No human has ever experienced the environment in which we are currently living. Consequently, those who create the mathematical models of the extremely complex climatic systems are often forced to make assumptions in their initial models based on past behavior, not expected, future behavior. Once models are completed, they are then tested against current observations and continually refined to include new variables. One variable of recent interest is model assumptions about the reflectivity of mixed-phase clouds.
Mid-altitude clouds with temperatures between 0oC (32oF) down to about -40o (-40oF – the only point where these two temperature scales are identical) are often “mixed-phase” clouds containing water in each of its’ 3 phases, ice crystals, vapor and supercooled liquid droplets. A large percentage of cloud ice crystals exhibit a cooling effect by reflecting incoming solar radiation back toward space, while a higher percentage of water vapor and cloud water droplets exhibit a warming, greenhouse effect.
Most climate models have assumed that the ratio of mixed-phase cloud ice to water was relatively stable, and that such clouds acted as a cooling factor. However, a recent study(1), using data from a research satellite found that mixed-phase clouds actually contain far less ice, and more water, than previously believed; and thus the cooling effect of clouds has been over-estimated by up to 1.3oC. As the atmosphere warms, this disparity may increase due to the fact that the atmosphere hold more water as the temperature increases.
A related study(2) which also used satellite data and Lidar (using laser pulses) observations to evaluate the ratio between mixed-phase cloud ice and water in 16 climate models found similar results, with only 3 of 16 models managing to “preform well” in replicating the observed correlations. The study found that “the best models were those that include more complex microphysics”, an observation that points out the need for more complexity of already extremely complex models, and for a greater understanding of little known natural processes.
To put these findings into context, the December, 2015 Paris agreement established a target of 1.5oC warming by the end of the century over preindustrial temperatures, while the IPCC has established a 2.0oC target for the same period. We have already exceeded a 1oC increase, leaving only a 1oC permissible increase for the IPCC target and 0.5oC for the Paris target. If the 1.3oC maximum impact of the obscure ratio of mixed-phase cloud ice to water is only half correct it will be impossible to meet the Paris target, even before the agreement is signed and in effect. And, it becomes exceedingly difficult, if not impossible, to reach the IPCC target.
Once again, the scientific predictions have been shown to be overly conservative, not “extreme”. Our science, based on the accumulated knowledge gained in the past is being over-run by the rapidly changing present.
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1. Tan, I., T. Storelvmo, and M.D. Zelinka. Observational constraints on mixed-phase clouds imply higher climate sensitivity. Science, Vol. 352, Issue 6282, 224-227. 2016.
2. Cesana, G., D. E. Waliser, X. Jiang, and J.-L. F. Li (2015), Multimodel evaluation of cloud phase transition using satellite and reanalysis data. J. Geophys. Res. Atmos., 120, 7871–7892. 2015
