In 2002, the Bush Administration unveiled its climate policy centered around the goal of reducing greenhouse gas intensity 18 percent by 2012.

“This step will set America on a path to slow the growth of our greenhouse gas emissions and, as science justifies, to stop and then reverse the growth of emissions,” the Bush White House said.

Ten years later (long after most have probably forgotten about it), data indicate the goal has been achieved, at least to an approximate degree.

To reach this conclusion, I analyzed carbon dioxide data from the Department of Energy and data from the Bureau of Economic Analysis .

I found - between the end of 2001 and the end of 2011 - carbon* (the most abundant greenhouse gas emitted in the U.S., 83% of greenhouse gas emissions) intensity dropped by 18 percent almost on the nose.

But let’s take a closer look at this notion of greenhouse gas “intensity” reductions and what they actually signify.

Intensity reductions are not necessarily real emissions reductions. They’re reductions in the ratio of greenhouse gas emissions to economic output, in this case Gross Domestic Product (GDP).

Advocates of the use of the intensity metric say - because it controls for the economy - it’s a true indicator of progress in producing energy more efficiently. Recessions don’t help you, economic booms don’t hurt you.

But critics rightly note reductions in GHG intensity do not necessarily imply absolute reductions in emissions. For absolute emissions reductions, emissions intensity has to decline faster than GDP increases, and considerably faster if emissions are to be reduced dramatically on short time scales.

In an analysis of the Bush policy, the Center for Climate and Energy Solutions (C2ES) noted greenhouse intensity declined 21 percent in the 1980s and 16 percent during the 1990s. But actual emissions went up. In 2000, GHG emissions were 14.1 percent above 1990 levels.

C2ES predicted more of the same when evaluating the Bush plan for 2002-2012. It wrote:

While this would represent a very modest improvement over the “business as usual” emissions projections for 2012 used by the Administration, it appears to continue the same trend of GHG-intensity reductions and GHG emissions increases experienced over the last two decades.

Interestingly though, for carbon dioxide at least (the dominant U.S. greenhouse gas), emissions actually decreased from 5.759 billion metric tons in 2001 to 5.471 billion metric tons in 2011.

Last week, the Associated Press (AP) reported a switch from coal burning to natural gas is the chief driver behind the emissions (and hence emissions intensity) reduction. Rather than directly crediting Bush or Obama administration policies, the AP attributed the switch to “market forces.”

Can natural gas save us from global warming? Wonkblog’s Brad Plumer says not so fast. He notes natural gas production can emit methane, another greenhouse gas (more potent than carbon dioxide, but emitted in much smaller quantities), complicating the question of whether the switch to natural gas can address climate change in the long run. Plumer summarizes the issue accordingly:

It’s still uncertain how big an improvement natural gas is over coal, because of those methane leaks. The good news is that those leaks can be plugged. The bad news is that even if the leaks are plugged, a flood of cheap natural gas isn’t, by itself, enough to prevent the planet from heating up significantly. There’s only so much more carbon the world can emit if it wants to avoid a 2°C rise in global temperatures. Natural gas can help avert drastic global warming, but only if paired with a broader set of efforts to curtail emissions.

* Note: I calculated carbon intensity has declined 18 percent between 2001 and 2011 which is not exactly the same as greenhouse gas intensity. But it’s not far off. According to EPA’s latest greenhouse gas emissions inventory, carbon dioxide represents 83 percent of U.S. emissions. And if you examine its time series of greenhouse gas intensity - see Figure ES-15: along the bottom line - you can eyeball a comparable reduction.