Publications by Myles Allen

Finding Ocean States That Are Consistent with Observations from a Perturbed Physics Parameter Ensemble

JOURNAL OF CLIMATE 31 (2018) 4639-4656

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

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


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

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


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

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%.

Warming caused by cumulative carbon emissions towards the trillionth tonne.

Nature 458 (2009) 1163-1166

MR Allen, DJ Frame, C Huntingford, CD Jones, JA Lowe, M Meinshausen, N Meinshausen

Global efforts to mitigate climate change are guided by projections of future temperatures. But the eventual equilibrium global mean temperature associated with a given stabilization level of atmospheric greenhouse gas concentrations remains uncertain, complicating the setting of stabilization targets to avoid potentially dangerous levels of global warming. Similar problems apply to the carbon cycle: observations currently provide only a weak constraint on the response to future emissions. Here we use ensemble simulations of simple climate-carbon-cycle models constrained by observations and projections from more comprehensive models to simulate the temperature response to a broad range of carbon dioxide emission pathways. We find that the peak warming caused by a given cumulative carbon dioxide emission is better constrained than the warming response to a stabilization scenario. Furthermore, the relationship between cumulative emissions and peak warming is remarkably insensitive to the emission pathway (timing of emissions or peak emission rate). Hence policy targets based on limiting cumulative emissions of carbon dioxide are likely to be more robust to scientific uncertainty than emission-rate or concentration targets. Total anthropogenic emissions of one trillion tonnes of carbon (3.67 trillion tonnes of CO(2)), about half of which has already been emitted since industrialization began, results in a most likely peak carbon-dioxide-induced warming of 2 degrees C above pre-industrial temperatures, with a 5-95% confidence interval of 1.3-3.9 degrees C.

A review of uncertainties in global temperature projections over the twenty-first century

Journal of Climate 21 (2008) 2651-2663

R Knutti, R Knutti, MR Allen, P Friedlingstein, JM Gregory, JM Gregory, GC Hegerl, GA Meehl, M Meinshausen, JM Murphy, GK Plattner, GK Plattner, SCB Raper, TF Stocker, PA Stott, H Teng, TML Wigley

Quantification of the uncertainties in future climate projections is crucial for the implementation of climate policies. Here a review of projections of global temperature change over the twenty-first century is provided for the six illustrative emission scenarios from the Special Report on Emissions Scenarios (SRES) that assume no policy intervention, based on the latest generation of coupled general circulation models, climate models of intermediate complexity, and simple models, and uncertainty ranges and probabilistic projections from various published methods and models are assessed. Despite substantial improvements in climate models, projections for given scenarios on average have not changed much in recent years. Recent progress has, however, increased the confidence in uncertainty estimates and now allows a better separation of the uncertainties introduced by scenarios, physical feedbacks, carbon cycle, and structural uncertainty. Projection uncertainties are now constrained by observations and therefore consistent with past observed trends and patterns. Future trends in global temperature resulting from anthropogenic forcing over the next few decades are found to be comparably well constrained. Uncertainties for projections on the century time scale, when accounting for structural and feedback uncertainties, are larger than captured in single models or methods. This is due to differences in the models, the sources of uncertainty taken into account, the type of observational constraints used, and the statistical assumptions made. It is shown that as an approximation, the relative uncertainty range for projected warming in 2100 is the same for all scenarios. Inclusion of uncertainties in carbon cycle-climate feedbacks extends the upper bound of the uncertainty range by more than the lower bound. © 2008 American Meteorological Society.

Towards constraining climate sensitivity by linear analysis of feedback patterns in thousands of perturbed-physics GCM simulations

Climate Dynamics 30 (2008) 175-190

BM Sanderson, C Piani, WJ Ingram, DA Stone, MR Allen

A linear analysis is applied to a multi-thousand member "perturbed physics" GCM ensemble to identify the dominant physical processes responsible for variation in climate sensitivity across the ensemble. Model simulations are provided by the distributed computing project, climate A principal component analysis of model radiative response reveals two dominant independent feedback processes, each largely controlled by a single parameter change. The leading EOF was well correlated with the value of the entrainment coefficient - a parameter in the model's atmospheric convection scheme. Reducing this parameter increases high vertical level moisture causing an enhanced clear sky greenhouse effect both in the control simulation and in the response to greenhouse gas forcing. This effect is compensated by an increase in reflected solar radiation from low level cloud upon warming. A set of 'secondary' cloud formation parameters partly modulate the degree of shortwave compensation from low cloud formation. The second EOF was correlated with the scaling of ice fall speed in clouds which affects the extent of cloud cover in the control simulation. The most prominent feature in the EOF was an increase in longwave cloud forcing. The two leading EOFs account for 70% of the ensemble variance in λ - the global feedback parameter. Linear predictors of feedback strength from model climatology are applied to observational datasets to estimate real world values of the overall climate feedback parameter. The predictors are found using correlations across the ensemble. Differences between predictions are largely due to the differences in observational estimates for top of atmosphere shortwave fluxes. Our validation does not rule out all the strong tropical convective feedbacks leading to a large climate sensitivity. © Springer-Verlag 2007.