User:William M. Connolley/References on attribution

Note: these refs are for my use. If you want to use them, feel free. If you know others I've missed, add a brief note on this page.

nb: I have not read most of these papers.

Stott PA, Tett SFB, Jones GS, Allen MR, Ingram WJ, Mitchell JFB (2000) Anthropogenic And Natural Causes Of Twentieth Century Temperature Change. SPACE SCIENCE REVIEWS 94 (1-2): 337-344 NOV 2000

We analyse spatio-temporal patterns of near-surface temperature change to provide an attribution of twentieth century climate change. We apply an "optimal detection" methodology to seasonal and annual data averaged over a range of spatial and temporal scales. We find that solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects.

Allen MR, Stott PA (2003) Estimating Signal Amplitudes In Optimal Fingerprinting , Part I : Theory. CLIMATE DYNAMICS 21 (5-6): 477-491 NOV 2003

There is increasingly clear evidence that human influence has contributed substantially to the large-scale climatic changes that have occurred over the past few decades. Attention is now turning to the physical implications of the emerging anthropogenic signal. Of particular interest is the question of whether current climate models may be over- or under-estimating the amplitude of the climate system's response to external forcing, including anthropogenic. Evidence of a significant error in a model-simulated response amplitude would indicate the existence of amplifying or damping mechanisms that are inadequately represented in the model. The range of uncertainty in the factor by which we can scale model-simulated changes while remaining consistent with observed change provides an estimate of uncertainty in model-based predictions. With any model that displays a realistic level of internal variability, the problem of estimating this factor is complicated by the fact that it represents a ratio between two incompletely known quantities: both observed and simulated responses are subject to sampling uncertainty, primarily due to internal chaotic variability. Sampling uncertainty in the simulated response can be reduced, but not eliminated, through ensemble simulations. Accurate estimation of these scaling factors requires a modification of the standard 'optimal fingerprinting' algorithm for climate change detection, drawing on the conventional 'total least squares' approach discussed in the statistical literature. Code for both variants of optimal fingerprinting can be found on http://www.climateprediction.net/detection.

Stott PA, Allen MR, Jones GS (2003) Estimating Signal Amplitudes In Optimal Fingerprinting . Part Ii : Application To General Circulation Models. CLIMATE DYNAMICS 21 (5-6): 493-500 NOV 2003

We show that there is a significant low bias in standard estimates of the amplitudes of climate change signals estimated by small ensembles of coupled ocean atmosphere general circulation models. This bias can be eliminated either by making larger ensembles of at least eight members or by employing total least squares regression (TLS) to take account of sampling uncertainty in model-simulated signals. Results using TLS agree with previous work using ordinary least squares regression (OLS) in showing that recent interdecadal warming trends in near-surface temperature are largely anthropogenic in origin. Consistent with previous results, we detect evidence of solar influence on surface temperature changes in the first half of the twentieth century. However the amplitudes of model-predicted signals in the observed record were previously underestimated by ordinary least squares regression. This implies that over the last 30 years the observations are consistent with a greater degree of greenhouse warming and sulfate cooling than previously thought and the early century warming is consistent with a greatly enhanced model response to solar changes with very little contribution from anthropogenic causes. The model-simulated response to solar forcing is, however, relatively weak and subject to large uncertainties. Contributions of both anthropogenic and natural forcings to the early century warming are therefore very poorly constrained.

Gillett NP, Zwiers FW, Weaver AJ, Stott PA (2003) Detection Of Human Influence On Sea - Level Pressure. NATURE 422 (6929): 292-294 MAR 20 2003

Greenhouse gases and tropospheric sulphate aerosols-the main human influences on climate-have been shown to have had a detectable effect on surface air temperature, the temperature of the free troposphere and stratosphere and ocean temperature Nevertheless, the question remains as to whether human influence is detectable in any variable other than temperature. Here we detect an influence of anthropogenic greenhouse gases and sulphate aerosols in observations of winter sea-level pressure (December to February), using combined simulations from four climate models. We find increases in sea-level pressure over the subtropical North Atlantic Ocean, southern Europe and North Africa, and decreases in the polar regions and the North Pacific Ocean, in response to human influence. Our analysis also indicates that the climate models substantially underestimate the magnitude of the sea-level pressure response. This discrepancy suggests that the upward trend in the North Atlantic Oscillation index (corresponding to strengthened westerlies in the North Atlantic region), as simulated in a number of global warming scenarios, may be too small, leading to an underestimation of the impacts of anthropogenic climate change on European climate.

Stott PA (2003) Attribution Of Regional - Scale Temperature Changes To Anthropogenic And Natural Causes. GEOPHYSICAL RESEARCH LETTERS 30 (14): art. no. 1728 JUL 16 2003

The causes of twentieth century temperature change in six separate land areas of the Earth have been determined by carrying out a series of optimal detection analyses. The warming effects of increasing greenhouse gas concentrations have been detected in all the regions examined, including North America and Europe. In most regions, cooling from sulfate aerosols counteracts some of the greenhouse warming, and there is some evidence for reduced net aerosol cooling in Asia, possibly as a result of warming from black carbon.

Stott PA, Jones GS, Mitchell JFB (2003) Do Models Underestimate The Solar Contribution To Recent Climate Change ?. JOURNAL OF CLIMATE 16 (24): 4079-4093 DEC 2003

Current attribution analyses that seek to determine the relative contributions of different forcing agents to observed near-surface temperature changes underestimate the importance of weak signals, such as that due to changes in solar irradiance. Here a new attribution method is applied that does not have a systematic bias against weak signals. It is found that current climate models underestimate the observed climate response to solar forcing over the twentieth century as a whole, indicating that the climate system has a greater sensitivity to solar forcing than do models. The results from this research show that increases in solar irradiance are likely to have had a greater influence on global-mean temperatures in the first half of the twentieth century than the combined effects of changes in anthropogenic forcings. Nevertheless the results confirm previous analyses showing that greenhouse gas increases explain most of the global warming observed in the second half of the twentieth century.

Karoly DJ, Braganza K, Stott PA, Arblaster JM, Meehl GA, Broccoli AJ, Dixon KW (2003) Detection Of A Human Influence On North American Climate. SCIENCE 302 (5648): 1200-1203 NOV 14 2003

Several indices of large-scale patterns of surface temperature variation were used to investigate climate change in North America over the 20th century. The observed variability of these indices was simulated well by a number of climate models. Comparison of index trends in observations and model simulations shows that North American temperature changes from 1950 to 1999 were unlikely to be due to natural climate variation alone. Observed trends over this period are consistent with simulations that include anthropogenic forcing from increasing atmospheric greenhouse gases and sulfate aerosols. However, most of the observed warming from 1900 to 1949 was likely due to natural climate variation.

Thorne PW, Jones PD, Tett SFB, Allen MR, Parker DE, Stott PA, Jones GS, Osborn TJ, Davies TD (2003) Probable Causes Of Late Twentieth Century Tropospheric Temperature Trends. CLIMATE DYNAMICS 21 (7-8): 573-591 DEC 2003

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.

Thorne PW, Jones PD, Osborn TJ, Davies TD, Tett SFB, Parker DE, Stott PA, Jones GS, Allen MR (2002) Assessing The Robustness Of Zonal Mean Climate Change Detection. GEOPHYSICAL RESEARCH LETTERS 29 (19): art. no. 1920 OCT 1 2002

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.

Braganza, K., D.J. Karoly, A.C. Hirst, P. Stott, R.J. Stouffer and S.F.B. Tett (2004), Simple Indices Of Global Climate Variability And Change - Part Ii : Attribution Of Climate Change During The Twentieth Century, Clim . Dyn . 22 ( 8 ): 823 - 838 Jul 2004

Five simple indices of surface temperature are used to investigate the influence of anthropogenic and natural (solar irradiance and volcanic aerosol) forcing on observed climate change during the twentieth century. These indices are based on spatial fingerprints of climate change and include the global-mean surface temperature, the land-ocean temperature contrast, the magnitude of the annual cycle in surface temperature over land, the Northern Hemisphere meridional temperature gradient and the hemispheric temperature contrast. The indices contain information independent of variations in global-mean temperature for unforced climate variations and hence, considered collectively, they are more useful in an attribution study than global mean surface temperature alone. Observed linear trends over 1950-1999 in all the indices except the hemispheric temperature contrast are significantly larger than simulated changes due to internal variability or natural (solar and volcanic aerosol) forcings and are consistent with simulated changes due to anthropogenic (greenhouse gas and sulfate aerosol) forcing. The combined, relative influence of these different forcings on observed trends during the twentieth century is investigated using linear regression of the observed and simulated responses of the indices. It is found that anthropogenic forcing accounts for almost all of the observed changes in surface temperature during 1946-1995. We found that early twentieth century changes (1896-1945) in global mean temperature can be explained by a combination of anthropogenic and natural forcing, as well as internal climate variability. Estimates of 'scaling factors' that weight the amplitude of model simulated signals to corresponding observed changes using a combined normalized index are similar to those calculated using more complex, optimal fingerprint techniques.

Christidis, N., P.A. Stott, S. Brown, G.C. Hegerl and J. Caesar (2005), Detection Of Changes In Temperature Extremes During The Second Half Of The 20th Century, Geophys . Res . Lett . 32 ( 20 ): Art . No . L20716 Oct 26 2005

Since 1950, the warmest and coldest days and nights of the year have become warmer. Comparing these observations with climate model simulations in an optimal detection analysis shows a significant human influence on patterns of change in extremely warm nights. Human influence on cold nights and days is also detected, although less robustly, but there is no detection of a significant human influence on extremely warm days. In the future , extreme temperatures are expected to intensify considerably, with adverse consequences for human health.

Santer, B.D., T.M.L. Wigley, A.J. Simmons, P.W. Kallberg, G.A. Kelly, S.M. Uppala and C. Ammann et al. (2004), Identification Of Anthropogenic Climate Change Using A Second - Generation Reanalysis, J . Geophys . Res . 109 ( D21 ): Art . No . D21104 Nov 4 2004

[ 1] Changes in the height of the tropopause provide a sensitive indicator of human effects on climate. A previous attempt to identify human effects on tropopause height relied on information from 'first-generation' reanalyses of past weather observations. Climate data from these initial model-based reanalyses have well-documented deficiencies, raising concerns regarding the robustness of earlier detection work that employed these data. Here we address these concerns using information from the new second-generation ERA-40 reanalysis. Over 1979 to 2001, tropopause height increases by nearly 200 m in ERA-40, partly due to tropospheric warming. The spatial pattern of height increase is consistent with climate model predictions of the expected response to anthropogenic influences alone, significantly strengthening earlier detection results. Atmospheric temperature changes in two different satellite data sets are more highly correlated with changes in ERA-40 than with those in a first-generation reanalysis, illustrating the improved quality of temperature information in ERA-40. Our results provide support for claims that human activities have warmed the troposphere and cooled the lower stratosphere over the last several decades of the 20th century, and that both of these changes in atmospheric temperature have contributed to an overall increase in tropopause height.