Statistically significant associations between trends in regional climate and
impacts have been documented in ~100 physical processes and ~450 biological
species or communities in terrestrial and polar environments. More than 90%
of the changes (~99% physical, ~80% biophysical) documented worldwide are consistent
with how physical and biological processes are known to respond to climate.
There are systematic trends of ecological change across major taxonomic groups
(amphibians, birds, insects, mammals, reptiles, and invertebrates) inhabiting
diverse climatic zones and habitats. The overall processes and patterns of observations
reveal a widespread and coherent impact of 20th-century climate changes on many
physical and biological systems (see Figure 19-2).
Expected directions of change relating to regional climate warming for physical
systems have been reported in studies documenting shrinkage of glaciers, decreases
in snow cover, shortening of duration of lake- and river-ice cover, declines
in sea-ice extent and thickness, lengthening of frost-free seasons, and intensification
of the hydrological cycle. Expected directions of change relating to regional
climate warming for biological systems have been reported in studies documenting
poleward and elevational shifts in distribution and earlier phenology (i.e.,
earlier breeding, emergence, flowering) in plant and animal species.
In general, geographic patterns of responses also conform to expectations relating
to regional climate change, as opposed to alternative explanations. Reported
cases of observed impacts are concentrated in high-latitude and high-altitude
physical and biological systems and tend to be in regions where observed regional
warming has been greatest and confounding factors often are at least partially
minimized. Although land-use change, pollution, and biotic invasions are widespread
anthropogenic influences, they are unlikely to cause the spatial patterns (e.g.,
skewed poleward and elevational range shifts) and temporal patterns (e.g., earlier
breeding and flowering) that are documented over the set of reported studies.
The sample of studies shown in Figure 19-2 was
drawn from a literature survey with keywords relating to climate trends and
observed trends in impacts. The time period of most of the studies includes
the recent warm period beginning in the late 1970s. The geographical distribution
of studies to date is biased toward Europe and North America but does include
evidence of observed impacts of regional climate change relating to physical
processes from all continents. The spottiness of biological evidence in other
regions may indicate that observed impacts of regional climate change are not
occurring, have not yet been detected, or are being masked by other changes,
such as urbanization. Many studies include multiple species and report on the
number of species that responded to regional climate changes as expected, not
as expected, or exhibited no change. Most of the biophysical studies included
in Figure 19-2 report on statistical tests of trends
in climate variable, trends in observed impacts, and relationships between the
two (see Chapter 5).
In Figure 19-2, ~16 studies examining glaciers,
sea ice, snow-cover extent/snowmelt, or ice on lakes or streams at more than
150 sites were selected. Of these ~150 sites, 67% (~100) show change in one
or more variable(s) over time. Of these ~100 sites, about 99% exhibit trends
in a direction that is expected, given scientific understanding of known mechanisms
that relate temperatures to physical processes that affect change in that variable.
The probability that this proportion of sites would show directional changes
by chance is much less than 0.00001.
There are preliminary indications that some social and economic systems have
been affected in part by 20th-century regional climate changes (e.g., increased
damages from flooding and droughts in some locations). It generally is difficult
to separate climate change effects from coincident or alternative explanations
for such observed regional impacts. Evidence from studies relating regional
climate change impacts on socioeconomic systems has been reviewed but is not
included in the summary figure because of the complexities inherent in those
The effects of regional climate change observed to date provide information
about the potential vulnerability of physical, biological, and socioeconomic
systems to climate change in terms of exposure, sensitivity, and adaptive capacity.
Some of the observed effects are adaptations. In some cases, observed impacts
are large relative to the levels of regional climate changes (e.g., large changes
in ecosystem dynamics with small changes in regional climate). In general, observations
of impacts agree with predictions that estimate more serious impacts at higher
GHG concentrations because the greater regional climate changes are associated
with stronger impacts.
Relating the observed impacts summarized here to the reasons for concern analyzed
in this chapter, we find the following:
- There is preliminary evidence that unique and threatened systems are beginning
to be affected by regional climate change (e.g., glaciers, polar environments,
- With regard to the distributional effects of observed impacts relating to
regional climate changes, most evidence to date comes from high-latitude and
high-altitude environments, where regional warming has been and is expected
to be more pronounced.
- Aggregate impacts of regional climate changes at the global level are difficult
to define, except in sectors in which there is a common metric, as in market
sectors. The many simultaneous factors and varying adaptive capacities make
extracting aggregate effects attributable to observed climate change difficult.
What can be stated in summary regarding the diverse set of impacts reported
to date is that there are cases of observed impacts in many diverse environments;
that they occur in a wide array of processes, species, and ecosystems; and that
the overall patterns and processes reveal a coherent impact of 20th-century
climate changes on many physical and biological systems.
- Impacts of extreme events have been implicated in many of the observations
summarized in this section, including increases in extreme precipitation events
in some locations.
- There is no current evidence in observed impacts that large-scale abrupt
changes already are occurring. Yet, paleoclimate evidence (see TAR
WGI Chapter 2) shows that such changes have occurred
in physical and biological systems in the past and therefore may occur with
a continuation of the current warming trend.