Global warming is the predicted result of increasing atmospheric CO2 causing a very small (~1-2%) decrease in the rate at which the Earth cools to outer space though infrared radiation. And the since temperature change of anything is always the result of net gains and losses of energy, a decrease in energy lost leads to warming.
The direct effect of that warming is only about 1 deg. C in the next 100 years, though (theoretically calculated, in response to an eventual doubling of CO2 late in this century). Climate models instead project 2 to 3 times as much warming as that, due to “positive feedbacks” in the climate system.
But the Earth hasn’t warmed as much as expected by the global warming pundits and their positive feedbacks, especially in the tropics where deep moist convection dominates the atmosphere’s response to forcing.
We know that water vapor is the main atmospheric gas which reduces the Earth’s ability to radiatively cool in the infrared (IR). And, unlike CO2, water vapor varies tremendously due to a variety of processes.
Increasing surface temperatures cause more evaporation which by itself increases the water vapor content of the atmosphere. Water vapor at low altitudes has indeed increased with warming, as I have shown here (over the oceans):
So, the simple-minded assumption has been that warming caused by increasing CO2 would cause more water vapor, which will enhance the radiative warming. That’s called positive water vapor feedback, which roughly doubles the amount of warming from the CO2 increase alone in climate models.
[Yes, I know that more water vapor evaporated from the surface cools the surface…that’s taken into account by the climate models, too.]
But for many years I have advocated the view that water vapor feedback on the long time scales of climate change might not be positive. Clearly, something is causing the current “pause” in global warming. The three most likely causes of the pause (in my view, not prioritized) are: (1) increasing cloud reflection reducing the solar input, or (2) decreasing water vapor (and maybe cirrus clouds) in the upper troposphere increasing the infrared output, or (3) an increase in ocean mixing sequestering extra heat in the deep ocean. Or, some combination of the three. (I’m not a big fan of other theories, like more aerosol reflection of sunlight from dirty Chinese coal, or problems with the CO2 theory itself. Not that they are necessarily wrong.)