The decrease is due mainly to fracking and the shale gas boom. There is so much cheap natural gas out there that utilities are switching power generation from coal to gas. It's a win for utilities, win for consumers, and win for the environment! If only it were that simple. For one thing, any change in the relative price of coal vs. natural gas can reverse the "progress" on emissions:
But there is a more serious problem. Let's suppose for now that fracking releases so much gas that the world stops burning coal, switching to gas instead. Heck, let's suppose that we stop burning oil as well, and convert all our fossil fuel use to gas! And let's imagine that this conversion will reduce the world's carbon footprint by 50%. Where does that get us?
Well... if the world was hurtling toward a cliff at 100mph, now we're speeding toward that very same cliff at 50mph. It means that all the climate-related problems we will be facing will take twice as long to materialize. That would certainly be good news.
But it would not really solve the problem. Humanity is currently releasing huge amounts of CO2 into the atmosphere. When similar amounts of CO2 were released "suddenly" in the past, bad things happened... 10 degrees (F) of warming, mass extinctions, long-term changes in the course of the history of life. Look, for example, at the end-Permian and end-Triassic extinctions. The extinction issues will likely persist long after all the CO2 we've released into the atmosphere gets mopped up into the rocks due to the long-term carbon cycle.
The problem here is that both of the above extinction events were the result of "sudden" releases of CO2. How sudden? Paleontologists think about a million years. Which is 10,000 times slower than we're releasing CO2 right now. It doesn't matter if we release that much CO2 over 100 years or 200 years, the end result will be the same. All that would have to change is Jim Hansen would have to change the title of his book from "Storms of my Grandchildren" to "Storms of my Great-Great-Grandchildren."
So let's quit the charade of how natural gas and fracking will save us. It will only ever get us halfway there. Instead, let's focus on REAL solutions to the climate problem, which require zero-carbon technologies. The best that natural gas will ever buy us is a little bit of time.
[By the same token, one could consider one of my favorite technologies, the electric bicycle. Suppose we stop driving cars and drive electric bikes instead. These use dramatically less fuel than automobiles, about 10% that of a Nissan Leaf. Heck, let's suppose we cut ALL our fossil fuel use by 90%. Would we still need to worry about climate change? Unfortunately, yes. Now the catastrophe that would have taken 100 years will take 1000 years to unfold. Maybe that's OK for some people, although the mass extinctions would still happen similarly in either case.]