Rates of Change at Centennial Timescales

Ice cores also provide the resolution and the record length necessary for examination of slower oscillations. Measures of O from the Camp Century and Dye 3 cores from Greenland both oscillate 4% at century timescales (equivalent to 6C), suggesting the existence of two quasi-stable modes of atmospheric circulation over this time frame [ Dansgaard et al., 1982]. Thompson [1989] compared cores across a global north-south transect from Greenland to the South Pole and showed distinct decadal and centennial oscillations consistent between Northern and Southern Hemisphere sites. Mayewski et al. [1993] showed not only the rapid fluctuations at the end of the Younger Dryas, but also peaks of ion concentration approximately every 400-500 years through the Pre-Boreal to the Older Dryas (11 to 14 kyr BP). Concentration of ammonium, which reflects biomass burning and other terrestrial exchanges [ Langford et al., 1992], and sulfate, a measure of volcanic and marine biospheric activity, aid understanding of past climates. Additional work involving the Greenland cores by Johnsen et al. [1992] produced a time line of volcanic eruptions and transitions between climatic era from the onset of the Bø lling interstadial (14.5 kyr BP) to the recent past. The record manifests event frequency of one volcano per century in the recent part of the record, with a few hundred years between events in the older portion of the core.

Evidence of century-scale climate variability is also preserved in lake sediments. Measures of total organic matter from laminated sediments of lakes in East Africa indicate that water level, salinity, and diatom species composition fluctuated greatly over the last deglaciation, reflecting synchrony of the Younger Dryas in high and low latitudes [ Roberts et al., 1993]. Williamson et al. [1993] also analyzed sediment cores from Lake Magadi (Kenya) and used magnetic parameters and lithology to determine lake level changes between 12-10 kyr BP. The authors concluded that the separation of Lake Magadi and nearby Lake Natron between 11 and 10 kyr BP provides evidence of an arid event in eastern equatorial Africa during the Younger Dryas. They also suggested that while separation of the lakes was fairly abrupt, the monsoon that fed the lakes progressively weakened over a period of a few centuries. Abrupt shifts from humid to arid conditions in the Sahel and the Sahara coincident with the Younger Dryas also occurred within a few centuries [ Gasse et al., 1990]. Carbon and nitrogen isotopic composition and bulk organic matter in sediment cores from Lake Bosumtwi, Ghana, also revealed dry periods coincident with the Younger Dryas [ Talbot and Johannessen, 1992]. This record, which covers the last 27.5 kyr, indicated that climatic change over this period of record induced significant variations in the size and chemistry of the lake as well as influencing the surrounding vegetation, which oscillated between forest and grassland at frequencies of hundreds to thousands of years.

A coupled atmosphere-ocean general circulation model was used by Manabe and Stouffer [1993] to explore century timescale effects of gradual increases in atmospheric CO concentrations to doubling and quadrupling endpoints. The model projected temperature increases of 3.5 and 7C over 500 years for the two scenarios (Figure 3), respectively, and a minimum sea level rise of 1 to 2 m over the same time period. The projected rates of change are thus consistent with those that occurred over the last deglaciation, but Broecker [1987] cautioned the architecture of extant models does not allow for key interactions (such as feedbacks from ecological change) which could cause the climate system to respond in a more abrupt fashion: this caution remains valid (see ``Thresholds and nonlinearities'' section).