Periodic Reporting for period 4 - ECCLES (Emergent Constraints on Climate-Land feedbacks in the Earth System)
Reporting period: 2022-04-01 to 2023-09-30
The carbon cycle is currently carrying-out an important service for humankind by absorbing about a half of our CO2 emissions. However, it is clear from the long-term measurements of CO2 in the atmosphere that the carbon cycle is sensitive to climate. The Earth System Models (ESMs) which appear in the reports of the Intergovernmental Panel on Climate Change now routinely include climate-carbon cycle feedbacks.
Unfortunately, the feedbacks from the land carbon cycle still vary by a factor of more than five, which impedes action to mitigate and adapt to climate change.
There are three major reasons for this uncertainty: (a) uncertainty in the sensitivity of climate to carbon dioxide; (b) uncertainty in the extent to which climate change will lead to a release of carbon from soil and vegetation; and (c) uncertainty in the extent to which CO2 increase will enhance photosynthesis and therefore increase the land carbon sink.
The ECCLES project addressed these uncertainties by searching for Emergent Constraints, which are essentially relationships between observable aspects of the Earth System and projected future changes, which are common across the full ensemble of models.
Emergent constraints are very attractive because they make use of the range of projections of future climate to reduce the uncertainty in the future climate of the real world. However, there is also a danger that misleading relationships could arise from blind data-mining of the multidimensional outputs of state-of-the-art models. ECCLES therefore focuses on emergent constraints that have a firm basis in mathematical theory and known physical mechanisms.
In short, ECCLES set-out to reduce uncertainties in climate-carbon projections using theory-based emergent constraints.
Unfortunately, the feedbacks from the land carbon cycle still vary by a factor of more than five, which impedes action to mitigate and adapt to climate change.
There are three major reasons for this uncertainty: (a) uncertainty in the sensitivity of climate to carbon dioxide; (b) uncertainty in the extent to which climate change will lead to a release of carbon from soil and vegetation; and (c) uncertainty in the extent to which CO2 increase will enhance photosynthesis and therefore increase the land carbon sink.
The ECCLES project addressed these uncertainties by searching for Emergent Constraints, which are essentially relationships between observable aspects of the Earth System and projected future changes, which are common across the full ensemble of models.
Emergent constraints are very attractive because they make use of the range of projections of future climate to reduce the uncertainty in the future climate of the real world. However, there is also a danger that misleading relationships could arise from blind data-mining of the multidimensional outputs of state-of-the-art models. ECCLES therefore focuses on emergent constraints that have a firm basis in mathematical theory and known physical mechanisms.
In short, ECCLES set-out to reduce uncertainties in climate-carbon projections using theory-based emergent constraints.
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.
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.
The ECCLES project carried-out innovative research, and stimulated debate, on the use of emergent constraints to reduce uncertainties in climate and carbon cycle projections. ECCLES showed that very high-climate sensitivities to carbon dioxide are unlikely to be true, and therefore that the Paris targets are still within our grasp. ECCLES also improved understanding of possible climate tipping point behaviour in a climate that is rapidly being forced from steady state (highlighting the phenomena of 'safe' overshoots and rate-dependent tipping).