Geoscience Reference
In-Depth Information
The interesting feature is that this model shows a “hysteresis loop,”
or what is sometimes called path dependence. The dashed lower line in
Figure 12 shows an alternative set of stable ice sheet sizes. Starting from
a low-elevation warm-world situation, the ice sheet responds differently
as the globe warms. For this case, you can follow the lower set of arrows.
Suppose the globe starts at an initial temperature of 6°C of warming or
more with a small ice sheet. As the globe cools from 6°C to 5°C, the ice
sheet hardly grows at all. Indeed, the ice sheet begins to recover only
when temperature declines below 3°C. Even if the globe returns to to-
day's temperature of around 1°C, the GIS grows to only one-fi fth of its
present volume. Finally, as the globe cools suffi ciently, the ice sheet re-
gains its current size.
Figure 12 is a striking example of the kind of instability that scien-
tists worry about. It shows how complex dynamic systems can move into
completely different states when they are pushed beyond some tipping
point. The behavior is similar to that of a tippy canoe at super-slow
motion—but much more frightening and consequential on a plane-
tary scale.
I must emphasize that, while the picture in Figure 12 comes from a
detailed computer model of the GIS, it is highly simplifi ed. Other mod-
els show different patterns. Scientists do not know for sure if there are
steep slopes like the one in Figure 12, or if the slippery slope is at 2 or 4
or 6°C, or if there might be many slippery slopes and many different solid
and dashed lines. However, the worrisome fi nding is that the strange tip-
ping behavior shown in Figures 11 and 12 has been found in different
areas of the earth's systems.
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The example of the GIS illustrates several points. First, all the sys-
tems involved in the analysis of tipping points are perplexing because
they involve poorly understood dynamics and nonlinear responses. We
generally do not know exactly where a tipping point is, or when we will
cross it, or whether we can climb back over the tipping point to the good
equilibrium with a large enough effort. If we use the analogy of the little
double-bottomed bowl in Figure 11, we need to understand exactly how
steep the sides of the bowl are, how much the bowl is being tipped, and
how deep the second bad equilibrium is. The fact is that we do not know
