The UnAustralian

Tuesday, May 13, 2003
The IPCC Emission Scenarios

The future effects of global warming are hugely dependent on humanities actions. If we keep on pumping large amounts of greenhouse gases (GHG) into the atmosphere, the warming will be large. If smaller amounts are introduced, the warming will be smaller.

As the IPCC has the job of predicting the effects of global warming, it must know future GHG emissions. Obviously this is impossible to know, so instead they have to settle for a range of likely emissions.

In order to generate a range of realistic emissions, the IPCC assembled a group of experts. Included were people such as Joseph M. Alcamo of the University of Kassel, Douglas McKay of Shell International Petroleum, William Pepper of ICF Consulting, and Nebojsa Nakicenovic of International Institute for Applied Systems Analysis (for a complete list see here). The emission scenarios were carried out by these people, and their work reviewed by an even larger group of experts. The report can be found here.

Initially four scenarios (called A1, A2, B1 and B2) were used to give a range of possible futures. I've described them in more detail here, but in a nutshell, the scenarios with an 1 involve rapid economic and technological growth which spreads to the developing world which leads to a rapid convergence between the developing and developed world. The scenarios with a 2 on the other hand, have a much slower rate of convergence. The world remains very heterogeneous. This leads to lower economic growth, especially in the third world. The B scenarios involve have a greater emphasis of ecologically sustainable technologies relative to the A scenarios. Additional the A1 scenario is divided up into three subgroups. One with a large emphasis on fossil fuels, another with a large emphasis on non-fossil fuels, and a third which is a hybrid of the other two. Each of these scenarios is essentially a story of the how the earth could develop. What will really happen is anyone guess, but chances are, it will fall somewhere inbetween these stories.

Once we have these stories, complex economic models are used to predict future growth, future energy usage, and future GHG emissions.

In order to predict future growth, we need an indicator of growth. The IPCC considered several indicators; Gross Domestic Product (GDP), green GDP, Purchasing Power Parities (PPP), and the Human Development Index (HDI). GDP was chosen as the indicator as it is already the most commonly used indicator in this sort of work.

With this in mind, the Special Report on Emissions Scenarios (SRES - no relation to SARS :) ) projected a range of future rates of GDP growth. I've already listed some of the range of numbers plus historical growth rates here.

The next step is to work out a relationship between energy usage and GDP rates. Fortunately there is a clear historical relationship between energy intensity (the amount of energy per dollar) and GDP (in US dollars). As the SRES state "Global energy intensities diverge across different scenarios, as shown in the shaded wedge in Figure 2-10. The wedge clearly illustrates a persistent inverse relationship between economic development and energy intensity across the wide range of scenarios in the database, despite numerous differences among them". The only real except to this is the Middle East, with it's disproportionally large amounts of cheap energy.

Knowing this relationship between energy intensity and gdp growth, it is easy to calculate future projections of energy use.

The next step is too look at where the energy comes from. Some energy sources such as coal produce large amounts of GHG, others such as wind or nuclear produce far less. Here we must introduce a concept called decarbonisation. Decarbonisation is the process of reducing the amount of carbon dioxide producing fuel sources from our total energy budget. Historically, this has been happening at about a rate of 0.3% per year for the last two centuries. At this stage a number assumptions of future rates of decarbonisation must be made. In some the scenarios (primarily the B's) the rate of decarbonisation is greatly increased. In others, particularly A2 with it's need for large amounts of cheap energy, it is significantly reduced. The range of rates of decarbonisation can be seen here.

With all of this data, it is possible to construct a range of future CO2 (and other GHG emissions) emissions. There is a very large range of CO2 emissions, illustrated here. As can been seen, in some scenarios, CO2 emissions are approx. 10 times 1990 levels, in others they are only a small fraction of 1990 levels. Overall the interventionist scenarios have much lower emissions than the non-interventionist scenarios (however some non-interventionists scenarios still have very low rates of CO2 emissions).
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