The chapter on Regional Climate Information in the third assessment report of the IPCC, Climate Change 2001: The Scientific Basis gives key information on the limits of the Atmosphere-Ocean General Circulation Models (AOCGM) used by the IPCC for climate projections. The models are evaluated by having them to reproduce the current climate characteristics. The deviations between the models' computed climate characteristics and the observed data are referred to as biases rather than errors. This is what the third assessment report has to say on regional climate and the models.

At the regional scale the models display area-average biases that are highly variable from region-to-region and among models, with sub-continental area-averaged seasonal temperature biases typically within 4°C and precipitation biases mostly between −40 and +80% of observations. (page 585)

The subcontinental areas being consider are as shown below.

Based on simulations of the future climate under increased concentration of carbon dioxide the task force concluded to the average conditions:

• "It is very likely that nearly all land areas will warm more rapidly than the global average, particularly those in high latitudes in the cold season; in Alaska, northern Canada, Greenland, northern Asia, and Tibet in winter and central Asia and Tibet in summer the warming will exceed the global mean warming by more than 40% (1.3 to 6.9°C for the range of models and scenarios considered). In contrast, the warming will be less than the global mean in south and Southeast Asia in June-July-August (JJA), and in southern South America in winter."
• "It is likely that precipitation will increase over northern mid-latitude regions in winter and over northern high latitude regions and Antarctica in both summer and winter. In December-January-February (DJF), rainfall will increase in tropical Africa, show little change in Southeast Asia and decrease in central America. There will be increase or little change in JJA over South Asia. Precipitation will decrease over Australia in winter and over the Mediterranean region in summer. Change in precipitation will be largest over the high northern latitudes.

The assessment report also had conclusions with respect to the variability of weather conditions

• Daily ranges of temperatures will likely decrease in winter but increase in summer in mid-latitude land areas.
• The variability of temperatures from year-to-year will likely decrease in winter but increase in summer in mid-latitude land areas.
• Future increases in the mean levels of precipitation will likely lead to increases in the variability of precipitation.
• Extreme precipitation intensies may increase in Europe, North America, South Asia, the Sahel in Africa, southern Africa .
• Increases in the occurrence of drought or dry spells are indicated in studies of Europe, North America and Australia. South Asia, Sahel in Africa, southern Africa, Australia and the South Pacific.

The exact phrasing of these conclusions is important. There is a different level of confidence involved in the use of the phrase it is very likely than in the phrase it is likely. These reflect more confidence than the phrase will likely and all three reflect greater confidence than the may in extreme precipitation intensies may increase. The lowest level of confidence appears in the phrase are indicated as in Increases in the occurrence of drought or dry spells are indicated. The Report does not spell out what the bases are for any of the levels of conclusions. The suspicion is unavoidable that the phrases may and are indicated reflect the judgements of the authors and would not bear up well under careful scrutiny. The public is left with the notion that hard science is predicting not only global warming but also more frequent extreme conditions of temperature and precipitation; i.e., flooding sometimes and drought other times.

There is a surprising lack of correspondence between the qualified conclusions of the the chapter on regional effects and the Summary for Policy Makers. Table 1 of the Summary says that the confidence of the projected climate change of Higher maximum temperatures and more hot days over nearly all land areas for the 21st century is Very likely. However the Summary says that the observation of such an occurrence of such a phenomenon in the climate of the latter half of the 20th century is only Likely. It is strange that the confidence of discerning this effect in the existing data is smaller than the confidence from projections from less-than-perfect computer models. The climate change of Higher minimum temperatures, fewer cold days and frost days over nearly all land areas which did not rate a Very likely in the chapter on regional effects does get such a rating in the Summary. This same disparity exists for the the climate change of More intense precipitation events which is characterized as Very likely, over many areas for the 21st century got only a may increase in the chapter on regional effects. The climate change of increased summer continental drying and increased risks of droughts in the 21st century rates a level of confidence of Likely in the Summary but in the chapter on regional effects it is characterized only by are indicated.