Recently a call went forth for super-brief "elevator pitches" about global warming. Now, I'm no expert on climate - despite having followed the science, events and drama for more than 20 years. Yet I do think I understand enough to pull out the critical details. Here (gulp) goes my stab at slimming the topic:
Global warming is often called "greenhouse warming", but it is difficult (for me anyway) to visualize the Earth's atmosphere as a humongous greenhouse. Would a better analogy help? I think that the effect is much more like a warming blanket, one that we are rapidly making much thicker.
The atmosphere we live in seems wispy, but is actually pretty substantial. If all this air was turned to a liquid, it would be about 32 feet, 10 meters, thick. Our air also contains a crucial but thin mix of heat-trapping gases, this special mix may be dilute (about 1 part per thousand), but it acts as a very effect blanket that warms our planet up by 33 °C, without this warming our Earth would be a chilly -18 °C on average.
Why do I call this trapping effect a blanket? Because the warming mechanism is similar: on a bed you supply the heat, which then must percolate through the blanket's fluff to the top of the covers and escape to the air. Similarly, on the Earth, sunlight warms the ground and ocean, and this heat has to slowly worm it's way up ten kilometers, to the top of the atmosphere before it can escape to space.
Now we know (because lots of teams have measured) that we've added a whole whack of heat-trapping gasses to the air. As best we can tell, over the last 100 years we've nearly doubled up Earth's heat-trapping blanket, and are firmly on our way to tripling it.
Luckily, the added blanket (or two) won't double-up the heating: we'd all be pretty sunk if the Earth were heading for average temperatures above +40 °C. With blankets one naturally gets diminishing returns; the first one gives the most warming, the second one, not so much. When all the numbers are crunched and really, really carefully checked, they predict Earth on average will warm by at least 2 - 3 °C (with the gasses added to date) and perhaps eventually hit 6 °C when the third blanket goes on.
And that is my "elevator pitch". I hope you find it useful, and ... please be kind.