IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

11.1.3 Some Unifying Themes

The basic pattern of the projected warming as described in Chapter 10 is little changed from previous assessments. Examining the spread across the MMD models, temperature projections in many regions are strongly correlated with the global mean projections, with the most sensitive models in global mean temperature often the most sensitive locally. Differing treatments of regional processes and the dynamical interactions between a given region and the rest of the climate system are responsible for some spread. However, a substantial part of the spread in regional temperature projections is due to differences in the sum of the feedbacks that control transient climate sensitivity (see also Chapter 10).

The response of the hydrological cycle is controlled in part by fundamental consequences of warmer temperatures and the increase in water vapour in the atmosphere (Chapter 3). Water is transported horizontally by the atmosphere from regions of moisture divergence (particularly in the subtropics) to regions of convergence. Even if the circulation does not change, these transports will increase due to the increase in water vapour. The consequences of this increased moisture transport can be seen in the global response of precipitation, described in Chapter 10, where, on average, precipitation increases in the inter-tropical convergence zones, decreases in the subtropics, and increases in subpolar and polar regions. Over North America and Europe, the pattern of subpolar moistening and subtropical drying dominates the 21st-century projections. This pattern is also described in Section 9.5.4, which assesses the extent to which this pattern is visible over land during the 20th century in precipitation observations and model simulations. Regions of large uncertainty often lie near the boundaries between these robust moistening and drying regions, with boundaries placed differently by each model.

High-resolution model results indicate that in regions with strong orographic forcing, some of these large-scale findings can be considerably altered locally. In some cases, this may result in changes in the opposite direction to the more general large-scale behaviour. In addition, large-area and grid-box average projections for precipitation are often very different from local changes within the area (Good and Lowe, 2006). These issues demonstrate the inadequacy of inferring the behaviour at fine scales from that of large-area averages.

Another important theme in the 21st-century projections is the poleward expansion of the subtropical highs, and the poleward displacement of the mid-latitude westerlies and associated storm tracks. This circulation response is often referred to as an enhanced positive phase of the Northern or Southern Annular Mode, or when focusing on the North Atlantic, the positive phase of the North Atlantic Oscillation (NAO). In regions without strong orographic forcing, superposition of the tendency towards subtropical drying and poleward expansion of the subtropical highs creates especially robust drying responses at the poleward boundaries of the five subtropical oceanic high centres in the South Indian, South Atlantic, South Pacific, North Atlantic and, less robustly, the North Pacific (where a tendency towards El-Niño like conditions in the Pacific in the models tends to counteract this expansion). Most of the regional projections of strong drying tendencies over land in the 21st century are immediately downstream of these centres (south-western Australia, the Western Cape Provinces of South Africa, the southern Andes, the Mediterranean and Mexico). The robustness of this large-scale circulation signal is discussed in Chapter 10, while Chapters 3, 8 and 9 describe the observed poleward shifts in the late 20th century and the ability of models to simulate these shifts.

The retreats of snow and ice cover are important for local climates. The difficulty of quantifying these effects in regions of substantial topographic relief is a significant limitation of global models (see Section, Box 11.3) and is improved with dynamical and statistical downscaling. The drying effect of an earlier spring snowmelt and, more generally, the earlier reduction in soil moisture (Manabe and Wetherald, 1987) is a continuing theme in discussion of summer continental climates.

The strong interactions between sea surface temperature gradients and tropical rainfall variability provides an important unifying theme for tropical climates. Models can differ in their projections of small changes in tropical ocean temperature gradients and in the simulation of the potentially large shifts in rainfall that are related to these oceanic changes. Chou and Neelin (2004) provide a guide to some of the complexity involved in diagnosing and evaluating hydrological responses in the tropics. With a few exceptions, the spread in projections of hydrological changes is still too large to make strong statements about the future of tropical climates at regional scales (see also Section 10.3). Many AOGCMs project large tropical precipitation changes, so uncertainty as to the regional pattern of these changes should not be taken as evidence that these changes are likely to be small.

Assessments of the regional and sub-regional climate change projections have primarily been based on the AOGCM projections summarised in Table 11.1 and an analysis of the biases in the AOGCM simulations, regional downscaling studies available for some regions with either physical or statistical models or both, and reference to plausible physical mechanisms.

To assist the reader in placing the various regional assessments in a global context, Box 11.1 displays many of the detailed assessments documented in the following regional sections. Likewise, an overview of projected changes in various types of extreme weather statistics is summarised in Table 11.2,

which contains information from the assessments within this chapter and from Chapter 10. Thus, the details of the assessment that lead to each individual statement can all be found in either Chapter 10, or the respective regional sections, and links for each statement are identifiable from Table 11.2.

Table 11.2. Projected changes in climate extremes. This table summarises key phenomena for which there is confidence in the direction of projected change based on the current scientific evidence. The included phenomena are those where confidence ranges between medium and very likely, and are listed with the notation of VL (very likely), L (likely), and M (medium confidence). maxTmax refers to the highest maximum temperature, maxTmin to the highest minimum temperature, minTmax to the lowest maximum temperature, and minTmin to the lowest minimum temperature. In addition to changes listed in the table, there are two phenomena of note for which there is little confidence. The issue of drying and associated risk of drought in the Sahel remains uncertain as discussed in Section The change in mean duration of tropical cyclones cannot be assessed with confidence at this stage due to insufficient studies.

Temperature-Related Phenomena


Change in phenomenon


Projected changes


Higher monthly absolute maximum of daily maximum temperatures (maxTmax) more hot / warm summer days


VL (consistent across model projections)

maxTmax increases at same rate as the mean or median1 over northern Europe,2Australia and New Zealand3



L (fairly consistent across models, but sensitivity to land surface treatment)

maxTmax increases more than the median over southern and central Europe,4 andsouthwest USA5



L (consistent with projected large increase in mean temperature)

Large increase in probability of extreme warm seasons over most parts of the world6


Longer duration, more intense, more frequent heat waves / hot spells in summer


VL (consistent across model projections)

Over almost all continents7, but particularly central Europe,8 western USA,9 East Asia10and Korea11


Higher monthly absolute maximum of daily minimum temperatures (maxTmin); more warm and fewer cold nights


VL (consistent with higher mean temperatures)

Over most continents12


Higher monthly absolute minimum of daily minimum temperatures (minTmin)


VL (consistent across model projections)

minTmin increases more than the mean in many mid- and high-latitude locations,13 particularly in winter over most of Europe except the southwest14


Higher monthly absolute minimum of daily maximum temperatures (minTmax), fewer cold days


L (consistent with warmer mean temperatures)

minTmin increases more than the mean in some areas15


Fewer frost days


VL (consistent across model projections)

Decrease in number of days with below-freezing temperatures everywhere16


Fewer cold outbreaks; fewer, shorter, less intense cold spells / cold extremes in winter


VL (consistent across model projections)

Northern Europe, South Asia, East Asia17



L (consistent with warmer mean temperatures)

Most other regions18


Reduced diurnal temperature range


L (consistent across model projections)

Over most continental regions, night temperatures increase faster than theday temperatures19


Temperature variability on interannual and daily time scales


L (general consensus across model projections)

Reduced in winter over most of Europe20

Increase in central Europe in summer21


1 Kharin and Zwiers (2005)

2 §, Supplementary Material Figure S11.23, PRUDENCE, Kjellström et al. (2007)

3 §, CSIRO (2001)

4 §, PRUDENCE, Kjellström et al. (2007)

5 §, Bell et al. (2004),

6 Table 11.1

7 §, Tebaldi et al. (2006), Meehl and Tebaldi (2004)

8 §, Barnett et al. (2006), Clark et al. (2006), Tebaldi et al. (2006), Gregory and Mitchell (1995), Zwiers and Kharin (1998), Hegerl et al. (2004), Meehl and Tebaldi (2004)

9 §, Bell et al. (2004), Leung et al. (2004)

10 §, Gao et al. (2002)

11 §, Kwon et al. (2005), Boo et al. (2006)

12 §, §

13 Kharin and Zwiers (2005)

14 §, Fig., PRUDENCE

15 §, Whetton et al. (2002)

16 Tebaldi et al. (2006), Meehl and Tebaldi (2004), §, PRUDENCE, §, CSIRO (2001), Mullan et al. (2001b)

17 §, PRUDENCE, Kjellström et al. (2007), §, Gao et al. (2002), Rupa Kumar et al. (2006)

18 §11.1.3

19 §, Bell et al. (2004), Leung et al. (2004), §, Rupa Kumar et al. (2006), Mizuta et al. (2005)

20 §, Räisänen (2001), Räisänen and Alexandersson (2003), Giorgi and Bi (2005), Zwiers and Kharin (1998), Hegerl et al. (2004), Kjellström et al. (2007)

21 §, PRUDENCE, Schär et al. (2004), Vidale et al. (2007)

Table 11.2. (continued)

Moisture-Related Phenomena





Projected changes


Intense precipitation events


VL (consistent across model projections; empirical evidence, generally higher precipitation extremes in warmer climates)

Much larger increase in the frequency than in the magnitude of precipitation extremes over most land areas in middle latitudes,22 particularly over northern Europe,23 Australia and NewZealand24

Large increase during the Indian summer monsoon season over Arabian Sea, tropical IndianOcean, South Asia25Increase in summer over south China, Korea and Japan26




Intense precipitation events


L (some inconsistencies across model projections)

Increase over central Europe in winter27Increase associated with tropical cyclones over Southeast Asia, Japan28




Changes in summer over Mediterranean and central Europe29



L decrease (consistent across model projections)

Iberian Peninsula30


Wet days


L (consistent across model projections)

Increase in number of days at high latitudes in winter, and over northwest China31

Increase over the Inter-Tropical Convergence Zone32

Decrease in South Asia33 and the Mediterranean area34


Dry spells (periods of consecutive dry days)


VL (consistent across model projections)

Increase in length and frequency over the Mediterranean area35, southern areas of Australia, New Zealand36



L (consistent across model projections)

Increase in most subtropical areas37

Little change over northern Europe38


Continental drying and associated risk of drought


L (consistent across model projections; consistent change in precipitation minus evaporation, but sensitivity to formulation of land surface processes)

Increased in summer over many mid-latitude continental interiors, e.g., central39 andsouthern Europe, Mediterranean area,40 in boreal spring and dry periods of the annual cycle over Central America41


22 §, Groisman et al. (2005), Kharin and Zwiers (2005), Hegerl et al. (2004), Semenov and Bengtsson (2002), Meehl et al. (2006)

23 §, Räisänen (2002), Giorgi and Bi (2005), Räisänen (2005)

24 §11.1.3, §, §, Huntingford et al. (2003), Barnett et al. (2006), Frei et al. (2006), Hennessy et al. (1997), Whetton et al. (2002), Watterson and Dix (2003), Suppiah et al. (2004), McInnes et al. (2003), Hennessy et al. (2004b), Abbs (2004), Semenov and Bengtsson (2002)

25 §, May (2004a), Rupa Kumar et al. (2006)

26 §, Gao et al. (2002), Boo et al. (2006), Kimoto et al. (2005), Kitoh et al. (2005), Mizuta et al. (2005)

27 §, PRUDENCE, Frei et al. (2006), Christensen and Christensen (2003, 2004)

28 §11.1.3, §, Kimoto et al. (2005), Mizuta et al. (2005), Hasegawa and Emori (2005), Kanada et al. (2005)

29 §, PRUDENCE, Frei et al. (2006), Christensen and Christensen (2004), Tebaldi et al. (2006)

30 §, PRUDENCE, Frei et al. (2006)

31 §, Gao et al. (2002), Hasegawa and Emori (2005)

32 Semenov and Bengtsson (2002)

33 § Krishna Kumar et al. (2003)

34 §, Semenov and Bengtsson (2002), Voss et al. (2002); Räisänen et al. (2004); Frei et al. (2006)

35 §, Semenov and Bengtsson, 2002; Voss et al., 2002; Hegerl et al., 2004; Wehner, 2004; Kharin and Zwiers, 2005; Tebaldi et al., 2006

36 §11.1.3, §, §, Whetton and Suppiah (2003), McInnes et al. (2003), Walsh et al. (2002), Hennessy et al. (2004c), Mullan et al. (2005)

37 §11.1.3

38 §, Beniston et al. (2007), Tebaldi et al. (2006), Voss et al. (2002)

39 §, Rowell and Jones (2006)

40 §11.1.3, §, Voss et al. (2002)

41 §11.1.3

Table 11.2. (continued)

Tropical Cyclones (typhoons and hurricanes)


Change in phenomenon


Projected changes


Increase in peak wind intensities


L (high-resolution Atmospheric GCM (AGCM) and embedded hurricane model projections)

Over most tropical cyclone areas42


Increase in mean and peak precipitation intensities


L (high-resolution AGCM projections and embedded hurricane model projections)

Over most tropical cyclone areas,43 South,44 East45 and southeast Asia46


Changes in frequency of occurrence


M (some high-resolution AGCM projections)

Decrease in number of weak storms, increase in number of strong storms47



M (several climate model projections)

Globally averaged decrease in number, but specific regional changes dependent onsea surface temperature change48

Possible increase over the North Atlantic49


Extratropical Cyclones


Change in phenomenon


Projected changes


Changes in frequency and position


L (consistent in AOGCM projections)

Decrease in the total number of extratropical cyclones50

Slight poleward shift of storm track and associated precipitation, particularly in winter51


Change in storm intensity and winds


L (consistent in most AOGCM projections, but not explicitly analysed for all models)

Increased number of intense cyclones52 and associated strong winds, particularly in winter over the North Atlantic,53 central Europe54 and Southern Island of New Zealand55



More likely than not

Increased windiness in northern Europe and reduced windiness in Mediterranean Europe56


Increased wave height


L (based on projected changes in extratropical storms)

Increased occurrence of high waves in most mid-latitude areas analysed, particularlythe North Sea57


42 Knutson and Tuleya (2004)

43 Knutson and Tuleya (2004)

44 §, Unnikrishnan et al. (2006)

45 §11.3.4, Hasegawa and Emori (2005)

46 §11.3.4, Hasegawa and Emori (2005), Knutson and Tuleya (2004)

47 Oouchi et al. (2006)

48 Hasegawa and Emori (2005)

49 Sugi et al. (2002), Oouchi et al. (2006)

50 §, Yin (2005), Lambert and Fyfe (2006), §, Lionello et al. (2002), Leckebusch et al. (2006), Vérant (2004), Somot (2005)

51 §11.1.3, Yin (2005), Lambert and Fyfe (2006)

52 §11.1.2, §, Yin (2005), Lambert and Fyfe (2006)

53 §, Leckebusch and Ulbrich (2004)

54 §, Zwiers and Kharin (1998), Knippertz et al. (2000), Leckebusch and Ulbrich (2004), Pryor et al. (2005a), Lionello et al. (2002), Leckebusch et al. (2006), Vérant (2004), Somot (2005)

55 §11.1.3, §

56 §, Lionello et al. (2002), Leckebusch et al. (2006), Vérant (2004), Somot (2005)

57 X.L. Wang et al. (2004)