Emergent Constraints on Climate-Carbon Cycle Feedbacks.

Purpose of review: Feedbacks between CO₂-induced climate change and the carbon cycle are now routinely represented in the Earth System Models (ESMs) that are used to make projections of future climate change. The inconclusion of climate-carbon cycle feedbacks in climate projections is an important advance, but has added a significant new source of uncertainty. This review assesses the potential for emergent constraints to reduce the uncertainties associated with climate-carbon cycle feedbacks.

Recent findings: The emergent constraint technique involves using the full ensemble of models to find an across-ensemble relationship between an observable feature of the Earth System (such as a trend, interannual variation or change in seasonality) and an uncertain aspect of the future. Examples focussing on reducing uncertainties in future atmospheric CO₂ concentration, carbon loss from tropical land under warming and CO₂ fertilization of mid- and high-latitude photosynthesis are exemplars of these different types of emergent constraints.

Summary: The power of emergent constraints is that they use the enduring range in model projections to reduce uncertainty in the future of the real Earth System, but there are also risks that indiscriminate data-mining, and systematic model errors could yield misleading constraints. A hypothesis-driven theory-led approach can overcome these risks and also reveal the true promise of emergent constraints-not just as ways to reduce uncertainty in future climate change but also to catalyse advances in our understanding of the Earth System.