Wednesday, 27 May 2020

Water, water everywhere

From the Arizona Daily Independent:

Water vapor is a powerful greenhouse gas, but its net effect in the atmosphere is to lower temperatures party because of convective heat transfer. Proponents of anthropogenic global warming (AGW) and most IPCC climate models assume the opposite (and that’s why climate model predictions diverge from reality).

AGW hypothesis: Carbon dioxide, a weak greenhouse gas, begins warming the planet. This warming evaporates water and so puts water vapor into the atmosphere which amplifies the warming effect. This is called a positive feedback.

At first look, this proposition seems logical and reasonable. But other properties of water vapor reduce temperatures and the net effect is a strong negative feedback. A positive feedback tends to destabilize a system, whereas, a negative feedback tends to keep a system in check. Just think for a minute, if water vapor had a net positive feedback effect, this planet would have had run-away global warming long ago. That alone should falsify the positive feedback hypothesis. But let’s look at some observational evidence for a negative feedback.

The graphic below compares four pairs of cities, each at about the same latitude so that each pair receives about the same amount of sunlight, and the cities are inland, away from possible tempering by sea breezes. The data is from the National Weather Service (the temperatures have been corrected for elevation differences). The difference between the pairs is that one city is in an arid climate, the other is in a humid climate. We see that the more humid city in each pair has a lower average annual temperature. The addition of water vapor to the atmosphere has a cooling effect in spite of water vapor being a greenhouse gas much more powerful than carbon dioxide.