Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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19.2.3. Conclusions

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 systems.

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:

  1. There is preliminary evidence that unique and threatened systems are beginning to be affected by regional climate change (e.g., glaciers, polar environments, rare species).
  2. 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.
  3. 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.
  4. 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.
  5. 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.

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