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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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J. M. Gregory, R. J. Stouffer, S. C. B. Raper, P. A. Stott, and N. A. Rayner

Abstract

A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5th percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analyses of observed interior-ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthropogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late nineteenth century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulation model, it provides an independent observationally based constraint on this important parameter of the climate system.

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P. D. Jones, S. C. B. Raper, and T. M. L. Wigley

Abstract

A new compilation of monthly mean surface air temperature data for the Southern Hemisphere for 1851–1984 is presented based on land-based meteorological station data. Where possible, the station data used in the analysis have been assessed for homogeneity. Only reliable or corrected station data have been used in calculating area averages. Grid point temperature estimates have been made by interpolating onto a 5° lat by 10° long grid for each month of the 134 years. For the period of best data coverage, 32% of the area of the Southern Hemisphere is covered. The reliability of hemispheric estimates is assessed for earlier periods when coverage is less than this maximum. Year-to-year estimates are considered reliable back to about 1890 and earlier estimates can indicate trends back to the 1860s. This new hemispheric compilation is compared with estimates of Southern Hemisphere marine data, and comparisons between land and marine data for both hemispheres are made and interpreted. The data show a long-term warming trend amounting to about 0.5°C over the past 100 years. The 1938–65 cooling trend that is so evident in the Northern Hemisphere data shows up only as a hiatus in the long-term warming of the Southern Hemisphere, pointing towards some hemispherically specific causal mechanism.

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P. D. Jones, S. C. B. Raper, R. S. Bradley, H. F. Diaz, P. M. Kellyo, and T. M. L. Wigley

Abstract

A new compilation of monthly mean surface air temperature for the Northern Hemisphere for 1851–1984 is presented based on land-based meteorological station data and fixed-position weather ship data. This compilation differs from others in two ways. First, a considerable amount of new data, previously hidden away in archives, has been included, thus improving both spatial and temporal coverage. Second, the station data have been analyzed to assess their homogeneity. Only reliable or corrected station data have been used in calculating area averages. Grid point temperature estimates have been made by interpolating onto a 5° latitude by 10° longitude grid for each month of the 134 years. In the period of best data coverage, 58% of the area of the Northern Hemisphere is covered by the available data network. (The remaining area is mainly ocean too far from land-based stations to warrant extrapolation.) The reliability of hemispheric estimates is assessed for earlier periods when coverage is less than this maximum. Year-to-year estimates are considered reliable back to about 1875. Estimates earlier than this are judged sufficiently good to indicate trends back to 1851. This new land-based hemispheric temperature curve is compared with recent estimates of Northern Hemisphere temperatures based on marine data. The two independent estimates agree well on the decadal time scale back to the start of the century, but important discrepancies exist for earlier times.

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S. C. B. Raper, T. M. L. Wigley, P. R. Mayes, P. D. Jones, and M. J. Salinger

Abstract

Antarctic temperature variations for 1957°82 have been objectively analyzed by gridding monthly data, from 16 stations, onto a 5° latitude by 10° longitude grid, from 65 to 90°S. These gridded data were used to calculate monthly values of the spatial mean temperature south of 65°S. The uncertainty in the area average is estimated to be 0.22°C for the annual values prior to 1970. After 1970 there is an additional uncertainty of about 0.10−0.16°C due to the cessation of Byrd station. The annual mean and summer areas averages show significant linear warming trends amounting to 0.74 and 0.77°C respectively. Spatial characteristics of the annual and seasonal temperature variations are described using principal components analysis of the station anomaly data. The first two principal components of the annual and winter data are similar PC1-winter is also similar to the winter pattern for linen trend found by van Loon and Williams for 1956–73. The warming trend associated with this pattern ceased in the mid-1970s.

Relationships between Antarctic temperatures and various parameters are investigated using linear trend and correlation analyses. Antarctic temperatures cannot be inferred from the long Orcadas record and the relationships between Antarctic temperatures and sea ice extent are complex. The most significant correlations between mean Antarctic temperature and sea ice extent averaged around Antarctica am found in spring; warm springs tend to be associated with anomalously large maximum sea ice extent. Lower Antarctic temperatures occur during summers and winters with strong westerlies (significant at the 0.1% level in summer and at the 0.1% level in winter).

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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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