3 - 4pm, 23rd February 2016, WebEX.
Title: In Search of the Holy Grail: an Emergent Constraint on Climate Sensitivity
3 - 4pm GMT, via WebEX.
Peter Cox, University of Exeter.
Earth System Modelling suffers from a significant timescale problem – we need to find constraints on the huge range of projected changes in the climate 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 look for relationships between the more extensive observations of short-term variability and the longer-term sensitivity of the climate to anthropogenic forcing. According to the Fluctuation-Dissipation Theorem (FDT), such relationships should be common in a large-class of systems including the climate (Leith, 1975). In principle it should even be possible to get good estimates of Earth System (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 accurate 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. It is called 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. A number of Emergent Constraints have been found, for example relating to snow-albedo feedbacks (Hall & Qu, 2007), sea-ice trends (Boe et al., 2009; Bracegirdle & Stephenson, 2014), and loss of tropical land-carbon under climate change (Cox et al., 2013). As a further example, I will describe a recently discovered emergent relationship between the increasing amplitude of the CO2 seasonal cycle in the atmosphere and CO2 fertilization of vegetation photosynthesis (Wenzel et al., submitted), which promises to significantly reduce the uncertainty in one of the key unknowns in Earth System Modelling.
However, the biggest unknown in climate projections is still the Equilibrium Climate Sensitivity (ECS), which determines how much the global temperature will increase if the CO2 concentration was stabilised at double its initial value. The ECS range quoted by the Intergovernmental Panel on Climate Change (IPCC) remains at 1.5-4.5K, essentially unchanged through 5 IPCC reports and more than 25 years of climate science. The recent “hiatus” in global warming has even led some researchers to conclude that ECS could be lower than 1.5K. I will show early results which hint at a relationship between equilibrium climate sensitivity and the interannual variability in surface temperature and the net radiative flux at the top of the atmosphere. The resulting Emergent Constraint suggests that ECS is unlikely to be less than 2K. This constraint appears to be robust to changes in the model ensemble (i.e. CMIP5 or QUMP ensemble) and to changes in the method of de-trending, but it is far from fully understood by the author (who will therefore welcome suggestions and challenges).
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