Publications by Myles Allen

Finding ocean states that are consistent with observations from a perturbed physics parameter ensemble

Journal of Climate American Meterological Society 31 (2018) 4639-4656

S Sparrow, R Millar, K Yamazaki, N Massey, AC Povey, A Bowery, RG Grainger, DCH Wallom, M Allen

A very large ensemble is used to identify subgrid-scale parameter settings for the HadCM3 model that are capable of best simulating the ocean state over the recent past (1980–2010). A simple particle filtering technique based upon the agreement of basin mean sea surface temperature (SST) and upper 700-m ocean heat content with EN3 observations is applied to an existing perturbed physics ensemble with initial conditions perturbations. A single set of subgrid-scale parameter values was identified from the wide range of initial parameter sets that gave the best agreement with ocean observations for the period studied. The parameter set, different from the standard model parameters, has a transient climate response of 1.68 K. The selected parameter set shows an improved agreement with EN3 decadal-mean SST patterns and the Atlantic meridional overturning circulation (AMOC) at 26°N as measured by the Rapid Climate Change (RAPID) array. Particle filtering techniques as demonstrated here could have a useful role in improving the starting point for traditional model-tuning exercises in coupled climate models.

A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation

npj Climate and Atmospheric Science Springer Nature 1 (2018) 16

M Allen, K Shine, J Fuglestvedt, R Millar, M Cain, D Frame, A Macey

While cumulative carbon dioxide (CO2) emissions dominate anthropogenic warming over centuries, temperatures over the coming decades are also strongly affected by short-lived climate pollutants (SLCPs), complicating the estimation of cumulative emission budgets for ambitious mitigation goals. Using conventional Global Warming Potentials (GWPs) to convert SLCPs to “CO2-equivalent” emissions misrepresents their impact on global temperature. Here we show that peak warming under a range of mitigation scenarios is determined by a linear combination of cumulative CO2 emissions to the time of peak warming and non-CO2 radiative forcing immediately prior to that time. This may be understood by expressing aggregate non-CO2 forcing as cumulative CO2 forcing-equivalent (CO2-fe) emissions. We show further that contributions to CO2-fe emissions are well approximated by a new usage of GWP, denoted GWP*, which relates cumulative CO2 emissions to date with the current rate of emission of SLCPs. GWP* accurately indicates the impact of emissions of both long-lived and short-lived pollutants on radiative forcing and temperatures over a wide range of timescales, including under ambitious mitigation when conventional GWPs fail. Measured by GWP*, implementing the Paris Agreement would reduce the expected rate of warming in 2030 by 28% relative to a No Policy scenario. Expressing mitigation efforts in terms of their impact on future cumulative emissions aggregated using GWP* would relate them directly to contributions to future warming, better informing both burden-sharing discussions and long-term policies and measures in pursuit of ambitious global temperature goals.

Higher CO2 concentrations increase extreme event risk in a 1.5C world

Nature Climate Change Nature Publishing Group 8 (2018) 604–608-

HS Baker, RJ Millar, Allen, DJ Karoly, U Beyerle, BP Guillod, D Mitchell, H Shiogama, SN Sparrow, T Woollings, MR Allen

The Paris Agreement1 aims to ‘pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels.’ However, it has been suggested that temperature targets alone are unable to limit the risks associated with anthropogenic emissions2, 3. Here, using an ensemble of model simulations, we show that atmospheric CO2 increase - a more predictable consequence of emissions compared to global temperature increase - has a significant impact on Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. Hence in an iterative climate mitigation regime aiming solely for a specific temperature goal, an unexpectedly low climate response may have corresponding ‘dangerous’ changes in extreme events. The direct impact of higher CO2 concentrations on climate extremes therefore substantially reduces the upper bound of the carbon budget, and highlights the need to explicitly limit atmospheric CO2 concentration when formulating allowable emissions. Thus, complementing global mean temperature goals with explicit limits on atmospheric CO2 concentrations in future climate policy would reduce the adverse effects of high-impact weather extremes.

The many possible climates from the Paris Agreement's aim of 1.5 °C warming.

Nature 558 (2018) 41-49

SI Seneviratne, J Rogelj, R Séférian, R Wartenburger, MR Allen, M Cain, RJ Millar, KL Ebi, N Ellis, O Hoegh-Guldberg, AJ Payne, C-F Schleussner, P Tschakert, RF Warren

The United Nations' Paris Agreement includes the aim of pursuing efforts to limit global warming to only 1.5 °C above pre-industrial levels. However, it is not clear what the resulting climate would look like across the globe and over time. Here we show that trajectories towards a '1.5 °C warmer world' may result in vastly different outcomes at regional scales, owing to variations in the pace and location of climate change and their interactions with society's mitigation, adaptation and vulnerabilities to climate change. Pursuing policies that are considered to be consistent with the 1.5 °C aim will not completely remove the risk of global temperatures being much higher or of some regional extremes reaching dangerous levels for ecosystems and societies over the coming decades.

The role of human influence on climate in recent UK winter floods and their impacts.

Nature Climate Change Nature Publishing Group (2016)

N Schaller, A Weisheimer

New use of global warming potentials to compare cumulative and short-lived climate pollutants

Nature Climate Change Nature Publishing Group (2016)

MR Allen, JS Fuglestvedt, KP Shine, A Reisinger, R Pierrehumbert, PM Forster

Parties to the United Nations Framework Convention on Climate Change (UNFCCC) have requested guidance on common greenhouse gas metrics in accounting for Nationally determined contributions (NDCs) to emission reductions1. Metric choice can affect the relative emphasis placed on reductions of ‘cumulative climate pollutants’ such as carbon dioxide versus ‘short-lived climate pollutants’ (SLCPs), including methane and black carbon2, 3, 4, 5, 6. Here we show that the widely used 100-year global warming potential (GWP100) effectively measures the relative impact of both cumulative pollutants and SLCPs on realized warming 20–40 years after the time of emission. If the overall goal of climate policy is to limit peak warming, GWP100 therefore overstates the importance of current SLCP emissions unless stringent and immediate reductions of all climate pollutants result in temperatures nearing their peak soon after mid-century7, 8, 9, 10, which may be necessary to limit warming to “well below 2 °C” (ref. 1). The GWP100 can be used to approximately equate a one-off pulse emission of a cumulative pollutant and an indefinitely sustained change in the rate of emission of an SLCP11, 12, 13. The climate implications of traditional CO2-equivalent targets are ambiguous unless contributions from cumulative pollutants and SLCPs are specified separately.

Attribution analysis of high precipitation events in summer in England and Wales over the last decade

CLIMATIC CHANGE 132 (2015) 77-91

FEL Otto, SM Rosier, MR Allen, NR Massey, CJ Rye, JI Quintana

Model structure in observational constraints on transient climate response

CLIMATIC CHANGE 131 (2015) 199-211

RJ Millar, A Otto, PM Forster, JA Lowe, WJ Ingram, MR Allen

Model structure in observational constraints on transient climate response

Climatic Change 131 (2015) 199-211

RJ Millar, A Otto, PM Forster, JA Lowe, WJ Ingram, MR Allen

© 2015, Springer Science+Business Media Dordrecht. The transient climate response (TCR) is a highly policy-relevant quantity in climate science. We show that recent revisions to TCR in the IPCC 5th Assessment Report have more impact on projections over the next century than revisions to the equilibrium climate sensitivity (ECS). While it is well known that upper bounds on ECS are dependent on model structure, here we show that the same applies to TCR. Our results use observations of the planetary energy budget, updated radiative forcing estimates and a number of simple climate models. We also investigate the ratio TCR:ECS, or realised warming fraction (RWF), a highly policy-relevant quantity. We show that global climate models (GCMs) don’t sample a region of low TCR and high RWF consistent with observed climate change under all simple models considered. Whether the additional constraints from GCMs are sufficient to rule out these low climate responses is a matter for further research.

weather@homedevelopment and validation of a very large ensemble modelling system for probabilistic event attribution


N Massey, R Jones, FEL Otto, T Aina, S Wilson, JM Murphy, D Hassell, YH Yamazaki, MR Allen

Anthropogenic influence on the changing likelihood of an exceptionally warm summer in Texas, 2011


DE Rupp, S Li, N Massey, SN Sparrow, PW Mote, M Allen

Reply to 'Drivers of the 2013/14 winter floods in the UK'

NATURE CLIMATE CHANGE 5 (2015) 491-492

C Huntingford, T Marsh, AA Scaife, EJ Kendon, J Hannaford, AL Kay, M Lockwood, C Prudhomme, NS Reynard, S Parry, JA Lowe, JA Screen, HC Ward, M Roberts, PA Stott, VA Bell, M Bailey, A Jenkins, T Legg, FEL Otto, N Massey, N Schaller, J Slingo, MR Allen



N Schaller, FEL Otto, GJ van Oldenborgh, NR Massey, S Sparrow, MR Allen

Potential influences on the United Kingdom's floods of winter 2013/14

NATURE CLIMATE CHANGE 4 (2014) 769-777

C Huntingford, T Marsh, AA Scaife, EJ Kendon, J Hannaford, AL Kay, M Lockwood, C Prudhomme, NS Reynard, S Parry, JA Lowe, JA Screen, HC Ward, M Roberts, PA Stott, VA Bell, M Bailey, A Jenkins, T Legg, FEL Otto, N Massey, N Schaller, J Slingo, MR Allen

Broad range of 2050 warming from an observationally constrained large climate model ensemble

Nature Geoscience 5 (2012) 256-260

DJ Rowlands, DJ Frame, D Ackerley, T Aina, BBB Booth, C Christensen, M Collins, N Faull, CE Forest, BS Grandey, E Gryspeerdt, EJ Highwood, WJ Ingram, S Knight, A Lopez, N Massey, F McNamara, N Meinshausen, C Piani, SM Rosier, BM Sanderson, LA Smith, DA Stone, M Thurston, K Yamazaki, Y Hiro Yamazaki, MR Allen

Incomplete understanding of three aspects of the climate system-equilibrium climate sensitivity, rate of ocean heat uptake and historical aerosol forcing-and the physical processes underlying them lead to uncertainties in our assessment of the global-mean temperature evolution in the twenty-first century 1,2. Explorations of these uncertainties have so far relied on scaling approaches 3,4, large ensembles of simplified climate models 1,2, or small ensembles of complex coupled atmosphere-ocean general circulation models 5,6 which under-represent uncertainties in key climate system properties derived from independent sources 7,9. Here we present results from a multi-thousand-member perturbed-physics ensemble of transient coupled atmosphere-ocean general circulation model simulations. We find that model versions that reproduce observed surface temperature changes over the past 50 years show global-mean temperature increases of 1.4-3 K by 2050, relative to 1961-1990, under a mid-range forcing scenario. This range of warming is broadly consistent with the expert assessment provided by the Intergovernmental Panel on Climate Change Fourth Assessment Report, but extends towards larger warming than observed in ensembles-of-opportunity 5 typically used for climate impact assessments. From our simulations, we conclude that warming by the middle of the twenty-first century that is stronger than earlier estimates is consistent with recent observed temperature changes and a mid-range 'no mitigation' scenario for greenhouse-gas emissions. © 2012 Macmillan Publishers Limited. All rights reserved.

Reconciling two approaches to attribution of the 2010 Russian heat wave

Geophysical Research Letters 39 (2012)

FEL Otto, N Massey, GJ Van Oldenborgh, RG Jones, MR Allen

In the summer 2010 Western Russia was hit by an extraordinary heat wave, with the region experiencing by far the warmest July since records began. Whether and to what extent this event is attributable to anthropogenic climate change is controversial. Dole et al. (2011) report the 2010 Russian heat wave was "mainly natural in origin" whereas Rahmstorf and Coumou (2011) write that with a probability of 80% "the 2010 July heat record would not have occurred" without the large-scale climate warming since 1980, most of which has been attributed to the anthropogenic increase in greenhouse gas concentrations. The latter explicitly state that their results "contradict those of Dole et al. (2011)." Here we use the results from a large ensemble simulation experiment with an atmospheric general circulation model to show that there is no substantive contradiction between these two papers, in that the same event can be both mostly internally-generated in terms of magnitude and mostly externally-driven in terms of occurrence-probability. The difference in conclusion between these two papers illustrates the importance of specifying precisely what question is being asked in addressing the issue of attribution of individual weather events to external drivers of climate. Copyright 2012 by the American Geophysical Union.



TC Peterson, PA Stott, S Herring, FW Zwiers, GC Hegerl, S-K Min, X Zhang, GJ van Oldenborgh, A van Urk, M Allen, C Funk, DE Rupp, PW Mote, N Massey, CJ Rye, R Jones, J Cattiaux, P Yiou, N Massey, T Aina, FEL Otto, S Wilson, RG Jones, N Christidis

Reconciling two approaches to attribution of the 2010 Russian heat wave

Geophysical Research Letters 39 (2012)

FEL Otto, N Massey, GJ Van Oldenborgh, RG Jones, MR Allen

Quantifying uncertainty in future Southern Hemisphere circulation trends

Geophysical Research Letters 39 (2012)

PAG Watson, DJ Karoly, MR Allen, N Faull, DS Lee

The Antarctic polar night jet has intensified during spring in recent decades due to stratospheric ozone depletion and rising greenhouse gas (GHG) concentrations and this has had substantial effects on the region's climate. GHG concentrations will rise over the 21st century whereas stratospheric ozone is expected to recover and there is uncertainty in future southern hemisphere (SH) circulation trends. We examine sensitivity to the physics parameterisation of the 21st century SH circulation projection of a coupled atmosphere-ocean General Circulation Model and the sensitivity of the contribution from stratospheric ozone recovery. Different model parameterizations give a greater range of future trends in the position of the tropospheric jet than has been found in previous multi-model comparisons. Ozone recovery causes a weakening and northward shift of the DJF tropospheric jet. Varying the physics parameterization affects the zonal wind response to ozone recovery of the SON stratosphere by ∼10% and that of the DJF troposphere by ∼25%. The projected future SAM index changes with and without ozone recovery and the SAM index response to ozone recovery alone are found to be strongly positively correlated with projected 21st century global warming. © 2012. American Geophysical Union. All Rights Reserved.

Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000.

Nature 470 (2011) 382-385

P Pall, T Aina, DA Stone, PA Stott, T Nozawa, AG Hilberts, D Lohmann, MR Allen

Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion (refs 5, 6). Although the flooding was deemed a 'wake-up call' to the impacts of climate change at the time, such claims are typically supported only by general thermodynamic arguments that suggest increased extreme precipitation under global warming, but fail to account fully for the complex hydrometeorology associated with flooding. Here we present a multi-step, physically based 'probabilistic event attribution' framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000. Using publicly volunteered distributed computing, we generate several thousand seasonal-forecast-resolution climate model simulations of autumn 2000 weather, both under realistic conditions, and under conditions as they might have been had these greenhouse gas emissions and the resulting large-scale warming never occurred. Results are fed into a precipitation-runoff model that is used to simulate severe daily river runoff events in England and Wales (proxy indicators of flood events). The precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.