For decades we’ve been told that net cloud radiative forcing is positive. This means that the the amount by which clouds cool the surface, by reflecting solar radiation back to space, is outweighed by the amount that clouds warm the surface, by re-radiating surface emitted IR back towards the ground. So cloud increase equals warmer surface See e.g IPCC AR5 on the subject:
The net radiative feedback is the sum of the warming and cooling feedbacks; the executive summary states “The sign of the net radiative feedback due to all cloud types is less certain but likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds.” They estimate the cloud feedback from all cloud types to be +0.6 W/m2°C (with an uncertainty band of −0.2 to +2.0), and continue, “All global models continue to produce a near-zero to moderately strong positive net cloud feedback.”
The closely related effective climate sensitivity has increased substantially in the latest generation of global climate models. Differences in the physical representation of clouds in models drive this enhanced sensitivity relative to the previous generation of models.
But now a new article on NASA’s climate website has thrown this into doubt. It tells us:
◆Understanding how greenhouse gases will affect clouds is crucial to forecasting climate change. But current computer climate models can’t handle the high resolution needed to simulate cloud dynamics worldwide.
◆A recent study suggests that if greenhouse gases raise the atmosphere’s temperature enough, stratocumulus clouds could disappear, causing a large spike in global temperature.
◆JPL is partnering with Caltech, MIT and the Naval Postgraduate School to build a new climate model that can take full advantage of Earth-observing satellites, with thousands of sub-models capable of simulating the dynamics of clouds and other small-scale phenomena.
That second point tells us that this new climate model has cloud radiative forcing as net negative, not positive. If cloud decreases, letting more sunlight to the surface, that will cause surface temperature to rise.
Watch the pea carefully here. Climate models have only been able to get any exciting amounts of global warming by assuming a large and positive water vapour feedback. Warming caused by extra CO2 causes more evaporation and increased amounts of airbourne water vapour, a more potent and prevalent ‘greenhouse gas’.
If global warming caused by more CO2 and WV feedback is going to raise the temperature to the point where clouds disappear, allowing the Sun to warm the surface making things even hotter, then there will be more evaporation of water from the surface. But it won’t be forming clouds apparently. Somehow there must be a switchover point where positive cloud radiative forcing becomes negative? It all seems very confused.
I think the modelers have painted themselves into a corner with positive cloud radiative feedback and are now trying to escape with this ‘new model’. The fundamental problem is that they think extra CO2 has caused the warming and cloud reduction as measured by satellites over the last 40 years, and are now trying to reconcile this.
But what if it was the reduction in cloud which caused the warming, rather than the other way round? After all, that’s what they’re now proposing as the outcome of reduced cloud, so why wouldn’t it have been the cause of the warming in the first place?
Ned Nikolov and Karl Zeller are writing up a new paper on cloud albedo and climate which will explore that possibility. Here’s a sneak preview:
See more at Ned Nikolov’s twitter moments.
via Tallbloke’s Talkshop
March 15, 2020 at 11:03AM