Climate change (5)

So, why do the IPCC projections of the global temperature in 2100 differ so widely from the numbers obtained assuming climate sensitivity derived from the historical records? This, ultimately, is the crux of the AGW debate. By accepting the IPCC assessments and the remedial measures they propose you – the reader of this post – are accepting the IPCC stance as correct. Do you know what assumptions underpin the 2.0C-4.5C temperature rise projection which justifies the need for drastic cuts to CO2 emissions?

The IPCC projections are based on the computer simulations run using programs which model global climate. The most advanced of these programs, General Circulation Models, are numerical representations of the physical processes in the atmosphere, ocean, cryosphere and land surface. In these models the Earth is divided into 3-D grids which extend above the ground surface (atmosphere) and below the ocean surface. The models can track the changes of the physical parameters within the grids over time, effectively trying to fast forward the changes to Earth climate using numeric models run on mainframe super-computers. In GCM’s the magnitude of the interactions between various physical processes are input parameters. This means the program operator has to let the model know say how much water vapour is created on the surface of the Earth depending on the soil moisture, air temperature, wind etc. If we include effects like ocean currents affecting air temperature and humidity, stratospheric jet streams moving huge volumes of air at high speed, wind shielding by mountain ranges a.s.o. the models grow in complexity exponentially.

It is critical to realise that the computers do not “know” say how much water vapour is created – it has to be worked out by whoever is running the program and input as a parameter. Some of these parameters, representing interactions between physical processes, are critical to the results produced by the models but poorly understood by the program operators. One such interaction is the effect of clouds on solar radiation reaching the Earth surface. The issue is enormously complex.


Low clouds appear to reflect more sunlight before it reaches the surface of the Earth (negative feedback), high clouds tend to trap the reflected heat (positive feedback). Additionally, the cloud cover changes constantly, in response to variations in the relative humidity of air, which in turn depends on the amount of water vapour, temperature and atmospheric pressure. From basic physics we know in which conditions water vapour will condense in the atmosphere but even the most sophisticated GCM models do not track the formation of individual clouds. Computer programs simply assume a certain feedback which ties the amount of water vapour in the atmosphere with the amount of solar heat reaching the surface of the Earth. The sense and magnitude of this feedback is user input.

Ok, so what is the effect water vapour has on the solar radiation through the cloud forming mechanism? The estimates vary widely from a strongly negative to a strongly positive feedback. In other words some scientists believe that more clouds will trap more heat, others claim that they will reflect more solar radiation before it reaches the surface of the Earth. There is no doubt that the doubling of CO2 in the atmosphere should increase the global temperatures by about 1.0C (primary forcing) but opinions vary on whether the water vapour pumped into the atmosphere by the additional heat will magnify this rise (positive feedback) or reduce it (negative feedback). Computer models will not produce the answer to this question – they merely process the answer which is input in them by the operators.

As you may have guessed all models used in the IPCC projections assume positive feedback from clouds. There are many other feedback parameters which have to be input into GCM’s and the uncertainties associated with each of them add to a potential error in the results. The bottom line is that the computer programs project 2.0-4.5C temperature rise by 2100 of which about 1.0C is the primary forcing (CO2) with the rest created by feedbacks. I will just remind here that an equivalent rise in CO2 between mid-1800s and the present times has only produced about 0.8C of warming. This is what you are signing up to by accepting the IPCC conclusions.

Climate change (6)


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