8.2 What are the upper limits of temperature increase and sea level rise?

The increase in global mean surface temperature over the next century depends on the emission scenarios, especially over the next five years and when global emissions peak and the rate at which they then decline. See Section 5 for more detail on future scenarios.

At current emission rates (especially since 2000), an increase of 2°C is unavoidable, and 4°C is probable. The upper end of the IPCC range in their Fourth Assessment Report (2007) was 6.4°C by 2100. The main uncertainties in the future are the economics, technology and rate of development – and hence the emissions pathway – hence the use of IPCC scenarios to explore this complex area and the range of temperature estimates covered by the scenarios.

For the science, the main uncertainty is ‘climate sensitivity’ – the amount of warming for a doubling of atmospheric CO₂ concentrations compared with pre-industrial levels (broadly, from 280 ppm to 560 ppm). The current ‘best estimate’ for this is 3°C (IPCC Fourth Assessment Report), but that has increased in each assessment report (see Section 5.6 – ‘So how much warming is likely?’). There is agreement that climate sensitivity is likely to be linear up to around 2–3°C warming, and that thereafter it is likely to become increasingly non-linear as positive feedbacks kick in (such as reduced uptake of atmospheric CO₂ by terrestrial sinks and the oceans).

Adjusting to warmer than 2°C worlds is likely to be challenging because of the uncertainties associated with the way that complex open systems like climate operate.

Sea level is more difficult to model. Current ‘best estimates’ are in the range 80 cm to 2 m by 2100. Major uncertainties include the rate at which the major ice sheets of Greenland and West/East Antarctica are likely to melt – and much current research is concentrated on understanding the dynamics of ice melting.

• Mark Lynas presents a credible view of possible future climate worlds in his popular science book Six Degrees.

• RealClimate endorses the scientific content of Mark Lynas's book Six Degrees and also discusses the question ‘How much will sea level rise?’.