I quoted a paper in the discussion of the previous article. Paper is by Hansen et al (“et al” a fancy way to say “and others.” The lead or sole author is the name cited and the full name of the paper and all the authors is quoted in the Bibliography at the end. Here endeth the lesson.)
Quote:[long history of climate science snipped]. . .most climate models are unrealistically insensitive to freshwater injected by melting ice and that ice sheet models are unrealistically lethargic in the face of rapid, large climate change [14].
We are certainly becoming aware that the long term meridional overturning circulation that distributes heat, CO2 and nutrients along the world’s oceans and warms Western Europe and northern North America is in severe trouble. It is in severe trouble because there is lots of fresh water from sea ice that is colder than seawater because of the salt content of the seawater.
Consequently—the meltwater sits on top of seawater and no longer sinks to the bottom, starting on its way to the Southern Ocean or the North Atlantic—no more overturning circulation.
Quote:Charney defined ECS as the eventual global temperature change caused by doubled CO2 if ice sheets, vegetation and long-lived GHGs are fixed (except the specified CO2 doubling). Other quantities affecting Earth’s energy balance—clouds, aerosols, water vapor, snow cover and sea ice—change rapidly in response to climate change. Thus, Charney’s ECS is also called the ‘fast-feedback’ climate sensitivity. Feedbacks interact in many ways, so their changes are calculated in global climate models (GCMs) that simulate such interactions. Charney implicitly assumed that change of the ice sheets on Greenland and Antarctica—which we categorize as a ‘slow feedback’—was not important on time scales of most public interest.
ECS defined by Charney is a gedanken concept* that helps us study the effect of human-made and natural climate forcings. If knowledge of ECS were based only on models, it would be difficult to narrow the range of estimated climate sensitivity—or have confidence in any range—because we do not know how well feedbacks are modeled or if the models include all significant real-world feedbacks. Cloud and aerosol interactions are complex, e.g. and even small cloud changes can have a large effect. Thus, data on Earth’s paleoclimate history are essential, allowing us to compare different climate states, knowing that all feedbacks operated.
*Gedanken concept = thought experiment.
Climate through the Cenozoic Era (Modern geological time, since the time of the meteorite that killed off the dinosaurs except birds) has been pretty well studied by paleoclimatologists and of course includes all the real feedbacks that are caused by and affect climate.
Quote:. . . .feedbacks occur in the real world and in GCMs. In our GCM the equilibrium response to 2 × CO2 was 4°C warming of Earth’s surface. Thus, the fraction of equilibrium warming due directly to the CO2 change was 0.3 (1.2°C/4°C) and the feedback ‘gain’, g, was 0.7 (2.8°C/4°C). Algebraically, ECS and feedback gain are related by(1)
We evaluated contributions of individual feedback processes to g by inserting changes of water vapor, clouds, and surface albedo (reflectivity, literally whiteness, due to sea ice and snow changes) from the 2 × CO2 GCM simulation one-by-one into a one-dimensional radiative-convective model [16], finding gwv = 0.4, gcl = 0.2, gsa = 0.1, where gwv, gcl, and gsa are the water vapor, cloud and surface albedo gains. The 0.2 cloud gain was about equally from a small increase in cloud top height and a small decrease in cloud cover. These feedbacks all seemed reasonable, but how could we verify their magnitudes or the net ECS due to all feedbacks?
Indeed, modelling the atmosphere with all its dynamics and feedbacks is very difficult, yet the models do pretty well.
Quote:We recognized the potential of emerging paleoclimate data. Early data from polar ice cores revealed that atmospheric CO2 was much less during glacial period . . . .However, when we employed CLIMAP boundary conditions including sea surface temperatures (SSTs), Earth was out of energy balance, radiating 2.1 W/m2 to space, i.e. Earth was trying to cool off with an enormous energy imbalance, equivalent to half of 2 × CO2 forcing.
Something was wrong with either assumed LGM conditions or our climate model.
Always checking hypothesis with reality is what science is all about.
Will have to be continued—dinner time!
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