Modelled past and expected future changes in concentrations of CO₂, CH₄, and N₂O above pre-industrial (1750) levels.

The model’s prediction of how much the four different country groups have and will contribute to changes in global mean surface temperature as a result of their greenhouse-gas emissions.

What if a polluter-pays principle is applied to global climate change? Greg Bodeker, Sylvia Nichol, and Kim Currie discuss the use of a simple climate model they have developed to answer this and other policy-relevant questions.

The Kyoto Protocol, which controls emissions of a range of greenhouse gases, is now in force. During the protocol’s first commitment period, which runs from 2008 to 2012, parties to the protocol aim to collectively reduce their greenhouse gas emissions to 5% below 1990 levels. The target for New Zealand is to equal 1990 emissions on average during this period. However, this was not the only option considered during the Kyoto Protocol negotiations. For example, Brazil made a proposal to apply a polluter-pays principle. Under this plan, the costs of reducing greenhouse gas emissions would be shared among nations based on their previous contributions to climate change. Over the past few years, NIWA has participated in an international study to investigate the scientific and methodological issues associated with this proposal. The primary tool we’ve used in this study has been a simple climate model which has recently been extended to permit application to a wider range of policy-relevant questions. See ‘How the simple climate model works’.

Applying the model to the Brazilian proposal

We’ve used the simple climate model to calculate regional contributions to past and potential future climate change for four country groups:

• OECD90: Countries that were members of the OECD in 1990. This includes Canada, the USA, most of the countries in Western Europe, Oceania, and Japan.
• REF: Eastern Europe and the former Soviet Union.
• ASIA: India, China, and Southeast Asia.
• ALM: Africa, Latin America, and the Middle East.

We extracted past carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions from a database and combined them with a future greenhouse-gas emissions scenario taken from a special report by the Intergovernmental Panel on Climate Change (IPCC). The resultant changes in the concentrations of these three gases, calculated using the simple climate model, are shown in the first graph. Before 1890, there are insufficient data on historical greenhouse gas emissions to say which country was responsible for the emissions; these emissions (shown in dark blue) are not ascribed to any country group. Note the different rates at which concentrations from these unattributed emissions decay with time in accordance with the different atmospheric lifetimes of the gases.

Our simple climate model suggests increases in global mean surface temperature of 0.2 °C by 1900, 0.8 °C by today, 1.8 °C by 2050, and 4.5 °C by the end of this century. Of course, these estimates depend on the scenario that is used to estimate future emissions of greenhouse gases.

The resulting contributions of the different country groups to these temperature changes are shown in the second graph. In 1900 the OECD90 and ASIA country groups contributed about equally to the small change in global mean surface temperature that had occurred then. This would have resulted from the large emissions of CH₄ from rice cultivation in Asia. However, by 1940 increased industrialisation and associated increases in CO₂ emissions from OECD countries result in OECD90 contributing 48% to the change in global mean surface temperature, with smaller contributions from ASIA (27%), ALM (19%), and REF (6%). By 2006, developing countries catch up somewhat, particularly REF, whose emissions now contribute 15% to global mean surface temperature change. By the end of the century, if the greenhouse gas emissions scenario we have used for future emissions occurs, the contribution of OECD90 decreases to 29%, with larger contributions from ASIA (34%), similar contributions from ALM (26%), and smaller contributions from REF (11%).

From this simple climate model study we can conclude that, while greenhouse gas emissions from developed countries have made a significant contribution to increases in global mean surface temperature observed to date, over the coming decades the contribution from developing countries is likely to increase.

Other applications and recent developments

We’ve used the simple climate model to address other interesting questions about climate–change policy, such as issues related to a per capita greenhouse-gas emissions quota for each country, alternatives to the ‘global warming potential’ (GWP) metric which is currently used to equate non–CO₂ emission to CO₂ emissions (the GWP for methane is 23), and to estimating future greenhouse gas emissions profiles needed to constrain changes in global mean surface temperature below a prescribed threshold.

Recently, we’ve extended the simple climate model to improve its treatment of carbon cycling within the ocean. This would permit, for example, studies of the effectiveness of actively pumping CO₂ into the deep ocean as a means of reducing its atmospheric concentrations. With ongoing development of this model, we expect that it will be applicable to an ever widening spectrum of climate-change policy studies.

Dr Greg Bodeker studies atmospheric processes and is based at NIWA in Lauder. Sylvia Nichol is at NIWA in Wellington, where she studies atmospheric emissions. Dr Kim Currie, based at NIWA in Dunedin, focuses her work on ocean–atmosphere interactions.