ECCLES set out to “reduce uncertainties in climate-carbon projections using Emergent Constraints - relationships between observable variability in the Earth System and future changes that are evident across an ensemble of models - providing a general theoretical basis for these, and identifying new constraints on the role of the land biosphere in the Earth System”. The ECCLES project made significant progress in these respects, as outlined below for each of the four main tasks as identified in the proposal.
Task 1: Theoretical basis for emergent constraints (Williamson, Nijsse, Cox)
When ECCLES began in 2017 there was no theoretical framework to guide the search for emergent constraints (ECs). As a result, there were published examples of indiscriminate data-mining of the multidimensional outputs from Earth System Models that lead to misleading emergent constraints. To mitigate against this risk, ECCLES set out to develop a sounder theoretical basis for emergent constraints. Good progress has been made in deriving relationships between global temperature variability and climate sensitivity (Williamson et al., Dynamics and Statistics of the Climate System, 2018). The ECCLES team and collaborators have also been influential in arguing for a sounder theoretical basis for emergent constraints among climate scientists (Hall et al., Nature Climate Change, 2019), carbon cycle modellers (Cox, Current Climate Change Reports, 2019) and theoretical physicists (Williamson et al., Reviews of Modern Physics, 2021).
Task 2: Identifying the key observed changes (Moore, Huntingford-CEH, Cox)
Task 2 aimed to identify the observed changes that could provide the necessary observational constraints. The work carried-out by Jon Moore focused on observations of individual tree-sizes within forests worldwide. This began with the USDA data on tree trunk diameters measured across forests in the US (Moore et al., 2018), and went on to look at similar data for the many RainFOR sites in South America (Moore et al., 2020). This analysis prompted ‘Demographic Optimality Theory’ (Moore et al., 2023), and also stimulated the development of the RED Dynamic Global Vegetation model (see ‘Interdisciplinary Advance 2’). Task 2 also benefitted hugely from a subcontract with Dr Chris Huntingford of the UK Centre for Ecology and Hydrology who analysed changing high-temperature extremes in climate models and in the real world (Huntingford et al., in review).
Task 3: Emergent constraints from temporal variations (Nijsse, Cox)
ECCLES began with a very high-profile letter reporting a Emergent Constraint on the key unknown in climate projections – the Equilibrium Climate Sensitivity (Cox et al., Nature, 2018). This paper stimulated much debate including three “Brief Communications Arising” and a response from Cox, Nijsse, Williamson and Huntingford (Cox et al., Nature 2018a). Peter Cox and Mark Williamson were both invited speakers at the AGU Fall meeting in December 2018 in Washington DC, where they presented aspects of this study. Shortly afterwards, Femke Nijsse published a paper showing a link between decadal climate variability and climate sensitivity in CMIP5 climate models (Nijsse et al., Nature Climate Change, 2019). Our subsequent analyses have not found such clear relationships between temperature variability and climate sensitivity in the more recent CMIP6 generation of models, for reasons that we are beginning to untangle (Williamson et al., in review). In general, we have found that many emergent constraints developed on the CMIP5 models do not apply in CMIP6 (Zechlau et al., 2022). However, a more straighforward and robust emergent constraint, that is evident for both CMIP5 and CMIP6 models, was published by the ECCLES group in 2020 (Nijsse et al., 2020). This paper has been influential in making the case that the IPCC ‘likely’ range of climate sensitivity should not be extended as far as the highest sensitivities produced by the CMIP6 climate models.
Task 4: Emergent constraints from spatial variations (Varney, Cox)
Most emergent constraints are still based on variations in time, such as interannual variability or trends. Task 4 of ECCLES set out to explore whether observed spatial patterns also imply a constraint on future changes. Based-on the pioneering work of Sarah Chadburn (Chadburn et al., Nature Climate Change, 2017), Rebecca Varney published a constraint on the temperature sensitivity of soil carbon based-on the observed pattern of soil carbon (Varney et al., Nature Communications, 2020). This paper remains a rare example of a published spatial emergent constraint.
Inputs to the IPCC AR6 (Cox, Ritchie)
ECCLES researchers made notable inputs to the the IPCC WG1 report, with a subsection on emergent constraints written for Chapter 5 by Peter Cox (Lead Author), and a subsection on tipping points written for Chapter 1 by Paul Ritchie (Contributing Author).The ECCLES project team have made excellent progress on these aims, as outlined below in terms of the 4 Tasks listed in the ‘Description of Action’ for the project.
