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Analyzing observed vs model simulations of global temperature

22 Jul 2020 | Science Notes

We are big admirers of CO2Science.org, which for many years has been sifting through the scientific literature and posting summaries of important articles on a wide variety of climate-related topics, with a special focus on biological effects of CO2 levels on plants. With their kind permission we are going to begin sharing with CDN readers some of their work here in our Science Items section. This week we start with two of their recent postings.

A new analysis reveals current state-of-the-art models overestimate the impact of volcanic aerosols and significantly underestimate the effect of natural solar variability on global temperatures...
Paper Reviewed by CO2Science:

Chylek, P., Folland, C., Klett, J.D. and Dubey, M.K. 2020. CMIP5 climate models overestimate cooling by volcanic aerosols. Geophysical Research Letters 47, e2020GL087047.

Chylek et al. (2020) introduce their work by noting climate model simulations of global mean surface temperature diverge from the observational record around the turn of the 21st century, with “the model warming being somewhat too high.” Two possible explanations for this discrepancy that they go on to investigate include (1) a failure of the models to properly account for aerosol forcings from volcanic eruptions and (2) a general underestimation of solar forcing.

The bulk of their analysis was focused on the first of these possible sources of error, where Chylek et al. compared the observed and model-simulated hemispheric mean temperature projections using a set of influencing factors, including anthropogenic greenhouse gases and aerosols, natural solar variability, volcanic eruptions and internal climate variability.

In describing their findings, the authors report that “climate models overestimate the cooling effect of volcanic activity and underestimate the effect of the variability of solar radiation.”
Altogether, Chylek et al. report “the volcanic aerosol regression coefficients of the CMIP5 [5th-generation climate models] simulations are consistently significantly larger (by 40-49%) than the volcanic aerosol coefficients of the observed temperature.” The hypothesized source of this discrepancy, they say, likely originates in faulty model parameterization of aerosol-cloud interactions within ice and mixed phase clouds. [CDN Note: we have discussed the importance of aerosol-cooling errors in climate models here.]

[In] the authors’ words, in addition to overestimating volcanic aerosol effects, “the CMIP5 climate models also significantly underestimate the effect of solar variability on the hemispherical and global temperature.”... “in CMIP5 models there is effectively no influence of solar variability on temperature, while the analysis of the observed temperature suggests quite a significant effect, especially on the Southern Hemisphere, consistent with the global results of Folland et al. (2018).”

Such large and significant discrepancies between observed and simulated hemispheric temperatures identified by Chylek et al. demonstrate that current state-of-the-art models are still not ready for primetime. Much more investigation and analysis must be conducted to identify and then correct these and likely many more other problems of which scientists are currently unaware before their projections can be trusted in policy formation.

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