The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 277

to 138 kybp shows a decrease in solar intensity as temperatures were rising. Curves
B, C, D, and E are suggestive of temperature variations over their corresponding
time periods. There are no indications of any significant time lags in comparing
solar intensity with temperature. One would have to conclude that Figure 10.1 is
somewhat supportive of the astronomical theory, although the question of time
lags (or lack thereof ) remains. However, the drop in solar intensity in the past
10,000 years has not produced a corresponding decrease in temperature.
One obvious conclusion from Figure 10.1 is that there are forces at play that
produce very large rapid changes in climate, and there does not seem to be any
way that slow ponderous changes in solar input could directly cause these sudden
changes. On the other hand, it is possible that solar input, or its rate of change,
could trigger other nonlinear effects that could introduce instability in the climate
under some conditions. The three likely candidates for such nonlinear effects are
(a) changes in average water vapor concentration, (b) changes in albedo due to
variation in ice/snow cover, and (c) changes in cloudiness. The potential triggers
to initiate such changes might include variability in the meridional overturning
circulation (MOC) or changes in the wind field (see Section 8.6.2). Observations
regarding Figure 10.1 are given in Table 10.1 . While there is fairly good correla-
tion between the overall envelope of temperature changes and the variability of
solar input to high northern latitudes, the wild gyrations superimposed on this
slowly varying background do not seem to be related to solar variability. But,
since these abrupt climate changes are almost as great in magnitude as longer
term secular changes, any explanation of

long-term secular changes would

Table 10.1. Comparison of Greenland temperature trends with solar inputs for the past
150,000 years.

Time period Greenland Solar input trend Comment

( kybp ) temperature

trend

140 to 125 Sharp rise Sharp rise Good correlation after 135,000 ybp .

However, temperature rise began while
insolation was decreasing

130 to 115 Warm, not

Sharp drop No correlation at all. Sharp drop in

much change insolation was not matched by a drop

in temperature

110 to 20 Several large Many oscillations Envelope of temperature data might

oscillations

but trending suggest solar trends

downward

20 to 10 Sharp rise Sharp rise Good correlation

10 to 0 Warm, not Sharp drop No correlation at all

much change