The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 273

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The astronomical theory and data compared

10.1 INTRODUCTION

In attempting to compare astronomical theory with historical climate data, a
number of issues arise. One issue is whether the relatively small percentage
changes that occur in solar intensity are sucient to cause major changes in
global climate. However, this question is complicated by the possibility that large
positive feedback forcings (albedo, ocean currents, wind changes, etc.) might be
initiated by smaller changes in solar intensity, leading to large climate changes
resulting from comparatively small perturbations. In this connection, it has been
postulated that there might be nonlinearities in the way that the climate responds
to perturbations, and there may be thresholds that cause discontinuous jumps to a
new state when crossed.
Another important question is how one should compare isotope time series
data with the time sequence of solar intensity variations? One relevant issue is
whether isotope data indicate local temperatures or global ice volume. Tempera-
ture measurements are indicative of current climate conditions and are likely to
fluctuate rather rapidly. Measurements of ice volume are cumulative and represent
an integration of past climate trends as expressed in the accumulation of ice. The
curves of ice accumulation vs. time tend to be less stochastic than temperature vs.
time curves. Variations in solar intensity take place over many thousands of years
and it is not immediately obvious how the timing of such variations should be
related to the time variability of temperature or the time variability of ice
volume—even assuming that astronomical forcing is the main driver of climate
change. Thus, in comparing the timing of changes in solar irradiance with the
timing of climate changes, one requires some sort of model that connects the two.
The process of deriving the chronology of isotope data is typically a montage
based on a number of different inputs. To the extent that some of these are tuned