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Climate Science: Investigating Climatic and Environmental Processes
Centennial Scale (<103 years)
Causes of Climate Change Over the Past 1000 Years
Forcing Factors

Millennial Temperature Reconstruction
Image showing various paleoclimate researchIn 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."

Causes of Climate Change Over the Past 1000 Years
Image of Mt. St. Helens from USGS
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 dramatic ways.

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.

(See large graph summarizing the research and Causes of Climate Change Over the Past 1000 Years

Image of globe

Internal influences on climatic variability at the centennial to millennial scales include
ocean and land carbon cycles, and the thermohaline circulation or THC which relates to ocean temperature and salt balance.

How 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.

Temporal Scope and Potential Information from Paleoclimatic Proxies for more on the types of information that various proxies provide to research scientists.

Millennial temperature reconstruction by Mann et. al.Millennial Temperature Reconstruction
The graph to the right is from the Mann et. al. (1999) study Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations.

Also see Robertson, et. al. (2001)Hypothesized Climate Forcing Time Series for the Last 500 Years

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.

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