw-down technology is developed, it would take time-scales in the order of centuries to millennia for natural CO2 sequestration to reduce the greenhouse effect to acceptable levels. The Garnaut Report acknowledges:
Large positive climate carbon feedbacks could result from the release of carbon from long-term sinks such as methane stored deep in ocean sediments and in frozen soils as temperatures increase (IPCC 2007a: 642). There is a high level of uncertainty about how the carbon cycle will respond to climate change.
However, the report admits feedbacks are difficult to quantify:
This causal chain does not explicitly include the feedbacks and non-linearities in the climate system that are important in its response to human forcings.
A major conclusion arising from the study of the recent history of the atmosphere is that carbon cycle feedbacks coupled with ice melt/water feedbacks constitute major amplifying mechanisms of initial relatively minor solar and greenhouse triggers (forcings). This observation is central to the view of the acceptability, or otherwise, of a 550 ppm CO2 atmosphere, for the following reasons:
- Based on a climate sensitivity of 3 degrees for a doubling of atmospheric CO2 levels, at 550 ppm CO2 which is twice the pre-industrial level of 280 ppm CO2, mean global temperatures will rise to about 3 degrees Celsius
- A rise of global temperature of 3 degrees Celsius implies sea level rise of about 25 metres +/- 12 metres, as recorded from the mid-Pliocene (3 million years ago) and consistent with sea level rise/temperature relations during glacial terminations
- Temperature rises to 3 degrees Celsius imply widespread desertification of mid-latitudes the agricultural centres of the world
- Natural sequestration of greenhouse gases occurs over time frames of centuries to millennia and no atmospheric mechanism is known which will stabilize CO2 levels over shorter periods
- In terms of the longevity of civilization, allowing CO2 levels to rise further than they already are (387 ppm) would prove to be a unidirectional process
A target of 450 ppm is dangerous, being the atmospheric greenhouse gas level at which the ice sheets began to form in the late Eocene some 34 million years ago.
A target of 550 ppm CO2 is a recipe for disaster.