Citizens' Forum on Public Policy

CLIMATE CHANGE

Current reserves of fossil fuels total more than four times the amount of carbon than we can afford to burn into the environment.

 
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THE CARBON BUDGET

Working Out the Carbon Budget

ECOLIMITS

In 1990 the United Nations Advisory Group on Greenhouse Gasses specified the following 'ecolimits for rates and magnitude of temperature and sea-level rise in order to protect both ecosystems and human systems from dangerous climate change:

Global mean temperature:

  • Maximum rate of 0.1 degree C per decade.
  • Maximum increase of 1.0 degree C.
They found that temperature increases beyond 1.0 degree C "may elicit rapid, unpredictable and non-linear responses that could lead to extensive ecosystem damage."

Sea-level rise:

  • Maximum rate of rise 20 mm per decade
  • Maximum 20 cm increase above 1990 levels
A 20 mm limit on sea level rise would "permit the vast majority of vulnerable ecosystems, such as natural wetlands and coral reefs to adapt. Beyond this rate of rise damage to ecosystems will rise rapidly."

CO2 CONCENTRATIONS

The rate and magnitude of global temperature increase are primarily governed by the amount of CO2  or its equivalent concentrated in the atmosphere.

If we continue to burn fossil fuels at current rates, CO2  concentrations will double by the year 2060. According to the Intergovernmental Panel on Climate Change, a doubling of CO2  in the atmosphere could cause temperatures to rise between 1.5 and 4.5 degrees C depending on how easily CO2  in the atmosphere causes global climate to change (climate sensitivity). If the Earth's climate is less sensitive to CO2  (or its equivalent in other gasses), the increase will be at the bottom of this range. If it is more sensitive it will be at the top. Although the 'best guess' used by most governments is 2.5 degrees C, the IPCC have noted that the actual pattern of temperature change best fits an assumption of 3.5 degrees C.

To stay within the ecological limits defined above, taking a climate sensitivity of 3.5 degrees C as a prudent and precautionary approach, levels of CO2 in the atmosphere have to be stabilized at or below 350 parts per million by volume (ppmv). This means bringing them down below current levels.

THE CARBON BUDGET

Given the knowledge that keeping the long-term temperature increase below 1.0 degree C requires stabilization of atmospheric CO2 at 350 ppmv it is possible to calculate a carbon budget giving the total amount of fossil fuels that can be burnt.

Assuming a climate sensitivity of 3.5 degrees C, and making the optimistic assumption that destruction of the world's forests is halted, the total amount of carbon that can be released from the burning of fossil fuels is 225 billion tonnes. Current reserves total more that four times this amount.

If deforestation continues at the present rate the total budget for fossil fuels is even lower--approximately 145 billion tonnes of carbon.

This appeared in The Ecologist, Vol. 29, No 2, March/April 1999, p127

 
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Last Updated May 23, 2001
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