Emergent Constraints on Climate-Carbon Cycle Feedbacks
Abstract:Earth System Models (ESMs) are designed to project changes in the climate-carbon cycle system over the coming centuries. However, ES modelling still suffers from a significant timescale problem – we need to find constraints on the huge range of projected changes in the climate-carbon system over the next century, but the contemporary observational data that we have relates to much shorter timescales. One way around this problem is to utilise longer-term palaeoclimate proxies to estimate the response of the ES on the timescales of interest, but here data availability and spatial coverage is often limiting. An alternative is to look for relationships between the more extensive observations of short-term variability and the longer-term sensitivity of the ES to anthropogenic forcing. According to the Fluctuation-Dissipation Theorem (FDT), such relationships should be common in a large-class of systems including the ES (Leith, 1975). In principle it should even be possible to get good estimates of ES sensitivities to external forcing purely by analysing the temporal correlations evident in climate observations - unfortunately this typically requires a prohibitively long time-series of observations.
An alternative approach to utilising the constraints embodied in short-term variability relies on “Emergent Constraints”. An Emergent Constraint is a relationship between some ES sensitivity to anthropogenic forcing and an observable feature of the ES. We call it emergent because it emerges from the ensemble of ESMs, and it is described as a constraint because it enables an observation to constrain the estimate of the sensitivity in the real world. As an example, we will describe emergent relationships between the sensitivity of tropical land-carbon storage to warming and the sensitivity of the annual growth-rate in atmospheric CO2 to tropical temperature anomalies, for both the current (CMIP5, Wenzel et al., 2014) and previous generation ESMs (C4MIP, Cox et al., 2013). When combined with contemporary observations of the atmospheric CO2 concentration and the tropical temperature, these relationships provide consistent constraints on the sensitivity of tropical land carbon to warming in the real climate system.