By Andy May
Global Climate Models (GCM) are used to compute the social cost of carbon dioxide emissions and to compute man’s contribution to recent global warming. The assertion that most of “climate change” is due to man’s influence is based solely on these models. They are also the sole basis for concluding “climate change” is dangerous. Just how accurate are they? How close are their predictions to observations?
Dr. Judith Curry has written an important white paper, for the layman, describing how the models work. It is easy to understand and well worth reading.
Her key conclusions:
GCMs have not been subject to the rigorous verification and validation that is the norm for engineering and regulatory science.
There are numerous arguments supporting the conclusion that climate models are not fit for the purpose of identifying with high confidence the proportion of the 20th century warming that was human-caused as opposed to natural.
There is growing evidence that climate models predict too much warming from increased atmospheric carbon dioxide.
Some portions of the GCMs are rooted in fundamental physics and chemistry, but there are thousands of atmospheric and surface processes that cannot be deterministically modeled and must be “parameterized” using simple empirical formulas based on observations. These empirical formulas are “tuned” or “calibrated” to make the models match observations. They are tweaked to match the twentieth century, especially the warming period from 1945 to 2000. Even with all of the tuning, the models do a very poor job matching the warming from 1910-1945.
Since all models are “tuned” to the twentieth century (see Voosen, et al., Science, 2016) and since the “more than half of warming is due to man” conclusion is based upon comparing two model runs “from 1951 to 2010” the validity of the computation of man’s influence is highly questionable. Dr. Curry points out:
“GCMs are evaluated against the same observations used for model tuning.”
This is not something that inspires confidence. Further, the Earth has been warming for 300 to 400 years, as Dr. Curry writes:
“Understanding and explaining the climate variability over the past 400 years, prior to 1950, has received far too little attention. Without this understanding, we should place little confidence in the IPCC’s explanations of warming since 1950.”
“Anthropogenic (human-caused) climate change is a theory in which the basic mechanism is well understood, but of which the potential magnitude is highly uncertain.”