Gareth S. Jones

Quantification of modelling uncertainties in a large ensemble of climate change simulations.

Comprehensive global climate models are the only tools that account for the complex set of processes which will determine future climate change at both a global and regional level. Planners are typically faced with a wide range of predicted changes from different models of unknown relative quality, owing to large but unquantified uncertainties in the modelling process. Here we report a systematic attempt to determine the range of climate changes consistent with these uncertainties, based on a 53-member ensemble of model versions constructed by varying model parameters. We estimate a probability density function for the sensitivity of climate to a doubling of atmospheric carbon dioxide levels, and obtain a 5–95 per cent probability range of 2.4–5.4 °C. Our probability density function is constrained by objective estimates of the relative reliability of different model versions, the choice of model parameters that are varied and their uncertainty ranges, specified on the basis of expert advice. Our ensemble produces a range of regional changes much wider than indicated by traditional methods based on scaling the response patterns of an individual simulation.

Attribution of twentieth century temperature change to natural and anthropogenic causes

Abstract  We analyse possible causes of twentieth century near-surface temperature change. We use an “optimal detection” methodology to compare seasonal and annual data from the coupled atmosphere-ocean general circulation model HadCM2 with observations averaged over a range of spatial and temporal scales. The results indicate that the increases in temperature observed in the latter half of the century have been caused by warming from anthropogenic increases in greenhouse gases offset by cooling from tropospheric sulfate aerosols rather than natural variability, either internal or externally forced. We also find that greenhouse gases are likely to have contributed significantly to the warming in the first half of the century. In addition, natural effects may have contributed to this warming. Assuming one particular reconstruction of total solar irradiance to be correct implies, when we take the seasonal cycle into account, that solar effects have contributed significantly to the warming observed in the early part of the century, regardless of any relative error in the amplitudes of the anthropogenic forcings prescribed in the model. However, this is not the case with an alternative reconstruction of total solar irradiance, based more on the amplitude than the length of the solar cycle. We also find evidence for volcanic influences on twentieth century near-surface temperatures. The signature of the eruption of Mount Pinatubo is detected using annual-mean data. We also find evidence for a volcanic influence on warming in the first half of the century associated with a reduction in mid-century volcanism.

Transient Climate Simulations with the HadGEM1 Climate Model: Causes of Past Warming and Future Climate Change

The ability of climate models to simulate large-scale temperature changes during the twentieth century when they include both anthropogenic and natural forcings and their inability to account for warming over the last 50 yr when they exclude increasing greenhouse gas concentrations has been used as evidence for an anthropogenic influence on global warming. One criticism of the models used in many of these studies is that they exclude some forcings of potential importance, notably from fossil fuel black carbon, biomass smoke, and land use changes. Herein transient simulations with a new model, the Hadley Centre Global Environmental Model version 1 (HadGEM1), are described, which include these forcings in addition to other anthropogenic and natural forcings, and a fully interactive treatment of atmospheric sulfur and its effects on clouds. These new simulations support previous work by showing that there was a significant anthropogenic influence on near-surface temperature change over the last century. They demonstrate that black carbon and land use changes are relatively unimportant for explaining global mean near-surface temperature changes. The pattern of warming in the troposphere and cooling in the stratosphere that has been observed in radiosonde data since 1958 can only be reproduced when the model includes anthropogenic forcings. However, there are some discrepancies between the model simulations and radiosonde data, which are largest where observational uncertainty is greatest in the Tropics and high latitudes. Predictions of future warming have also been made using the new model. Twenty-first-century warming rates, following policy-relevant emissions scenarios, are slightly greater in HadGEM1 than in the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) as a result of the extra forcing in HadGEM1. An experiment in which greenhouse gases and other anthropogenic forcings are stabilized at 2100 levels and held constant until 2200 predicts a committed twenty-second-century warming of less than 1 K, whose spatial distribution resembles that of warming during the twenty-first century, implying that the local feedbacks that determine the pattern of warming do not change significantly.

Do Models Underestimate the Solar Contribution to Recent Climate Change

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.

A New HadGEM3-A-Based System for Attribution of Weather- and Climate-Related Extreme Events

AbstractA new system for attribution of weather and climate extreme events has been developed based on the atmospheric component of the latest Hadley Centre model. The model is run with either observational data of sea surface temperature and sea ice or estimates of what their values would be without the effect of anthropogenic climatic forcings. In that way, ensembles of simulations are produced that represent the climate with and without the effect of human influences. A comparison between the ensembles provides estimates of the change in the frequency of extremes due to anthropogenic forcings. To evaluate the new system, reliability diagrams are constructed, which compare the model-derived probability of extreme events with their observed frequency. The ability of the model to reproduce realistic distributions of relevant climatic variables is another key aspect of the system evaluation. Results are then presented from analyses of three recent high-impact events: the 2009/10 cold winter in the United K...

Human contribution to rapidly increasing frequency of very warm Northern Hemisphere summers

[1] The European summer of 2003 was exceptionally warm, and there is evidence that human influence has at least doubled the risk of such a hot summer. It is possible that by the 2040s, summers over southern Europe will be as warm or warmer 50% of the time. Because of the related socioeconomic impacts, there is growing interest in investigating changes in climate extremes across the world and how they may change in the future. We examine observed and simulated summer temperatures over a set of regions covering the Northern Hemisphere. Simulated changes are consistent with observed changes over the vast majority of regions when the climate simulation includes changes in anthropogenic and natural influences. We detect the dominant influence of anthropogenic factors on observed warming in almost every region, which has led to a rapidly increasing risk of hot summers. We show that hot summers which were infrequent 20-40 years ago are now much more common and that our projections indicate that the current sharp rise in incidence of hot summers is likely to continue.

Are observed decadal changes in intermediate water masses a signature of anthropogenic climate change

Recent observations have shown relatively large changes in the temperature and salinity of intermediate water masses in the ocean on decadal timescales. We compare the observed changes with simulations of the coupled climate model HadCM3. In simulations driven by anthropogenic forcing, we see significant changes in intermediate waters in the Southern Ocean which are similar in both pattern and magnitude to the observations. Our results suggest that the observed changes are most likely to be a signal of anthropogenic climate change. The strong signal in the Southern Ocean, which is detectable in the model from the 1980s, is in marked contrast with the intermediate waters of the Northern hemisphere oceans, where internal climate variability is large and a signal of anthropogenic climate change is not detectable in the model until 2020 at the earliest. Our results suggest that intermediate waters, particularly those of the Southern hemisphere, are a potentially sensitive indicator of anthropogenic climate change, and could be an important part of a climate monitoring network.

The upper end of climate model temperature projections is inconsistent with past warming

Climate models predict a large range of possible future temperatures for a particular scenario of future emissions of greenhouse gases and other anthropogenic forcings of climate. Given that further warming in coming decades could threaten increasing risks of climatic disruption, it is important to determine whether model projections are consistent with temperature changes already observed. This can be achieved by quantifying the extent to which increases in well mixed greenhouse gases and changes in other anthropogenic and natural forcings have already altered temperature patterns around the globe. Here, for the first time, we combine multiple climate models into a single synthesized estimate of future warming rates consistent with past temperature changes. We show that the observed evolution of near-surface temperatures appears to indicate lower ranges (5–95%) for warming (0.35–0.82 K and 0.45–0.93 K by the 2020s (2020–9) relative to 1986–2005 under the RCP4.5 and 8.5 scenarios respectively) than the equivalent ranges projected by the CMIP5 climate models (0.48–1.00 K and 0.51–1.16 K respectively). Our results indicate that for each RCP the upper end of the range of CMIP5 climate model projections is inconsistent with past warming.

Sensitivity of global-scale climate change attribution results to inclusion of fossil fuel black carbon aerosol

It is likely that greenhouse gas emissions caused most of the global mean warming observed during the 20th century, and that sulphate aerosols counteracted this warming to some extent, by reflecting solar radiation to space and thereby cooling the planet. However, the importance of another aerosol, namely black carbon, could be underestimated. Here we include fossil fuel black carbon aerosol in a detection and attribution analysis with greenhouse gas and sulphate aerosols. We find that most of the warming of the 20th Century is attributable to changes in greenhouse gases offset by net aerosol cooling. However the pattern of temperature change due to black carbon is currently indistinguishable from the sulphate aerosol pattern of temperature change. The attribution of temperature change due to greenhouse gases is not sensitive to the inclusion of black carbon. We can be confident about the overall attribution of total aerosols, but less so about the contributions of black carbon emissions to 20th century climate change. This work presents no evidence that black carbon aerosol forcing outweighed the cooling due to sulphate aerosol.

Assessing the robustness of zonal mean climate change detection

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.