Why Climate Change Effect Studies are Pessimistic

Hundreds of studies have been published on the effects of future climate change on various ecosystems or species, but the conclusions are often easily disputed. Why are these results so shaky? This study shows that there are systematic gaps in the methods used, and that these gaps all tend to falsely increase the potential negative impacts of climate change and inflate the certainty of the studies.

In this study, criteria for conducting reliable and useful assessments of impacts of future climate were suggested. The major steps involve: clearly defining an emissions scenario; selecting a climate model; evaluating climate model skill and bias; quantifying General Circulation Model (GCM) between-model variability; selecting an ecosystem model and assessing uncertainty; properly considering transient vs. equilibrium responses; including effects of CO₂ on plant response; evaluating implications of simplifying assumptions; and considering animal linkage with vegetation. A sample of the literature was surveyed in light of these criteria. Many of the studies reviewed used climate simulations that were >10 years old and not representative of best current models. Future effects of elevated CO₂ on plant drought resistance and productivity were generally included in growth model studies but not in niche (habitat suitability) studies, causing the latter to forecast greater future adverse impacts or even negative impacts when positive effects are likely. Overly simplified spatial representation was frequent and caused the existence of refugia to be underestimated. Few studies compared multiple climate simulations and ecosystem models (including parametric uncertainty), leading to a false impression of precision and potentially arbitrary results due to high between-model variance. No study assessed climate model retrodictive skill or bias. For niche models, the equilibrium assumption (that species must, following warming, instantly move to a new geographic region corresponding to their niche model, or else die) is unsupported by any experimental data. That is, no evidence exists in most cases that projected climate change will cause a species to die out in their current range. Instead, geographic displacement should take hundreds to thousands of years for plants and result from gradual competitive processes. Thus the risk of extinction is grossly exaggerated in most studies. Overall, no current studies in the survey met all of the proposed criteria and typically have very little quantification of uncertainty. The net effect is a strong bias in this literature toward predicting negative consequences of climate change.

Article available free at http://onlinelibrary.wiley.com/doi/10.1002/ece3.7/pdf