Sensitivity and response time of natural systems to climatic change in the late quaternary

Abstract

Although the leading theory for the cause of climatic change — the Milankovitch perturbations in insolation — indicates variable rates of change at different latitudes, any discussion of the sensitivity and response time of different natural systems is most practical when the climatic change is considered to be relatively abrupt and global rather than gradual and regional. The oxygen-isotope record in glacial ice should have the swiftest response time, but the stratigraphic record in ice sheets at particular localities may be obscured or confused if the glacier flow is irregular. The oxygen-isotope stratigraphy of ocean sediment cores is a sensitive reflection primarily of contemporaneous ice volume, and to a lesser extent ocean temperature. Ocean temperature, however, is recorded more specifically by microfossils in ocean sediments and may reflect global insolation directly, except that in the North Atlantic the influence of glacial meltwater and various feedbacks may override the insolation factor. The differential lags in the response of isotopes and faunas to insolational change are apparent in the ocean sediment stratigraphy. Ice sheets themselves at their terminus may respond to climatic change only slowly if a change in snow accumulation is the factor, because of the time involved in building a thickness sufficient for flow to great distances, but if wastage near the margin is the factor then the response may be more rapid. Some glaciers, however, may advance rapidly without regard to climatic change, as in the case of surging, or they may retreat just as rapidly if they terminate in deep water, through iceberg formation. World sea level is depressed with glacial growth and thus reflects the volume of ice sheets, but isostatic changes in the crust complicate the sea level response not only locally beneath the ice load but elsewhere as well. The pluvial lakes of the American Southwest, correlated in their high levels with intervals of glaciation, show a direct response to climatic patterns resulting from the expansion of the North American ice sheets and thus may lag with respect to the primary climatic change that causes ice-sheet growth. North African lakes, on the other hand, respond to the intensity of monsoonal circulation, which is controlled directly by insolational shifts. The response of plants and insects to deglaciation can be immediate in the case of pioneers on newly exposed terrain, although the deglaciation itself may lag behind climatic change. Succession can follow rapidly, but then more slowly as other plants immigrate from great distances. It is heare where the more realistic assumption of continuing climatic change may be necessary to explain the dynamics of long-range vegetational change following the last deglaciation. The distribution and abundance of vertebrate populations reflect primarily their food resources, and the ‘disharmonious’ faunas of the last glacial period in North America — with components that no longer have overlapping ranges — perhaps may be explained by vegetational associations that likewise may have no modern analogues. The cause may be a different climatic seasonality. Wastage of the ice-sheet — itself lagging behind the climatic change — may have changed the seasonal climatic patterns sufficiently to bring about the precipitous extinction of many vertebrate species.