That excellent question is posed at Watts Up With That. The significance of any story depends largely on when you decide to begin it. Warmists like to talk about the last 100 years or less, but the Earth’s climate has–needless to say–a much longer history than that.
This chart, based on ice cores, shows temperatures over the last 450,000 years. It reminds us that we are lucky to be living during an inter-glacial period. Just the blink of an eye ago, in geologic time–say, 15,000 years–the place where I am now typing was buried under ice a half-mile deep:
If that time scale seems a little too long, let’s look at the Holocene Interglacial, the “warm” interval in which we are now living:
Two points worth noting: 1) Since the end of the last Ice Age, it has been warmer than it is now around 90% of the time. 2) The Earth is still emerging from the Little Ice Age that ended circa 1850.
And if you want to get really micro, this chart from NASA GISS shows no net warming over the last 12 years (other data sets show no warming over the last 17 years):
So, when did global warming begin? There have been many cycles of warming and cooling over the Earth’s history, and even within the brief time that has elapsed during the current interglacial period. At the moment, for better or worse, the Earth is not warming. But one thing we can be absolutely sure of is that at some point in the future the Earth will get warmer; cooler, too.