This review paper summarizes recent research work on abrupt climatic changes and oscillations. The climatic system is viewed as a dissipative, highly non-linear system, under non-equilibrium conditions, and, as such, should be expected to have some unusual properties. These unusual properties include bifurcation points with marked instability just before the point, magnification of semi-periodic oscillations around bifurcation points, and variations in the strength of teleconnections with distance from equilibrium. These properties are discussed and illustrated for the climatic system using both the historical, Holocene and glacial climatic records. It is found that there are abrupt climatic changes and oscillations on all time-scales. The amplitudes and frequencies of climate variability and teleconnections are found to vary between different time periods. A number of persistent oscillations exist, particularly one about 1500 years, but their amplitudes vary considerably between time periods. The Holocene appears to be no more climatically benign than the similar period in the Eemian. The importance of the North Atlantic thermohaline circulation for generating abrupt climatic changes in Europe, particularly in association with sudden pulses of fresh water, is illustrated. The concept of antiphase temperature changes between the North and South Atlantic is discussed. Externally generated abrupt climatic deteriorations owing to explosive volcanic eruptions and variations in solar irradiance are also discussed.
The astronomical theory of the ice ages is investigated using a simple climate model which includes the ice sheets explicitly. A one-level, zonally averaged, seasonal energy-balance equation is solved numerically for sea-level temperature T as a function of latitude and month (similar to North, 1975). Seasonally varying snow cover (which affects planetary albedo) is included diagnostically by parameterizing monthly snowfall and snowmelt in simple ways. The net annual accumulation and ablation on the ice sheet surface at each latitude are computed using the same parameterizations as for snow cover above (with T corrected for ice sheet height using a lapse rate of -6.5OC km-I). Treatment of the ice sheets follows Weertman (1976) with ice flow approximated as perfect plasticity, which constrains the ice sheet profiles to be parabolic. The northern hemisphere's ice sheet is constrained to extend equatorward from 75ON (corresponding to the Arctic Ocean shoreline).Model ice age curves are generated for the last several I00 Kyears by computing the seasonal climate as above once every 2 Kyears, with insolation calculated from actual Earth orbit perturbations. The change in ice sheet size for each 2 Kyear time step depends only on the net annual snow budget integrated over the whole ice sheet surface. In these model runs, the equatorward tip of the northern hemisphere's ice sheet oscillates through -7 O in latitude, correctly simulating the phases and approximate amplitude of the higher frequency components (-43 Kyear and 22 Kyear) of the deep-sea core data (Hays et al., 1976). However, the model fails to simulate the dominant glacial-interglacial cycles (-100 to 120 Kyear) of this data. The sensitivity of the model ice age curves to various parameter changes is described, but none of these changes significantly improve the fit of the model ice age curves to the data. In the concluding section we generalize about the types of mechanisms that might yield realistic glacial-interglacial cycles.
Estimates made of lowland precipitation and evaporation in Britain during the last (Devensian) glaciation suggest that during the cold periods of the Devensian precipitation was probably low, between 260 and 370 mm/thermal maximum of the Upton Warren Interstadial lowland precipitation was probably in the range 450 to 650 mm/year. Two summer precipitation regimes are identified during the cold periods, one with high values and the other with low. The high summer precipitation variant leads to moist conditions with July and August precipitation values similar to those at the present day, and global circulation models suggest that the moist regime may have existed at the time of the maximum advance of the ice sheets. On the other hand, the low summer precipitation variant leads to a dry summer with wind action creating aeolian deposits, and this variant probably existed at earlier times in the glacial period. About 6500 yr B.P., in the Atlantic period, forest conditions probably caused increased evaporation which more than compensated for the increased precipitation of the time, causing low runoff conditions. The clearance of British forests by man since 6500 yr B.P. has probably led to an absolute increase in runoff values even though precipitation values have fallen.
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