2.2. Detection of Response to Climate Change by Using Indicator Species or
Climate change may cause responses in many human and natural systems, influencing
human health (disease outbreaks, heat/cold stress), agriculture (yield, pest
outbreaks, crop timing), physical systems (glaciers, icepack, streamflow), and
biological systems (distributions/abundances of species, timing of events).
In intensely human-managed systems, the direct effects of climate change may
be either buffered or so completely confounded with other factors that they
become impossible to detect. Conversely, in systems with little human manipulation,
the effects of climate change are most transparent. Systems for which we have
a good process-based understanding of the effects of climate and weather events,
and have had minimal human intervention, may act as indicators for the more
general effects of climate change in systems and sectors where they are less
2.2.1. Detection in Natural Systems
188.8.131.52. Predicted Physical Responses to Climatic Warming
The cryosphere is very sensitive to climate change because of its proximity
to melting. Consequently, the size, extent, and position of margins of various
elements of the cryosphere (sea ice, river and lake ice, snow cover, glaciers,
ice cores, permafrost) are frequently used to indicate past climates and can
serve as indicators of current climate change (Bradley and Jones, 1992; Fitzharris,
1996; Everett and Fitzharris, 1998). In particular, former glacier extent is
indicative of past glacials and the Little Ice Age. At high latitudes and high
altitudes, ice cores have provided high-resolution annual (and, in some cases,
seasonal) records of past precipitation, temperatures, and atmospheric composition.
These records stretch back for many hundreds of years, well before the instrumental
period, so they have proven to be very valuable in documenting past climates.
Borehole measurements provide data on permafrost warming. Later freeze-up and
earlier breakup of river and lake ice is measurable at high latitudes.
Interpretation of climate change resulting from changes in the cryosphere is
seldom simple. For example, in the case of glaciers, glacier dynamics and extent
are influenced by numerous factors other than climate. Different response times
are observed for the same climate forcing, so some glaciers can be in retreat
while others are advancing. Changes in glacier size can be caused by changes
in temperature or in precipitationor even a nonlinear combination of both.
Similarly, changes in sea ice can be a result of changes in ice dynamics (winds,
currents) as much as thermodynamics (temperature). Thus, attribution of the
exact nature of climate change from changes in the cryosphere is quite complicated.
With many measures of the cryosphere, there frequently is large interannual
variability. This makes determination of possible anthropogenic climate trends
difficult to distinguish from the natural noise of the data. Another problem
is that high-resolution records usually are not available, except from polar
or high-altitude ice cores. Changes in the extent of sea ice and seasonal snow
are best observed with satellites, but such records are relatively short (from
about 1970), so long-term climate change is difficult to distinguish from short-term,
natural variability. The first records of cryospheric extent and changes often
come from documentary sources such as old diaries, logbooks of ships, company
records, and chronicles (Bradley and Jones, 1992). Although these sources are
fraught with difficulty of interpretation, they clearly demonstrate climate
changes such as the medieval warm period and the Little Ice Age (see Section
5.7, Chapter 16, and Section 19.2).