Climate Change: The Past

Global Climate changes, and their consequences for human populations, are known from studies of the climate of the recent geological past. Instrumental climate records generally do not capture the full range of climate variability and long-term trends (i.e., climate change) because they are mostly confined to the past 150 years. This period is shorter than many cycles of climate variation and corresponds to the interval of time during which human activities have significantly increased the atmospheric concentrations of greenhouse gases. Proxy climate data span millions of years and thus define what is possible in terms of natural climate change and variability, providing the context for the current global warming. A plot of Northern Hemisphere temperatures for the past millennium, reconstructed mainly from tree rings, is one of the most convincing and widely used illustrations of the unusual rate of recent warming. It demonstrates that the 1990s was the warmest decade of the past millennium, and 1998 the warmest single year. The past 200 years have also seen the coldest years of the past several millennia because the earth recently emerged from the Little Ice Age (approximately 1450 to 1850 CE). The climate system is extremely complex, with many factors interacting over a range of time scales and with teleconnections over long distances. Over millions of years, the earth's climate history is related to the drifting of continents and resulting changes in the configuration of the continents and ocean basins. Periodicity in the position of the earth relative to the sun is a dominant cause of shorter-term climate variation over thousands of years, the scale of ice ages. Superimposed on these long cycles of solar irradiance are shorter-term fluctuations in the sun's energy (sunspot cycles) and climate fluctuations caused by the effects of volcanic eruptions on the composition of the atmosphere. Annual to decadal climate variation, the time scale of most relevance to humans, is sensitive to internal oscillations in the climate system such as fluctuations in sea surface temperatures associated with ocean circulation anomalies like ENSO (El Nino Southern Oscillation).

Past climate is inferred from geological and biological archives, which preserve a measurable response to climate; the longest paleoclimate records are derived from glaciers and ocean sediments. In continental interiors like Saskatchewan, the common sources of proxy climate data are the minerals and the plant and animal fossils that accumulate in lakes. Other archives include tree rings, buried Soils and terrestrial sediments, and ground temperatures. While these proxies record different types of climate signal, a general picture of post-glacial climate emerges that includes an interval of warmer drier climate about 5,000 to 7,500 years ago, in the middle of the Holocene (the last 10,000 years), and the cooler conditions of the Little Ice Age of the past millennium. In Saskatchewan, variations in climate over the past several millennia are reflected in the migration of the boundary between grassland and forest, and fluctuations in the level and salinity of lakes. Tree rings record prolonged droughts prior to Euro-Canadian settlement; these long droughts affected sand dunes activity, the Fur Trade, and the health of Aboriginal people. The tree rings and other climate proxies suggest that the climate of the 20th century was relatively favourable for the settlement of Saskatchewan because it lacked the sustained droughts of preceding centuries.

David Sauchyn

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Further Reading

Mann, M.E., R.S Bradley and M.K Hughes. 1999. “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties and Limitations,” Geophysical Research Letters 26: 759-62; Sauchyn, D.J., E. Barrow, R. F. Hopkinson and P. Leavitt. 2002. “Aridity on the Canadian Plains,” Géographie physique et quaternaire 56: 247-59; Sauchyn, D.J., J. Stroich and A. Beriault. 2003. “A Paleoclimatic Context for the Drought of 1999-2001 in the Northern Great Plains,” The Geographical Journal 169: 158-67.