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The IPCC Fourth Assessment Report (AR4) has been subjected to both scientific and non scientific (process based) criticism. Scientific criticism can broadly be broken down into criticism that the report is too conservative or too optimistic. Process criticism does not concern the science, but it can effect the science.

Scientific criticism

Scientific criticism can broadly be broken down into criticism that the report is too conservative or too optimistic. In this case, the view that the IPCC is conservative means the IPCC did not go far enough, it understated the state of the science or the consequences of global warming. Conversely, those who view the IPCC as optimistic, think that the IPCC overstated the state of the science and oversold the consequences of global warming.

AR4 is too conservative

• Scientists, including former U.S. Department of Energy member Joseph Romm, have claimed that the report underestimates positive feedbacks that could lead to a runaway greenhouse effect, thus greatly underestimating the future warming and its effects. The report is also said to be out of date because it omits recent observations such as the release of greenhouse gases, including methane, from thawing tundra.

The actual report gives a warning that positive feedbacks could release more carbon dioxide in a yet uncertain magnitude, but it does not mention gases with an even greater global warming potential like methane: "Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilization level of atmospheric carbon dioxide concentration."

AR4 is too optimistic

• Shortly after publication of the AR4 Summary for Policymakers, The libertarian Fraser Institute issued an alternative "Independent Summary for Policymakers" (ISPM) drawing skeptical conclusions. An analysis by climate scientists writing for noted climate science blog RealClimate describes it as "profoundly ignorant" of IPCC processes, and stated that it is better to think of the ISPM as the "Incorrect Summary for Policymakers".

Process criticism

• In January 2005, Dr. Chris Landsea who was already an author on the 2001 report (TAR), withdrew his participation in the Fourth Assessment Report claiming that the IPCC had become "politicized" and that the IPCC leadership simply dismissed his concerns. He published an open letter explaining why he was resigning and to "bring awareness to what I view as a problem in the IPCC process".

The conflict centers around Dr. Kevin Trenberth's public contention that global warming was contributing to "recent hurricane activity", which Landsea described as a "misrepresentation of climate science while invoking the authority of the IPCC". He has stated that the process of producing the Fourth Assessment Report is "motivated by pre-conceived agendas" and "scientifically unsound".

Landsea writes that "the IPCC leadership said that Dr. Trenberth was speaking as an individual even though he was introduced in the press conference as an IPCC lead author."

• Roger A. Pielke (Jr), the University of Colorado professor who originally published Landsea's letter, expressed his opinion that the report "maintain consistency with the actual balance of opinion(s) in the community of relevant experts" but notes the political influence on the process of negotiation of the report's contents:

"The open atmosphere of negotiations in the IPCC is probably something that should be revised. How anyone can deny that political factors were ever-present in the negotiations isn't paying attention."

1. For example, see Joseph Romm's December 2006 book, Hell and High Water: Global Warming, pp. 65-72, and his interview on Fox News on January 31 2007.
2. Cite error: The named reference WG1SPM was invoked but never defined (see the help page).
3. Fraser Institute fires off a damp squib
4. ^ http://sciencepolicy.colorado.edu/prometheus/archives/science_policy_general/000318chris_landsea_leaves.html
5. http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/001085follow_up_ipcc_and_.html