|Working Group II: Impacts, Adaptation and Vulnerability|
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19.2.2. Synthesis of Observed Impacts
There is an accumulating body of evidence of observed impacts relating to regional climate changesprimarily rising temperature across a broad range of affected physical processes and biological taxaand widespread geographical distribution of reported effects (see Figure 19-2 and accompanying notes). In many cases, reported changes are consistent with functional understanding of the climate-impact processes involved. Cases of no change or change in unexpected direction are noted, as are possible alternative explanations and confounding factors, where available.
The hydrological cycle is expected to respond to regional climate warming through changes in the energy balance of glaciers and the depth and extent of snow cover, earlier snowmelt runoff, seasonal changes in freezing and thawing of lakes and rivers, and intensification of precipitation and evaporative processes. For the most part, evidence of regional climate change impacts on elements of the hydrological cycle is consistent with expected responses to warming temperatures and intensification of hydrological regimes (see Chapters 4 and 5, and TAR WGI).
Evidence for such changes in the 20th century includes recession of glaciers on all continents (e.g., Hastenrath, 1995; Ames and Hastenrath, 1996; Dowdeswell et al., 1997; Dyurgerov and Meier, 1997; Haeberli and Beniston, 1998; Greene et al., 1999; Kaser, 1999; Krabill et al., 1999; Serreze et al., 2001). There have been decreases in the extent of snow cover (10% since the late 1960s and 1970s) in the northern hemisphere (e.g., Groisman et al., 1994; Serreze et al., 2001). Since the late 1940s, snowmelt and runoff have occurred increasingly earlier in northern and central California (Dettinger and Cayan, 1995). Annual duration of lake- and river-ice cover in the middle and high latitudes of the northern hemisphere has been reduced by about 2 weeks and is more variable (Schindler et al., 1990; Magnuson et al., 2000; Kratz et al., 2001).
Also reported is increased frequency of extreme rainfall in the middle and high latitudes of the northern hemisphere, including the United States (Karl and Knight, 1998), the UK (Osborn et al., 2000), and most extratropical land areas except China (Groisman et al., 1999).
Ecological theory predicts several types of species and community responses
to changing regional climate in plants and animals: changes in ecosystem structure
and dynamics, including shifts in ranges and distributions; altered phenology;
effects on physiology; and genetic evolutionary responses (see Chapters
2 and 5). Changes in disturbance (e.g., fires, wind
damage) also may be occurring but are not included in this review (see Chapters
5 and 6). Evidence from plants and animals documents
all of these types of ecological responses to regional warming, especially poleward
and elevational shifts in species ranges and earlier timing of reproduction.
Reviews of recent changes in biological systems also have documented examples
of these different types of responses, consistent with process-level understanding
Much of the evidence of vegetation change relating to regional climate change comes from responses to warming at high-latitude and high-altitude environments, where confounding factors such as land-use change may be minimized and where climate signals may be strongest (see TAR WGI Chapter 12). Increases in species richness were found at 21 of 30 high summits in the Alps; remaining summits exhibited stagnation or a slight decrease (Grabherr et al., 1994; Pauli et al., 1996). However, Körner (1999) suggests that grazing, tourism, and nitrogen deposition may be contributing to such observed migrations. Hasenauer et al. (1999) found significant increases in diameter increments of Norway spruce across Austria related to increased temperatures from 1961 to 1990. In North America, Barber et al. (2000) linked reduced growth of Alaskan white spruce to temperature-induced drought stress, and Hamburg and Cogbill (1988) propose that historical declines in red spruce in the northeastern United States are related to climatic warming, possibly aggravated by pollution and pathogen factors.
In more temperate ecosystems, Bradley et al. (1999) documented phenological advances in flowering date in 10 herbaceous and tree species and no change in 26 such species related to local warming in southern Wisconsin over the periods 1936-1947 and 1976-1998. Menzel and Fabian (1999) document extension of the growing season for 12 tree and shrub species at a network of sites throughout Europe, which they attribute to warming temperature. Alward and Detling (1999) found reorganization of a shortgrass steppe ecosystem in a semi-arid site in Colorado related to increased spring minimum temperatures, although the responses of C3 and C4 species did not occur as expected.
Regarding regional changes in precipitationwhich are much more uncertain with regard to future climatereorganization of a semi-arid ecosystem in Arizona, including increases in woody shrubs, has been associated with increases in winter precipitation (Brown et al., 1997); retraction of mesic species to areas of higher rainfall and lower temperature has been attributed to a long-term decline in rainfall in the West African Sahel (Gonzalez, 2001).
Temperature change-related effects in animals have been documented within all major taxonomic groups (amphibians, birds, insects, mammals, reptiles, and invertebrates) and on all continents (see Chapter 5). Terrestrial evidence in animals that follows process-level understanding of responses to warming includes poleward and elevational changes in spatial distribution, alterations in species abundance and diversity, earlier phenology (including advances in timing of reproduction), and physiological and genetic adaptations.
Poleward and elevational shifts associated with regional warming have been documented in the ranges of North American, British, and European butterfly species (Parmesan, 1996; Ellis, 1997; Ellis et al., 1997; Parmesan et al., 1999), birds (Thomas and Lennon, 1999), and insects (Fleming and Tatchell, 1995). Prop et al. (1998) found that increasing spring temperatures and changes in agricultural practices in Norway have allowed barnacle geese (Branta leucopsis) to move northward and invade active agricultural areas. Changes in species distribution and abundance of amphibians, birds, and reptiles in Costa Rica have been associated with changing patterns of dry-season mist frequency and Pacific sea-surface temperatures (SST) (Pounds et al., 1999; Still et al., 1999).
Earlier timing of reproduction has been found for many bird species (Mason, 1995; Crick et al., 1997; McCleery and Perrins, 1998; Crick and Sparks, 1999; Slater, 1999) and amphibians (Beebee, 1995; Reading, 1998) in the UK and Europe (Winkel and Hudde, 1996, 1997; Ludwichowski, 1997; Forchhammer et al., 1998; Visser et al., 1998; Bergmann, 1999). Zhou et al. (1995) found a warming trend in the spring to be associated with earlier aphid flights in the UK. Also in the UK, Sparks (1999) has associated arrival times of bird migration to warmer spring temperature. Bezzel and Jetz (1995) and Gatter (1992) document delays in the autumn migratory period in the Alps and Germany, respectively.
In North America, Brown et al. (1999) document earlier egg-laying in Mexican jays (Aphelocoma ultramarina) associated with significant trends toward increased monthly minimum temperatures in Arizona. Dunn and Winkler (1999) found that the egg-laying date of North American tree swallows advanced by as much as 9 days, associated with increasing air temperatures at the time of breeding. Bradley et al. (1999) document phenological advances in arrival dates for migratory birds in southern Wisconsin, associated with earlier icemelt of a local lake and higher spring temperature.
Post et al. (1997) and Post and Stenseth (1999) document differential selection of body size in red deer throughout Norway from 1965 to 1995. Male red deer have been getting larger and females smaller, correlated with warming trends and variations in the North Atlantic Oscillation (NAO). Post and Stenseth (1999) also report on the interactions of plant phenology, northern ungulates (red deer, reindeer, moose, white-tailed deer, musk oxen, caribou, and Soay sheep), and the NAO. Jarvinen (1994) found that increased mean spring temperatures in Finnish Lapland are associated with mean egg volume of the pied flycatcher. De Jong and Brakefield (1998) found shifts in color patterns (black with red spots versus red with black spots), most likely related to thermal budgets of ladybird beetles (Adalia bipunctata) in The Netherlands, coinciding with an increase in local ambient spring temperatures. The potential for rapid adaptive responses and their genetic costs to populations has been studied by Rodriguez-Trelles and Rodriguez (1998), who found microevolution and loss of chromosomal diversity in Drosophila in northwestern Spain as the local climate warmed.
In coastal zones and marine ecosystems, there is evidence of changes in physical
and biological systems associated with regional trends in climate, especially
warming of air temperatures and SST (see Chapters 4, 5,
and 6). However, separating out responses of marine ecosystems
to variability caused by large-scale ocean-atmosphere phenomena, such as ENSO
and NAO, from regional climate changes is a challenge (e.g., Southward et al.,
1995; McGowan et al., 1998, 1999; Sagarin et al., 1999). Variations caused by
ENSO and NAO per se are not considered climate change, but multi-decadal trends
of change in ENSO or NAO frequency and intensity are climate changes, according
to the IPCC definition.
Changes in the physical systems of coastal zones related to regional warming trends include trends in sea ice and coastal erosion. Since the 1950s, Arctic sea-ice extent has declined by about 10-15%; in recent decades, there has been about a 40% decline in Arctic sea-ice thickness during late summer to early autumn and a considerably slower decline in winter (e.g., Maslanik et al., 1996; Cavalieri et al., 1997; Johannessen et al., 1999; Rothrock et al., 1999; Vinnikov et al., 1999; Serreze et al., 2001). No significant trends in Antarctic sea-ice extent are apparent (see TAR WGI).
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