Last week I discussed Al Gore’s call for an “immediate freeze” on U.S. carbon dioxide emissions. I dismissed this as being in the realm of fantasy, but the notion of freezing U.S. carbon dioxide emissions motivated me to investigate the issue a bit further. The following data and analyses report what I’ve learned.

Data on projected carbon dioxide emissions is available from the U.S. Energy Information Agency (EIA) here (xls). It is presented in tons of carbon dioxide so it needs to be converted to tons of carbon (divide by 3.664). In 2006 the U.S. is projected to emit 1.63 gigatons of carbon (GtC). This is projected to increase to 2.21 GtC by 2030. EIA projections go to 2030, so that is what I use below.

What accounts for this increase? There are two important factors. One is the projected increase in U.S. population and the other is the projected increase in per capita emissions. I gathered data on projected population increases from the U.S. Census here (xls) and data on projected per capita carbon dioxide emissions here (pdf). I also gathered data on projected immigration from the Congressional Budget Office here (pdf) (note that in the calculations below I use the Social Security Administration’s Intermediate projections). These various data allow the projections to be disaggregated. Here is what I found.

U.S. Carbon Dioxide Emissions 2007-2030

Projected population growth accounts for 64% of the increased U.S. carbon emissions.

Of this growth 40.5% comes from births in the U.S. and 23.5% comes from immigration (i.e., 40.5 + 23.5 = 64).

The remaining 36% comes from an increase in the per capita production of carbon dioxide which EIA estimates to increase by 11% by 2030. (Note that the per capita increase is not included in the above estimates of the effects of population growth. If included they raise the values by about 2.9% — 43.4% — and 1.1% — 24.6% — respectively).

How much carbon emissions are we talking about under an immediate U.S. freeze?

The accumulated U.S. emissions in excess of its 2006 value 2007-2030 equal 7.0 GtC. What would it mean to global carbon emissions if the United States were in fact frozen at their 2006 levels? In 2030 accumulated global emissions would be 228 GtC versus 235 GtC (world data here in xls).

But let’s go further, what if the United States were to become immediately carbon neutral starting in 2007? Through 2030 accumulated global carbon emissions would then be 189.0 GtC. In 2030 global emissions would be 9.72 GtC, or about equivalent to what the world is projected to see with the U.S. under business-as-usual in 2018.

What to conclude from all of this? Here are a few things:

1. The majority of increasing emissions in the United States comes from its population growth. About 37% of this increase (i.e., 23.5/64) is due to emissions from immigration. It is not inaccurate to say that through immigration the United States is “offsetting” the emissions from other parts of the world to some degree, since their net emissions will decrease to to emigration. But it is also true that most (if not all) immigrants are coming to the U.S. from countries with far lower per capita emissions, so there is a net increase in global emissions from immigration to the U.S. Of course, the factors which lead the U.S. to such high emissions in the first place are what drive much of the motivation for immigration. Will policy makers talk of stopping immigration as a climate policy? I doubt it, but it is interesting to consider.

2. Per capita increases in carbon emissions, at 11% by 2030 seem quite small and in principle could be relatively easily addressed through improvements in efficiency. Transfer and adoption of many European practices to the U.S. would I think be more than sufficient to meet an 11% goal.

3. An immediate freeze of U.S. carbon dioxide emissions, even if possible, would have exceedingly little effect on the overall challenge of reducing global carbon emissions. It would reduce the accumulated global total by 2030 by about 3% and delay any projected effects of climate change by about 6 months.

4. If we go to the extreme and assume that the U.S. becomes carbon neutral in 2007, this would have a clearly discernible effect on accumulated emissions but really wouldn’t much change the overall challenge of reducing global carbon emissions. If the U.S. were carbon neutral starting in 2007 then it would reduce the accumulated global total by 2030 by about 20% and delay any of the projected effects of climate change by about 12 years. The relative importance of the U.S. as a contributor of to carbon dioxide emissions is projected to decrease from 21.9% of global annual emissions in 2006 to 18.6% in 2030.

All of the projections above from EIA, Census, SSA, and CBO are made in the face of considerable uncertainties. For example, this recent paper (in PDF and peer-reviewed journal version) suggests that, under some scenarios, demographic factors may in fact lead to a decrease in U.S. per capita emissions as the population ages.

Whatever the future holds, it is clear from this data that while the United States is the largest contributor to carbon dioxide emissions, it nonetheless produces a small share of total global emissions. Given that the majority of its emissions come from its growing population, this places the U.S. at a disadvantage with countries with slower rates of population growth when emissions reductions are accounted on a national basis (discussed here). A continued discussion of climate policy in terms of nations seems to be more divisive than anything else from the standpoint of policy development. Politically, however, the focus on the U.S. does serve a function in both domestic and international politics and in my view goes far beyond the issues related to climate change.