After 20 minutes I finally located the quote, which appears as a caveat in the caption to a particular figure, and not as a blanket statement regarding our state of knowledge of land use change impacts. It's the caption to Figure 11.24 on p. 95 of the report of Working Group III – Mitigation of Climate Change, Chapter 11, Agriculture, Forestry and Other Land Use (AFOLU). The full caption reads (with the above quoted section in bold),
Figure 11.24. Estimates of GHGLUC emissions – GHG emissions from biofuel production-induced LUC (as gCO2eq/MJfuel produced) over a 30-year time horizon organized by fuel(s), feedstock, and study. Assessment methods, LUC estimate types and uncertainty metrics are portrayed to demonstrate the diversity in approaches and differences in results within and across any given category. Points labeled ‘a’ on the Y-axis represent a commonly used estimate of lifecycle GHG emissions associated with the direct supply chain of petroleum gasoline (frame A) and diesel (frame B). These emissions are not directly comparable to GHGLUC because the emission sources considered are different, but are potentially of interest for scaling comparison. Based on Warner et al. (2013). Please note: These estimates of global LUC are highly uncertain, unobservable, unverifiable, and dependent on assumed policy, economic contexts, and inputs used in the modelling. All entries are not equally valid nor do they attempt to measure the same metric despite the use of similar naming conventions (e.g., ILUC). In addition, many different approaches to estimating GHGLUC have been used. Therefore, each paper has its own interpretation and any comparisons should be made only after careful consideration. *CO2eq includes studies both with and without CH4 and N2O accounting.
I wonder whether they made the original source very hard to track down quite intentionally. Googling "Bioenergy and Climate Change Mitigation: An Assessment" mostly gives hits on biofuel promotion web pages.
If there really is a standalone IPCC document titled "Bioenergy and Climate Change Mitigation: An Assessment," please direct me to it. Thanks!
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