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Q10: Can you explain why people don't talk about leaving carbon in the ground? How much of the carbon (in the form of fossil fuels) still unused, needs to be left in the ground to avoid catastrophic global warming?
Submitted by Rod Thorn

A: At present there is no numeric figure decided on what constitutes dangerous interference with the climate system. Current carbon dioxide concentrations have not been exceeded for the past 420,000 years and possibly not for 20 million years. The world has entered new territory as far as carbon dioxide levels go.

Sometime in the next several decades the world has to stop growth in emissions and then bring global concentrations down below current levels. Given population increases and economic growth this is a big task for everyone worldwide.
- Ian Carruthers, Australian Greenhouse Office

A: A feasible target would be to stabilise emissions at 50% more than current levels. It would be tough to do much more than that. In this case the warming would still be significant but may reduce the risk of higher degrees of warming, for example the 1 to 6 degree warming would be reduced to a 1 to 3.5 degree warming.

- Dr Penny Whetton, Leader, Climate Impacts Group and Joint leader, Climate and Weather: Applications and Impacts group, CSIRO Atmospheric Research

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