19.2.2. Synthesis of Observed Impacts
There is an accumulating body of evidence of observed impacts relating to regional
climate changes—primarily rising temperature across a broad range of affected
physical processes and biological taxa—and 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.
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Figure 19-2: Observed impacts of temperature-related regional
climate change in the 20th century:
- Hydrology and Glaciers—Glacier retreat, decrease in snow-cover
extent/earlier snowmelt, reduction in annual duration of lake and river
ice
- Sea Ice—Decline in sea-ice extent and thickness
- Animals—Poleward and elevational shifts in range, alteration
in species abundance, changes in phenology (including earlier reproduction
and migration), physiological and morphological adaptation
- Plants—Change in abundance and diversity, change in phenology
(including earlier flowering), change in growth.
Studies that cover large areas and use remote-sensing methods are indicated.
About 50 studies were selected, according to the following criteria: (1)
hydrology/sea-ice studies that report long-term trends in observed variables
(time periods of studies range from ~20 to 150 years), and (2) terrestrial
and marine ecosystem studies that associate trends in observed change(s)
with trends in regional climate data for Ž20 years (time periods
of studies range from ~20 to 50 years). Of the ~100 physical processes
and ~450 biophysical species that exhibited change, more than 90% (~99%
physical, ~80% biophysical) are consistent with well-known mechanisms
of system responses to climate.
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19.2.2.1. Hydrology
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).
19.2.2.2. Terrestrial Ecosystems
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
(Hughes, 2000).
19.2.2.2.1. Vegetation
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 precipitation—which are much more uncertain
with regard to future climate—reorganization 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).
19.2.2.2.2. Animals
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.
19.2.2.3. Coastal Zones and Marine Ecosystems
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.
19.2.2.3.1. Physical processes
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).
