Reconciling Climate Model/Data Discrepancies: The Case of the ‘Trees That Didn’t Bark’
Estimates of Equilibrium Climate Sensitivity (ECS)—the temperature with a doubling of CO2—usually are between 1.5 °C and 5 °C warming, with a mid-range/most likely value close to 3 °C. Notably, comparisons of simulations of temperature changes over the past millennium with paleoreconstructions of past temperatures tend to suggest an ECS value toward the lower end of the range closer to 2 °C, which deserves closer scrutiny. In this chapter, I review evidence that paleoreconstructions may selectively underestimate the cooling signal associated with large explosive volcanic eruptions of the past millennium, due to their reliance on tree-ring data from tree-line-proximal environments. Once large volcanic episodes are masked, these comparisons instead support a mid-range of ~3 °C of ECS. Regardless of the reason for the model/paleodata discrepancy, ECS estimates—I argue—should not rely on comparisons that include these episodes.
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