Climatic and Environmental Processes Centennial
years) Causes of Climate Change Over the Past
Forcing Factors Millennial
looking for climate processes and the forces that influence them at periods
ranging from 100 to 1000 years, paleoclimatologists splice instrumented
data with calibrated proxy data such from tree rings, cores from icecaps,
glaciers, marine and lake sediments layers, and corals, and evidence of
vegetation change found in pollen samples and packrat middens.
Some climate patterns or possible millennial scale oscillations have been
observed in the paleo record that are not necessarily operating today.
For example, scientists examining the long-term climate record from ice
cores have noted a ~900 year oscillation that has appeared in the North
Atlantic. In their article "Holocene climate variability on centennial-to-millennial
time scales," Schulz and Paul
(2002) note that "Proxies of atmospheric temperature and humidity
from Greenland and northern/central Europe show evidence for 900 year
climate oscillations between 3,000 and 8,500 years ago. The magnitude
of the climate perturbations in Europe was probably large enough to affect
human societies, especially since they occurred during the important transition
from hunting- gathering life style to sedentary agriculture."
of Climate Change Over the Past 1000 Years
Image of Mount St. Helens from USGS
While the climate of
the past 1000 years may have its own unique qualities, such as
the growth in human population, it does serve as a fascinating
case study of how climate varies in both subtle and sometimes
According to the research of Crowley
(2000), between 40-65% of decadal-scale
temperature variations during the past 1000 years prior to 1850
were caused by changes in solar irradiance and volcanism. While
individual volcanoes usually only impact climate for a year or
so, clustered eruptions can perturb the climate system for longer
periods of time.
Reconstructions of temperatures and climate forcing are crucial
for developing and testing climate models that can separate natural
climate variability from the impact of human activities such as
the release of carbon dioxide from the combustion of fossil fuels
and changes in land cover.
Measured Scientists rely on Paleoclimatic Proxy records to reconstruct
variability and climate patterns at the 1000 year scale.
Cores from coral reefs, ice, ocean and lake sediments can provide
an array of information including temperature, precipitation,
chemical composition of air or water, biomass or vegetation,
volcanic eruptions and solar activity with
varying degrees of accuracy and detail.
The figure below is from Mann,
M. E., Rutherford, S., et. al. (2003). Optimal Surface Temperature Reconstructions
Using Terrestrial Borehole Data in JGR Atmospheres. Figure shows comparisons
between different Northern Hemisphere temperature reconstructions and instrumental
record. Shown are smoothed (40 year lowpassed) reconstructions and, in the
case of the Mann et al reconstruction, the associated 95% confidence interval.
Shown for comparison are the HPS00 reconstruction, and the really-weighted
mean of gridded HPS00 borehole reconstructions.