According to the United Nations, people were responsible for more than 27 billion metric tons of CO2 emissions in 2004. Figure 1 shows a breakdown of global emissions by country for the top 10 emitters in 2004 (and while the data has not been fully assembled, emissions in the developing world have risen substantially since 2004, so the current picture is actually substantially worse than that shown in Figure 1).

This gives us a starting point for looking at how to reduce emissions – and the picture is already complicated. In its Climate Change 2007: Synthesis Report, the IPCC estimated that limiting temperature rise to something between 2.4 and 2.8 °C requires reducing global CO2 emissions by something between 30% and 60% from year 2000 levels (or between about 35% and 65% below 2004 levels) by 2050. For the sake of argument, let’s assume we need to reduce emissions by 50% from 2004 levels by 2050.

But the top 4 countries together account for about 50% of global emissions on their own. So if we eliminated the emissions of the top 4 global emitters (and, by the way, kept the rest of the world from increasing their emissions), we might have a shot at keeping global temperature rise to something in the neighborhood of 2.5 °C. That seems unlikely on the face of it …

But start to dig further in the data and the picture quickly gets even more complicated. Figure 2 shows the per capita emissions by country for the top 10 emitters. The most obvious observation from this figure: the average person in the US is responsible for more than 10 times as much CO2 emissions every year as the average person in India – and more than 5 times as much as those in China. In other words, if every person in the United States cut their emissions by 80%, we’d still emit more on average than the average person in China and more than twice as much as the average person in India. Ask yourself: What’s the likelihood that China and India will buy into any emissions reductions if the United States only commits to reducing emissions by 80%?

More to the point, though: what should global per capita emissions be if we want to reduce total emissions by 50%? To answer than, note that we’ve also added a line for the global average per capita emissions in Figure 2 (which is about 4¼ tonnes of CO2 per year). The first thing to notice is that neither China nor India even emits as much per capita as the global average – and, even so, they account for 23% of global CO2 emissions. But more importantly: if we assume that, in the end, we should expect global average per capita emissions to be similar and we shoot for a 50% reduction, then the U.S. needs to reduce emissions by 90%. China only needs to shoot for a 45% reduction, though, and India’s per capita emissions could actually increase by about 75%.

In a prior post I estimated that the economic cost to the US of achieving a 50% reduction was more than $300 billion per year. It’s safe to assume that the cost to achieve a 90% reduction will be much higher. We’ll take a look at some of the technologies for achieving these reductions in future posts.