## lördag 29 december 2012

### Most Likely IPCC AR5 Very Likely Uncertain

The leaked IPCC AR5 states:
• Despite considerable advances in climate models and in understanding and quantifying climate feedbacks, the assessed literature still supports the conclusion from AR4 that climate sensitivity is likely in the range 2–4.5°C, and very likely above 1.5°C. The most likely value remains near 3°C.
• A few studies argued for low values of climate sensitivity, but almost all of them have received criticism in the literature.
• Equilibrium climate sensitivity, transient climate response and climate feedbacks are useful concepts to characterize the response of a model to an external forcing perturbation. However, there are limitations to the concept of radiative forcing.
• Projections of climate change are uncertain, firstly because they are primarily dependent on scenarios of future anthropogenic and natural forcings, secondly because of incomplete understanding and inadequate models of the climate system and finally because of the existence of natural climate variability
• The term climate projection tacitly implies these uncertainties and dependencies.
• Nevertheless, as greenhouse gas concentrations continue to rise, we expect to see future changes to the climate system that are greater than those already observed and attributed to human activities.
• It is possible to understand future climate change using models and to use models to quantify likely outcomes and uncertainties dependent on assumptions about future forcing scenarios.
We read contradictory statements expressing that the most likely value of climate sensitivity is 3 C, while projections of climate change are uncertain. In short, IPCC AR5 tells the world:
• It is uncertain that climate sensitivity very likely is above 1.5 C and most likely is 3C.
One may ask if it is in fact very uncertain or most uncertain that the value is very likely above 1.5 C or most likely is  3 C?

In any case, it seems to be very likely that the terminology developed by IPCC most likely does not represent considerable advances in climate models and in understanding

Recall that the IPCC climate sensitivity of 3 C is obtained from a starting guess of radiative sensitivity of 1 C from a guess of radiative forcing of 4 W/m2, combined with a freely invented thermodynamic feedback factor of 3, giving 3 C as an inventive guess work.