Publications by Myles Allen

The blame game.

Nature 432 (2004) 551-552

MR Allen, R Lord

Detection and attribution of changes in 20th century land precipitation

Geophysical Research Letters 31 (2004) L10203 1-4-

FH Lambert, PA Stott, MR Allen, MA Palmer

Observed globally-averaged land precipitation changes over the 20th century are compared with simulations of the HadCM3 climate model using an "optimal fingerprinting" method. We find that observed changes in precipitation are too large to be consistent with model-generated internal variability and are consistent with (attributable to) the combination of natural and anthropogenic forcings applied to the model. By comparing precipitation observations to shortwave and longwave forcing timeseries, we find that most of the forced variation in precipitation appears to be driven by natural shortwave forcings. We are unable to detect a response to anthropogenic longwave forcings in isolation. Finally, we seek to explain these results in terms of perturbations to the energy budget of the troposphere. Copyright 2004 by the American Geophysical Union.

Solar forcing of climate: model results

ADV SPACE RES 34 (2004) 343-348

MA Palmer, LJ Gray, MR Allen, WA Norton

The role of stratospheric ozone changes in determining the climate response to solar forcing is investigated using a version of the Unified Model from the UK Meteorological Office which includes a mixed-layer ocean of constant depth (HadSM3). Two experiments have been performed, both of which include a wavelength-dependent reduction in total solar irradiance (TSI) of 7.5 W m(-2) (0.55%). The second experiment includes, in addition, an estimate of the accompanying stratospheric ozone changes. A large change in TSI is used (approximately a factor of two greater than the 'best-guess' change between present mean levels and the Maunder Minimum) to demonstrate the sensitivity of the climate system to this forcing. Results show that in the annual mean, the temperature response of the model is enhanced by the inclusion of the ozone changes, by approximately 15-20%. We compare results from our TSI and ozone experiment to those of Shindell et al. [Science 294 (2001) 2149] who performed a similar study with the GISS GCM. Temperature changes are greater in our simulation, as expected from the larger magnitude forcing, however the circulation response is very different: our results do not resemble the Arctic Oscillation, whilst those of Shindell et al. [loc. cit.] project strongly onto this leading mode of variability. The lack of a fully resolved stratosphere in our model is a potential reason for this distinction. To test this possibility, we repeated our combined irradiance and ozone experiment using identical model formulations, but with different vertical extents: the first extends to a height of 5 hPa, the second to 0.01 hPa. Both simulations produce a relatively weak surface pressure response to solar forcing that does not strongly resemble the Arctic Oscillation. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Probable causes of late twentieth century tropospheric temperature trends

Climate Dynamics 21 (2003) 573-591

PW Thorne, PW Thorne, PD Jones, SFB Tett, MR Allen, DE Parker, PA Stott, GS Jones, TJ Osborn, TD Davies

We assess the most probable causes of late twentieth century (1960-1994) tropospheric temperature changes. Optimal detection techniques are used to compare observed spatio-temporal patterns of near-surface and tropospheric temperature change with results from experiments performed with two different versions of the Hadley Centre climate model. We detect anthropogenic forcings, particularly well-mixed greenhouse-gases, with a less certain sulfate aerosol cooling influence. More limited evidence exists for a detectable volcanic influence. Our principal results do not depend upon the choice of model. Both models, but particularly HadCM3, appear to overestimate the simulated climate response to greenhouse gases (especially at the surface) and volcanoes. This result may arise, at least in part, due to errors in the forcings (especially sulfate) and technical details of our approach, which differs from previous studies. We use corrected and uncorrected versions of the radiosonde record to assess sensitivity of our detection results to observational uncertainties. We find that previous corrections applied to the radiosonde temperature record are likely to have been sub-optimal in only taking into account temporal consistency. However, the choice of corrected or uncorrected version has no systematic effect upon our main conclusions. We show that both models are potentially internally consistent explanations of observed tropospheric temperatures.

Estimating signal amplitudes in optimal fingerprinting, part I: theory

Climate Dynamics 21 (2003) 477-491

MR Allen, Peter A. Stott

Modelling the atmospheric response to doubled CO2 and depleted stratospheric ozone using a stratosphere-resolving coupled GCM

Quarterly Journal of the Royal Meteorological Society 129 (2003) 947-966

NP Gillett, MR Allen, KD Williams

We investigate the atmospheric response to doubled CO2 and stratospheric ozone depletion in three versions of a general-circulation model with differing vertical resolution and upper-boundary heights. We find that an approximate doubling of the vertical resolution below 10 hPa reduces the temperature response to a doubling of CO2 from 3.4 K to 2.5 K. Much of this difference is associated with changes in the cloud response. All model versions show an increase in the Arctic Oscillation index in response to a doubling of CO2, but the increase is no larger in the model with an upper boundary at 0.01 hPa than in the standard model with a top level at 5 hPa. All models also show general stratospheric cooling in response to doubling CO2. However, unlike some other authors, we find no cooling in the Arctic winter vortex below around 10 hPa in the stratosphere-resolving model, and a weakening of the zonal winds throughout this region. This effect is due to enhanced upward propagation of planetary waves from the troposphere, and is an effect found only in the northern hemisphere, probably because of its larger zonal asymmetries. All models show a small but significant surface cooling in response to a reconstruction of 1998 stratospheric ozone depletion, and an increase in the Antarctic Oscillation index in the southern summer. The cooling extends through most of the atmosphere, and reaches a maximum in the region of the Antarctic ozone hole in November and December.

Climate forecasting: possible or probable?

Nature 425 (2003) 242-

MR Allen

Assessing the relative roles of initial and boundary conditions in interannual to decadal climate predictability

JOURNAL OF CLIMATE 15 (2002) 3104-3109

M Collins, MR Allen

The role of stratospheric resolution in simulating the Arctic Oscillation response to greenhouse gases

Geophysical Research Letters 29 (2002) 138-1 - 138-4-

NP Gillett, MR Allen, KD Williams

The Arctic Oscillation index has increased significantly over the past forty years, and such an increase has been simulated in response to greenhouse gas increases in several climate models. However, it has been suggested that an atmospheric model with an upper boundary in the upper stratosphere or mesosphere is required to simulate a realistic response, and that predictions made with standard climate models are hence unreliable. Here we show that a climate model with a 30-km upper boundary shows no increase in its surface Arctic Oscillation response to doubled carbon dioxide when its upper boundary is raised to 80 km. Neither model version shows a significant Arctic Oscillation response to stratospheric ozone depletion.

How linear is the Arctic Oscillation response to greenhouse gases?


NP Gillett, MR Allen, RE McDonald, CA Senior, DT Shindell, GA Schmidt

Quantifying uncertainties in climate system properties with the use of recent climate observations

SCIENCE 295 (2002) 113-117

CE Forest, PH Stone, AP Sokolov, MR Allen, MD Webster

Constraints on future changes in climate and the hydrologic cycle.

Nature 419 (2002) 224-232

MR Allen, WJ Ingram

What can we say about changes in the hydrologic cycle on 50-year timescales when we cannot predict rainfall next week? Eventually, perhaps, a great deal: the overall climate response to increasing atmospheric concentrations of greenhouse gases may prove much simpler and more predictable than the chaos of short-term weather. Quantifying the diversity of possible responses is essential for any objective, probability-based climate forecast, and this task will require a new generation of climate modelling experiments, systematically exploring the range of model behaviour that is consistent with observations. It will be substantially harder to quantify the range of possible changes in the hydrologic cycle than in global-mean temperature, both because the observations are less complete and because the physical constraints are weaker.

Assessing the robustness of zonal mean climate change detection

Geophysical Research Letters 29 (2002) 26-21

PW Thorne, PD Jones, TJ Osborn, TD Davies, SFB Tett, DE Parker, PA Stott, GS Jones, MR Allen

We assess the robustness of previous optimal detection and attribution studies considering zonal-mean temperatures. Principal results, which have consistently pointed towards a demonstrable anthropogenic influence on recently observed upper air temperatures, are confirmed. Importantly our detection results are not critically dependent on the inclusion of stratospheric as well as tropospheric temperatures. We find that detection is dependent on input field pre-processing choices, and on the choice of detection algorithm. There are a number of cases where either no signals are detected, or results fail a consistency test.

Reconciling two approaches to the detection of anthropogenic influence on climate

Journal of Climate 15 (2002) 326-329

NP Gillett, GC Hegerl, MR Allen, PA Stott, R Schnur

Anthropogenic influences on surface temperature over the second half of the twentieth century are examined using output from two general circulation models (HadCM2 and ECHAM3). Optimal detection techniques involve the comparison of observed temperature changes with those simulated by a climate model, using a control integration to test the null hypothesis that all the observed changes are due to natural variability. Two recent studies have examined the influence of greenhouse gases and the direct effect of sulfate aerosol on surface temperature using output from the same two climate models but with many differences in the methods applied. Both detected overall anthropogenic influence on climate, but results on the separate detection of greenhouse gas and sulfate aerosol influences were different. This paper concludes that the main differences between the results can be explained by the season over which temperatures were averaged, the length of the climatology from which anomalies were taken, and the use of a time-evolving signal pattern as opposed to a spatial pattern of temperature trends. This demonstration of consistency increases confidence in the equivalence of the methodologies in other respects, and helps to synthesize results from the two approaches. Including information on the temporal evolution of the response to different forcings allows sulfate aerosol influence to be detected more easily in HadCM2, whereas focusing on spatial patterns gives better detectability in ECHAM3.

Detecting anthropogenic influence with a multi-model ensemble


NP Gillett, FW Zwiers, AJ Weaver, GC Hegerl, MR Allen, PA Stott

Sensitivity analysis of the climate of a chaotic ocean circulation model

Quarterly Journal of the Royal Meteorological Society 128 (2002) 2587-2605

DJ Lea, TWN Haine, MR Allen, JA Hansen

We explore sensitivity analyses of ocean circulation models by comparing the adjoint and direct-perturbation methods. We study the sensitivity of time-averaged inter-gyre vorticity transport to the imposed wind-stress curl in an eddy-permitting reduced-gravity ocean of a double gyre. Two regimes exist: a non-chaotic regime for low wind-stress curl, and a chaotic regime for stronger wind forcing. Direct-perturbation methods are found to converge, with increasing integration time, to a stable 'climate' sensitivity in both the chaotic and non-chaotic regimes. The adjoint method converges in the non-chaotic regime but diverges in the chaotic regime. The divergence of adjoint sensitivity in the chaotic regime is directly related to the chaotic divergence of solution trajectories through phase-space. Thus, standard adjoint sensitivity methods cannot be used to estimate climate sensitivity in chaotic ocean circulation models. An alternative method using an ensemble of adjoint calculations is explored. This is found to give estimates of the climate sensitivity of the time-mean vorticity transport with O(25%) error or less for integration times ranging from one month to one year. The ensemble-adjoint method is particularly useful when one wishes to produce a map of sensitivities (for example, the sensitivity of the advective vorticity transport to wind stress at every point in the domain) as direct sensitivity calculations for each point in the map are avoided. However, an ensemble-adjoint of the variance of the vorticity transport to wind-stress curl fails to estimate the climate sensitivity. We conclude that the most reliable method of determining the climate sensitivity is the direct-perturbation method, but ensemble-adjoint techniques may be of use in some problems.

Towards objective probabalistic climate forecasting.

Nature 419 (2002) 228-

MR Allen, DA Stainforth

Estimation of natural and anthropogenic contributions to twentieth century temperature change

Journal of Geophysical Research Atmospheres 107 (2002)

SFB Tett, GS Jones, PA Stott, DC Hill, JFB Mitchell, MR Allen, WJ Ingram, TC Johns, CE Johnson, A Jones, DL Roberts, DMH Sexton, MJ Woodage

Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well-mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an "optimal detection" methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.

Evidence for nonlinearity in observed stratospheric circulation changes

Journal of Geophysical Research Atmospheres 106 (2001) 7891-7901

NP Gillett, NP Gillett, MP Baldwin, MP Baldwin, MR Allen, MR Allen

The leading mode of variability of the lower atmosphere circulation in the Northern Hemisphere is a largely zonally symmetric mode known as the Arctic Oscillation. We calculate Arctic Oscillation (AO) indices on a range of levels from 1000 to 10 hPa by means of a principal component analysis of National Centers for Environmental Prediction daily geopotential height anomalies. We find the apparent downward propagation of anomalies noted by other authors to be statistically significant compared to a red noise model. By examining histograms of these indices for each month, we note that the distribution of the index is generally close to Gaussian in the troposphere. In the stratosphere, however, the index is negatively skewed in the winter and positively skewed in the spring. We conclude that the positive skewness in April results from the coexistence of distinct summer and winter circulation states, and by examining polar stratospheric temperatures, we conclude that the negative skewness in January may be due to the radiatively determined limit on the vortex strength. This radiative limit responds relatively slowly to anthropogenic forcing, whereas changes in planetary wave forcing could have a much faster impact on the number of warm events. This suggests a hypothesis that the vortex strength may respond nonlinearly to anthropogenic forcing, which is supported by an observed change in the shape of the histograms of 20-200 hPa AO indices in January over the past 40 years. Copyright 2001 by the American Geophysical Union.

Allowing for solar forcing in the detection of human influence on tropospheric temperatures


DC Hill, MR Allen, PA Stott