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P. D. Jones
and
T. M. L. Wigley

Abstract

The reliability of the Australian (June 1972–April 1985) and NOTOS (1957–62) gridded monthly-mean, mean sea level pressure datasets over Antarctica is examined by comparison with station data from 29 sites over the continent. After rejecting about 30% of the months in both sets of gridded data, the remaining “good” months are used in a principal component motion technique to reconstruct gridded data from the station data for 1957 to 1985. The regression technique uses the “good” Australian data for calibration and verifies the statistical relationships developed between station and grid point pressure data with the “good” NOTOS data. The reconstructions are shown to be reliable over all of Antarctica between 60° and 75°S except in the area to the east of the Ross Sea and adjacent areas of the southern Pacific Ocean.

The reconstructions are used to compare the NOTOS data with the more recent Australian gridded pressure data. Major differences between the two datasets are found over eastern Antarctica and the extreme southern Pacific and adjacent areas of western Antarctica. The first problem region was found to be related to extrapolation of the NOTOS data beyond their region of reliability as defined by the original published maps. The second problem region has a 10 mb difference between the two datasets, with the NOTOS data higher than the Australian. As this is the region of poorest data coverage anywhere in the world, the difference is difficult to resolve. In contrast, comparisons with the Taljaard et al. (1969) climatology show that this dataset contains fundamental spatial inconsistencies, and its further use cannot be recommended.

A composite dataset linking the Australian, NOTOS and the reconstructed data can be produced for the whole region except for the southern Pacific and wet Antarctic region. This extended dataset is used to examine changes in pressure patterns between the January 1957–May 1972 and June 1972–April 1985 periods. Some of the changes in temperature that have occurred over this period can be explained by changes in surface circulation patterns.

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T. M. L. Wigley
,
S. J. Smith
, and
M. J. Prather

Abstract

Reactive gas emissions (CO, NOx, VOC) have indirect radiative forcing effects through their influences on tropospheric ozone and on the lifetimes of methane and hydrogenated halocarbons. These effects are quantified here for the full set of emissions scenarios developed in the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. In most of these no-climate-policy scenarios, anthropogenic reactive gas emissions increase substantially over the twenty-first century. For the implied increases in tropospheric ozone, the maximum forcing exceeds 1 W m−2 by 2100 (range −0.14 to +1.03 W m−2). The changes are moderated somewhat through compensating influences from NOx versus CO and VOC. Reactive gas forcing influences through methane and halocarbons are much smaller; 2100 ranges are −0.20 to +0.23 W m−2 for methane and −0.04 to +0.07 W m−2 for the halocarbons. Future climate change might be reduced through policies limiting reactive gas emissions, but the potential for explicitly climate-motivated reductions depends critically on the extent of reductions that are likely to arise through air quality considerations and on the assumed baseline scenario.

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T. M. L. Wigley
and
S. C. B. Raper

Abstract

Projections of future warming in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) are substantially larger than those in the Second Assessment Report (SAR). The reasons for these differences are documented and quantified. Differences are divided into differences in the emissions scenarios and differences in the science (gas cycle, forcing, and climate models). The main source of emissions-related differences in warming is aerosol forcing, primarily due to large differences in SO2 emissions between the SAR and TAR scenarios. For any given emissions scenario, concentration projections based on SAR and TAR science are similar, except for methane at high emissions levels where TAR science leads to substantially lower concentrations. The new (TAR) science leads to slightly lower total forcing and slightly larger warming. At the low end of the warming range the effects of the new science and the new emissions scenarios are roughly equal. At the high end, TAR science has a smaller effect and the main reason for larger TAR warming is the use of a different high-end emissions scenario, primarily changes in SO2 emissions.

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Aiguo Dai
,
T. M. L. Wigley
,
B. A. Boville
,
J. T. Kiehl
, and
L. E. Buja

Abstract

The Climate System Model, a coupled global climate model without “flux adjustments” recently developed at the National Center for Atmospheric Research, was used to simulate the twentieth-century climate using historical greenhouse gas and sulfate aerosol forcing. This simulation was extended through the twenty-first century under two newly developed scenarios, a business-as-usual case (ACACIA-BAU, CO2 ≈ 710 ppmv in 2100) and a CO2 stabilization case (STA550, CO2 ≈ 540 ppmv in 2100). Here we compare the simulated and observed twentieth-century climate, and then describe the simulated climates for the twenty-first century. The model simulates the spatial and temporal variations of the twentieth-century climate reasonably well. These include the rapid rise in global and zonal mean surface temperatures since the late 1970s, the precipitation increases over northern mid- and high-latitude land areas, ENSO-induced precipitation anomalies, and Pole–midlatitude oscillations (such as the North Atlantic oscillation) in sea level pressure fields. The model has a cold bias (2°–6°C) in surface air temperature over land, overestimates of cloudiness (by 10%–30%) over land, and underestimates of marine stratus clouds to the west of North and South America and Africa.

The projected global surface warming from the 1990s to the 2090s is ∼1.9°C under the BAU scenario and ∼1.5°C under the STA550 scenario. In both cases, the midstratosphere cools due to the increase in CO2, whereas the lower stratosphere warms in response to recovery of the ozone layer. As in other coupled models, the surface warming is largest at winter high latitudes (≥5.0°C from the 1990s to the 2090s) and smallest (∼1.0°C) over the southern oceans, and is larger over land areas than ocean areas. Globally averaged precipitation increases by ∼3.5% (3.0%) from the 1990s to the 2090s in the BAU (STA550) case. In the BAU case, large precipitation increases (up to 50%) occur over northern mid- and high latitudes and over India and the Arabian Peninsula. Marked differences occur between the BAU and STA550 regional precipitation changes resulting from interdecadal variability. Surface evaporation increases at all latitudes except for 60°–90°S. Water vapor from increased tropical evaporation is transported into mid- and high latitudes and returned to the surface through increased precipitation there. Changes in soil moisture content are small (within ±3%). Total cloud cover changes little, although there is an upward shift of midlevel clouds. Surface diurnal temperature range decreases by about 0.2°–0.5°C over most land areas. The 2–8-day synoptic storm activity decreases (by up to 10%) at low latitudes and over midlatitude oceans, but increases over Eurasia and Canada. The cores of subtropical jets move slightly up- and equatorward. Associated with reduced latitudinal temperature gradients over mid- and high latitudes, the wintertime Ferrel cell weakens (by 10%–15%). The Hadley circulation also weakens (by ∼10%), partly due to the upward shift of cloudiness that produces enhanced warming in the upper troposphere.

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Gerald A. Meehl
,
Warren M. Washington
,
T. M. L. Wigley
,
Julie M. Arblaster
, and
Aiguo Dai

Abstract

Ensemble experiments with a global coupled climate model are performed for the twentieth century with time-evolving solar, greenhouse gas, sulfate aerosol (direct effect), and ozone (tropospheric and stratospheric) forcing. Observed global warming in the twentieth century occurred in two periods, one in the early twentieth century from about the early 1900s to the 1940s, and one later in the century from, roughly, the late 1960s to the end of the century. The model's response requires the combination of solar and anthropogenic forcing to approximate the early twentieth-century warming, while the radiative forcing from increasing greenhouse gases is dominant for the response in the late twentieth century, confirming previous studies. Of particular interest here is the model's amplification of solar forcing when this acts in combination with anthropogenic forcing. This difference is traced to the fact that solar forcing is more spatially heterogeneous (i.e., acting most strongly in areas where sunlight reaches the surface) while greenhouse gas forcing is more spatially uniform. Consequently, solar forcing is subject to coupled regional feedbacks involving the combination of temperature gradients, circulation regimes, and clouds. The magnitude of these feedbacks depends on the climate's base state. Over relatively cloud-free oceanic regions in the subtropics, the enhanced solar forcing produces greater evaporation. More moisture then converges into the precipitation convergence zones, intensifying the regional monsoon and Hadley and Walker circulations, causing cloud reductions over the subtropical ocean regions, and, hence, more solar input. An additional response to solar forcing in northern summer is an enhancement of the meridional temperature gradients due to greater solar forcing over land regions that contribute to stronger West African and South Asian monsoons. Since the greenhouse gases are more spatially uniform, such regional circulation feedbacks are not as strong. These regional responses are most evident when the solar forcing occurs in concert with increased greenhouse gas forcing. The net effect of enhanced solar forcing in the early twentieth century is to produce larger solar-induced increases of tropical precipitation when calculated as a residual than for early century solar-only forcing, even though the size of the imposed solar forcing is the same. As a consequence, overall precipitation increases in the early twentieth century in the Asian monsoon regions are greater than late century increases, qualitatively consistent with observed trends in all-India rainfall. Similar effects occur in West Africa, the tropical Pacific, and the Southern Ocean tropical convergence zones.

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Gerald A. Meehl
,
Warren M. Washington
,
Caspar M. Ammann
,
Julie M. Arblaster
,
T. M. L. Wigley
, and
Claudia Tebaldi

Abstract

Ensemble simulations are run with a global coupled climate model employing five forcing agents that influence the time evolution of globally averaged surface air temperature during the twentieth century. Two are natural (volcanoes and solar) and the others are anthropogenic [e.g., greenhouse gases (GHGs), ozone (stratospheric and tropospheric), and direct effect of sulfate aerosols]. In addition to the five individual forcing experiments, an additional eight sets are performed with the forcings in various combinations. The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar). However, the signature of globally averaged temperature at any time in the twentieth century is a direct consequence of the sum of the forcings. The similarity of the response to the forcings on decadal and interannual time scales is tested by performing a principal component analysis of the 13 ensemble mean globally averaged temperature time series. A significant portion of the variance of the reconstructed time series can be retained in residual calculations compared to the original single and combined forcing runs. This demonstrates that the statistics of the variances for decadal and interannual time-scale variability in the forced simulations are similar to the response from a residual calculation. That is, the variance statistics of the response of globally averaged temperatures in the forced runs are additive since they can be reproduced in the responses calculated as a residual from other combined forcing runs.

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Gerald A. Meehl
,
William D. Collins
,
Byron A. Boville
,
Jeffrey T. Kiehl
,
T. M. L. Wigley
, and
Julie M. Arblaster

Abstract

The global warming resulting from increased CO2 is addressed in the context of two regional processes that contribute to climate change in coupled climate models, the “El Niño–like” response (slackening of the equatorial Pacific SST gradient) and sea-ice response at high latitudes. The National Center for Atmospheric Research (NCAR) Climate System Model (CSM) response is compared with results from a coupled model that produces comparatively greater global warming, the NCAR U.S. Department of Energy (DOE) global coupled model. In an experiment where atmospheric CO2 is increased 1% yr−1 compound, globally averaged surface air temperature increase near the time of CO2 doubling for the CSM is 1.43°C (3.50°C for the DOE model). Analysis of a simple coupled model shows the CSM equilibrium sensitivity to doubled CO2 is comparable to that from the slab ocean version (about 2.1°C). One process that contributes to global warming (estimated to be about 5% in one slab ocean model), as well as to significant Pacific region climate effects, is the El Niño–like response. It is a notable feature in the DOE model and some other global coupled models but does not occur in the CSM. The authors show that cloud responses are a major determining factor. With increased CO2, there are negative net cloud-forcing differences in the western equatorial Pacific in the CSM and DOE models, but large positive differences in the DOE model and negative differences in the CSM in the eastern equatorial Pacific. This produces asymmetric cloud radiative forcing contributing to an El Niño–like response in the DOE model and not in the CSM. To remove the amplifying effects of ocean dynamics and to identify possible parameter-dependent processes that could contribute to such cloud forcing changes, the authors analyze slab ocean versions of the coupled models in comparison with a slab ocean configuration of the atmospheric model in the CSM [Community Climate Model Version 3 (CCM3)] that includes prognostic cloud liquid water. The latter shows a change in sign (from negative to positive) of the net cloud forcing in the eastern equatorial Pacific with doubled CO2, similar to the DOE model, in comparison with the CCM3 version with diagnostic cloud liquid water. Atmospheric Model Intercomparison Project (prescribed SST) experiments show that all three atmospheric models (DOE, CCM3 with diagnostic cloud liquid water, and CCM3 with prognostic cloud liquid water) perform poorly relative to observations in terms of cloud radiative forcing, though CCM3 with prognostic cloud liquid water is slightly superior to the others. Another process that contributes to climate response to increasing CO2 is sea-ice changes, which are estimated to enhance global warming by roughly 20% in the CSM and 37% in the DOE model. Sea-ice retreat with increasing CO2 in the CSM is less than in the DOE model in spite of identical sea-ice formulations. Results from the North Atlantic and Greenland–Iceland–Norwegian (GIN) Sea region show that the surface energy budget response is controlled primarily by surface albedo (related to ice area changes) and cloud changes. However, a more important factor is the poleward ocean heat transport associated with changes in meridional overturning in the GIN Sea. With increased CO2, the transport of warmer water from the south into this region in the DOE model is greater in comparison with that of the CSM. This leads to a larger ice reduction in the DOE model, thus also contributing to the enhanced contribution from ice albedo feedback in the DOE model in comparison with the CSM.

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R. Knutti
,
M. R. Allen
,
P. Friedlingstein
,
J. M. Gregory
,
G. C. Hegerl
,
G. A. Meehl
,
M. Meinshausen
,
J. M. Murphy
,
G.-K. Plattner
,
S. C. B. Raper
,
T. F. Stocker
,
P. A. Stott
,
H. Teng
, and
T. M. L. Wigley

Abstract

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.

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