Every year, the Energy Information Administration puts out its forecast of energy trends in the United States. And each year since 2009, the agency has become steadily more optimistic that the country will restrain its carbon-dioxide emissions in the decades ahead.

In its latest forecast, the EIA expects U.S. carbon emissions from all energy sources to stay below 2005 levels through 2040 and beyond. Note how the projections keep pushing downward with each passing year:

Why do the forecasts keep changing? The EIA lists five factors: 

* The outlook for economic growth keeps getting bleaker, which pushes down forecasts for energy growth.
* Fuel-economy standards are improving, people are driving less, and alternatives to gasoline (electric cars, for instance) are starting to appear.
* Oil and coal are expected to get more expensive, which will rein in their use.
* Natural gas, by contrast, is getting cheaper, thanks to the fracking boom. Natural gas generally produces less carbon dioxide than coal when burned for electricity.
* The electricity sector is getting more efficient and people are using more renewables, thanks in part to state-level laws.

So that's good news for climate change, right? Perhaps, but not necessarily. Here are a few of the caveats I'd throw in:

1) The United States is still falling short of its climate pledges. At the moment, EIA says, the country is on pace for a roughly 9 percent cut in emissions by 2020. But let's put that in context: Back in 2009, President Obama pledged to cut U.S. carbon-dioxide emissions 17 percent by 2020.  It will be tough to meet that promise unless Obama uses the EPA to drive even deeper cuts in emissions. Here's one suggestion for doing so.

2) The drop in emissions could prove fleeting if natural gas prices rise. Right now, natural gas is incredibly cheap, thanks to widespread shale-gas drilling in places like Eagle Ford and the Marcellus. That cheap gas is displacing coal and driving down carbon-dioxide emissions. But if low natural gas prices are unsustainable or if shale gas is less plentiful than thought, then coal will make a comeback and emissions will likely rise again.

3) The graph leaves out things like methane emissions. Carbon-dioxide isn't the only planet-warming greenhouse gas. There's also methane, which lingers in the atmosphere for a shorter period than carbon dioxide, but is far, far more effective at trapping heat. And that's a concern when it comes to natural gas, as leaks from drilling and pipelines can put more methane in the air. Now, natural gas can still be superior to coal when methane is taken into account, but it needs to be taken into account.

4) Focusing solely on the drop in U.S. emissions ignores other countries. The United States may be pumping out less carbon-dioxide, but emissions in China are soaring. And the United States may bear part of the responsibility there, by shipping factories overseas and importing the goods. One study in 2010 found that the United States "outsourced" about 11 percent of its emissions this way.

But that's just another way of saying that some sort of global climate treaty will be necessary to tackle climate change. (The recent round of U.N. talks in Doha didn't achieve much.) The drop in U.S. emissions helps, but it's only a tiny first step — and one that could prove fleeting.