Historical responsibility for climate impact

If we assume that the climate has been damaged by human activity, and that someone needs to fix it, who should pay? Some people say "the polluter should pay." The preceding pictures showed who's doing the polluting today. But it isn't the rate of CO2 pollution that matters, it's the cumulative total emissions; much of the emitted carbon dioxide (about one third of it) will hang around in the atmosphere for at least 50 or 100 years. If we accept the ethical idea that "the polluter should pay" then we should ask how big is each country's historical footprint. The next picture shows each country's cumulative emissions of CO2, expressed as an average emission rate over the period 1880-2004.

population (billions)

Congratulations, Britain! The UK has made it onto the winners' podium. We may be only an average European country today, but in the table of historical emitters, per capita, we are second only to the USA.

OK, that's enough ethics. What do scientists reckon needs to be done, to avoid a risk of giving the earth a 2 °C temperature rise (2 °C being the rise above which they predict lots of bad consequences)? The consensus is clear. We need to get off our fossil fuel habit, and we need to do so fast. Some countries, including Britain, have committed to at least a 60% reduction in greenhouse-gas emissions by 2050, but it must be emphasized that 60% cuts, radical though they are, are unlikely to cut the mustard. If the world's emissions were gradually reduced by 60% by 2050, climate sci-

entists reckon it's more likely than not that global temperatures will rise by more than 2 ° C. The sort of cuts we need to aim for are shown in figure 1.8. This figure shows two possibly-safe emissions scenarios presented by Baer and Mastrandrea (2006) in a report from the Institute for Public Policy Research. The lower curve assumes that a decline in emissions started in 2007, with total global emissions falling at roughly 5% per year. The upper curve assumes a brief delay in the start of the decline, and a 4% drop per year in global emissions. Both scenarios are believed to offer a modest chance of avoiding a 2 °C temperature rise above the pre-industrial level. In the lower scenario, the chance that the temperature rise will exceed 2 °C is estimated to be 9-26%. In the upper scenario, the chance of exceeding 2 °C is estimated to be 16-43%. These possibly-safe emissions trajectories, by the way, involve significantly sharper reductions in emissions than any of the scenarios presented by the Intergovernmental Panel on Climate Change (IPCC), or by the Stern Review (2007).

These possibly-safe trajectories require global emissions to fall by 70% or 85% by 2050. What would this mean for a country like Britain? If we subscribe to the idea of "contraction and convergence," which means that all countries aim eventually to have equal per-capita emissions, then Britain needs to aim for cuts greater than 85%: it should get down from its current 11 tons of CO2e per year per person to roughly 1 ton per year per o

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Figure 1.8. Global emissions for two scenarios considered by Baer and Mastrandrea, expressed in tons of CO2 per year per person, using a world population of six billion. Both scenarios are believed to offer a modest chance of avoiding a 2 °C temperature rise above the pre-industrial level.

carbon dioxide carbon dioxide

World greenhouse-gas emissions

Agricultural by-products: 12.5%

World greenhouse-gas emissions

Agricultural by-products: 12.5%

Land use, biomass burning: _10%

Figure 1.9. Breakdown of world greenhouse-gas emissions (2000) by cause and by gas. "Energy" includes power stations, industrial processes, transport, fossil fuel processing, and energy-use in buildings. "Land use, biomass burning" means changes in land use, deforestation, and the burning of un-renewed biomass such as peat. "Waste" includes waste disposal and treatment. The sizes indicate the 100-year global warming potential of each source. Source: Emission Database for Global Atmospheric Research.

nitrous oxide

person by 2050. This is such a deep cut, I suggest the best way to think about it is no more fossil fuels.

One last thing about the climate-change motivation: while a range of human activities cause greenhouse-gas emissions, the biggest cause by far is energy use. Some people justify not doing anything about their energy use by excuses such as "methane from burping cows causes more warming than jet travel." Yes, agricultural by-products contributed one eighth of greenhouse-gas emissions in the year 2000. But energy-use contributed three quarters (figure 1.9). The climate change problem is principally an energy problem.

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