Today many governments and scientists agree that the world needs to keep below an average global temperature rise of 2°C (3.4°F) above pre-industrial times (circa 1800).

This takes into account that the world has already warmed by 0.7°C since pre-industrial times. The margin for manoeuvre becomes tighter.

The 2°C threshold is based on the best available science – the third assessment report of the Intergovernmental Panel on Climate Change (IPCC).

This threshold has been accepted by many governments, including the Prime ministers and Presidents of all 25 European Union member states.

The science

Over 2,000 scientists contributed to the IPCC Third Assessment Report published in 2001. The graph below shows at what temperature a warming world starts to become really dangerous.

The red line marks the 2°C threshold and has been added by WWF. It is the red line we must not cross. We need to keep world average temperatures below this mark to avoid catastrophic climate change.

The future is in our hands like never before – if we act now we can turn the tide and avoid dangerous climate change.

The World needs to keep below an average global temperature rise of 2°C in order to prevent the worst impacts of global warming.
© Malte Meinshausen

The difficult question is at what level CO₂ concentrations in the atmosphere will have to peak – or be stabilized - to prevent 2°C being exceeded?

At ‘Avoiding Dangerous Climate Change’, a 3-day conference in the UK held in early 2004, Malte Meinshausen from ETH Zurich and others presented their assessment of this question.

If CO₂ concentrations were to stay below 400ppm (parts per million), staying below 2°C would be likely (probability of exceeding 2°C ranges between 8% and 57%, depending on the model). However, given current energy and power infrastructures, it is very unlikely that we can keep concentrations that low.

A level of 550ppm is very unlikely to keep us below 2°C and could even mean overshooting 4°C. At 475ppm the prospect of staying below 2°C is still rather slim.

If 475ppm was the peak and a rapid decrease followed, by the year 2100, we have at least options to stabilise at a temperature 2°C higher than pre-industrial times.

Note: Current concentrations of CO₂ are at over 380 ppm. These measurements include the effect of other greenhouse gases by attributing global warming values equivalent to CO₂; greenhouse gas warming capacities are expressed in CO₂ equivalents.

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Source:
Meinshausen, M, What does a 2°C target mean for greenhouse gas concentrations? In ‘Avoiding Dangerous Climate Change’, 2006, Cambridge University Press

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