Sunday, 6 December 2009

Cumulative CO2 emissions: the 1,000 Gt budget for limiting global warming to 2C

Almost one quarter of the fifty year CO2 budget for the 2C temperature threshold has been used between 2000 and 2008

The probability of exceeding temperature thresholds as global warming progresses can be related to cumulative CO2 emissions - the year on year sum of CO2 emissions over a defined period of time.

Underlying the negotiations at COP15 is the issue of how to share, between nations, a finite and limited budget of CO2 that can be emitted into the atmosphere over the next fifty years.

Meinshausen et al analysed the probability of exceeding a 2C temperature threshold over the period 2000-2050 in relation to cumulative CO2 emissions and concluded that 

'Limiting cumulative CO2 emissions over 2000–50 to 1,000 Gt CO2 yields a 25% probability of  warming exceeding 2C, and a limit of 1,440 Gt CO2 yields a 50% probability'

If the world is to stay within a 2C warming threshold the cumulative CO2 emissions budget to be shared between all nations over the next fifty years is just 1,000 Gt CO2 or an average of 20 Gt per year. World CO2 emissions in 2008 were 31.6 Gt.

If a 50% risk of exceeding 2C warming is considered acceptable then a budget of 1,440 Gt CO2 could be shared out over the next fifty years.

Emissions of CO2 from the countries with the largest emissions over the period 1990-2008 are shown in Figure 20.1. Cumulative CO2 emissions on a plot of CO2 emissions vs. years can be represented by the 'area under the curve'. For each country the total area up to its trend line, read from the base line, represents the cumulative emissions for the period 1990-2008 and the trend lines show how CO2 emissions have changed over time.

        Figure 20.1 CO2 Emissions: Cumulative emissions landscape 1990-2008  

At present the USA has the largest cumulative emissions area for the period 1990-2008 but China's emissions trend, with annual emissions starting to exceed those of the USA in 2008, means that in future China is likely to become the country with both the largest annual and  largest cumulative CO2 emissions.

Figure 20.2 Cumulative CO2 emissions 2000

Figure 20.2 shows contributions to cumulative CO2 emissions from countries with the largest emissions in the shorter period between 2000and 2008 and that world cumulative CO2 emissions for this eight year period were ~250 Gt.

Concerted efforts will be needed to limit warming to 2C

Using Meinshausen's criterion of 1,000 Gt CO2 emitted between 2000 and 2050 corresponding to a 25% probability of staying below 2C, we find that because ~250 Gt CO2 has already been emitted between 2000 and 2008, all countries of the world, during the next forty or so years, should budget to emit no more than ~750 Gt CO2

In relation to the planet's fifty year 1,000 Gt CO2 'budget', that almost 250 Gt has already been emitted in the first eight years is staggering.

On this analysis one quarter of the fifty-year CO2 budget has already been used up in the last eight years, so radical change is needed if the world is not to go beyond the 2C warming threshold.

An unprecedented agreement at the COP15 Copenhagen summit, which sets aside national self interest and leads to concerted efforts from the all the world's nations, will be needed to stay within a global 1,000 Gt CO2  'budget' and limit warming to 2C.


Greenhouse-gas emission targets for limiting global warming to 2°C
Meinshausen M et al.  Nature 458, 1158-1162 (30 April 2009)
BP Statistical Review of World Energy 2009

Note i) One giga tonne Gt = one thousand million tonnes
Note ii) The data used here, from the 2009 BP Statistical Review of World Energy, differ slightly from data used by Meinshausen who took world CO2 emissions for the period 2000-2006 to be 234 Gt CO2.

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