Appendix Greenhouse gases

It's worth clarifying a technical detail that leads to much confusion. Each greenhouse gas has a different potential to cause climate warming, first because of its molecular structure, which determines just how effective it is in absorbing and emitting heat at different wavelengths, and secondly because each gas has a different atmospheric residence time before being chemically changed or absorbed by the oceans or biosphere.

To measure their relative warming effects, climate scientists convert greenhouse gases other than carbon dioxide (CO2) into a 'carbon dioxide equivalent' (CO2-e). So, for example, in terms of its heat-trapping potential a molecule of methane (CH4) is (over 20 years) 23 times more potent than a molecule of CO2. Together they are measured by the number of parts per million (ppm) of carbon dioxide equivalent (CO2-e) in the atmosphere. In the scientific literature and the policy discussion it is common for people to switch without notice between talking about CO2 concentrations and concentrations of all greenhouse gases, measured in CO2-e. The confusion has even found its way into official documents, yet it is vitally important.

For easy reference, the following table sets out the

Level

CO2 concentration in the atmosphere (ppm)

Corresponding

CO2-e concentration (ppm)

Eventual warming most likely associated with this concentration (°C)

Pre-industrial

280

0°C

Hansen target

350

445a

2.0°C

Current (2009)

387

455b

2.2°C

EU target

380

450

2.0°C

Stern target

450

550

3.0°C

Anderson and Bows' 'optimistic' scenario

530

650

4.0°C

Source: IPCC, 'Summary for Policymakers' in Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, 2007, Table SPM5 and various others.

a. If annual CO2 emissions were reduced to below zero to get to 350 ppm (i.e. net withdrawals from the atmosphere using some form of carbon sequestration) then non-CO2 gases would in all likelihood be lower than indicated.

b. The Stern report notes it is currently (2006) at 430 ppm CO2-e. If account is taken of aerosols and other offsetting factors the net effect of all anthropogenic forcing is estimated to be 375 ppm CO2-e (Stern, The Economics of Climate Change, p. 193).

equivalence levels for CO2 and CO2-e along with the expected levels of warming associated with the levels most commonly discussed. (The temperatures are mean estimates and should be regarded as having considerable uncertainty.) The corresponding levels of CO2-e can only be approximations as they depend on the emissions of non-CO2 gases over the period taken for CO2

to reach the indicated concentrations. If, for example, methane emissions were reduced by more than anticipated then a lower level of CO2-e would be associated with each level of CO2.

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