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An About-face on the Impact of Landcover Change on Historic Temperatures

Paper Reviewed
Lejeune, Q., Davin, E.L., Gudmundsson, L., Winckler, J. and Seneviratne, S.I. 2018. Historical deforestation locally increased the intensity of hot days in northern mid-latitudes. Nature Climate Change 8: 386-390.

According to Lejeune et al. (2018), previous research has led the IPCC to conclude that the biogeophysical impacts of historical deforestation have induced a cooling effect on the temperature record of the northern mid-latitudes ever since pre-industrial times. However, a new analysis by the five European researchers now challenges that assumption.

As their contribution to the topic, Lejeune et al. used "observational data to constrain the historical impact of deforestation on hot extremes in 11 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) that simulate the climate effect of landcover changes," ultimately estimating "the local impacts of historical deforestation on mean daily maximum surface air temperature (TX) in the warm season, as well as its yearly maximum value (TXx) from 1861-2000, compared with a pre-industrial control period." And what did their analysis reveal?

The multi-model mean revealed that historical deforestation "has led to local increases in TXx over extensive parts of North America, Eurasia and South Asia, but also southern South America, eastern Australia and southeastern Africa during present-day (1981-2000) compared with pre-industrial conditions." Continuing, Lejeune et al. report that "the strongest deforestation-induced warming of TXx has occurred over North America and Eurasia, where it reaches 0.3°C on average (over areas that have been at least moderately deforested) and up to 1°C locally over the Great Plains." Consequently, the authors estimate that the relative contribution of the biogeophysical effects of deforestation on the observed 20th century temperature rise "remained as high as 56% (20-115%) over North America and 32% (between 22% and 7 times higher, depending on the models) over Eurasia on average between 1920 and 1980."

In commenting on their findings, Lejeune et al. state that, "contrary to many previous studies, which suggested that the biogeophysical effects of historical deforestation had mitigated daytime hot extremes over mid-latitudinal regions, this observation-constrained analysis of CMIP5 models shows that they have actually led to significant local increases in TXx over many areas in the world," where they "were responsible for at least half of the warming of TXx over most deforested mid-latitudinal regions by 1980."

The results of this analysis thus reveal that the IPCC has incorrectly assumed the magnitude and even the sign of the temperature forcing associated with historical landcover change. Consequently, one is left wondering just how much of the 20th century warming is really due to rising atmospheric CO2 and other greenhouse gases once the correct impact of landcover change is factored in. The answer -- not much!

Posted 18 January 2019