On Thursday, 17 September 2020 at 09:21:00 UTC+1, Spike wrote:

Whenever I see 'HadCrut' I think of Clmategate.

Whenever I see Climategate I think of a scientist suffering from the timewasting tactics of a climate change denier.


More nonsense. When the temperature drops water vapour decreases and CO2 becomes the main greenhouse gas.

So we should be worried about water vapour rather than CO2. At what
level of water vapour does the crossover take place? How does that
relate to temperature?

There is no crossover point. At 0C there is virtually no water vapour (WV) and so the WV greenhouse effec is virtually nil, and the greenhouse effect depends on the CO2 concentration. As the temperature rises the concentration of water vapour increases until its effect overtakes CO2 and eventually it dominates, as it does in the tropics at sea level.

Note we are talking about local surface temperature, not global temperature.. Thus in the tropics, the snow-capped peak of Kilimanjaro is melting primarily because of the effect of increased CO2, not because the region is warming.

At 0C water vapour is insignificant, so over ice sheets CO2 is the main greenhouse gas. That is why the Arctic is warming three times faster than elsewhere. When the Arctic sea ice melts completely water vapour will take over the greenhouse effect and the NH will warm even faster!

Water vapour is part of a positive feedback loop. When the surface temperature rises (because of increased CO2) then more water is evaporated and water vapour increases. This raises the air temperature even further and produces more water vapour. This runaway effect only ends when enough of the water vapour condenses forming clouds which reflect away the incoming solar radiation.

When the Arctic sea ice has melted, water vapour will take over the greenhouse effect and the NH will warm even faster!

To sum it all up, CO2 concentration is not the only thing that affects global temperature. Solar radiation and ithe albedo from ice sheets and clouds play a major role. But the later depend on water vapour which depends on CO2.

HTH, Alastair.