Jökull - 01.01.2005, Síða 9

Global Warming: Take Action or Wait?
Figure 6. The record of air temperature kept in the Summit, Greenland ice cores (Stuiver and Grootes, 2000). –
Samsætuhlutföll súrefnissamsæta í ískjarna frá hábungu Grænlandsjökuls endurspegla sveiflur í hitastigi.
southward shift of the thermal equator resulting from
the severe northern cooling. The former was caused
by the lengthened presence of snow cover in the north
temperate region which delayed the summer warming
of Asia and thereby weakened the monsoons (Barnett
et al., 1988).
This second example not only reveals one of na-
ture’s hidden amplifiers, but also makes it clear that
interactions between the ocean and atmosphere give
rise to what might be termed as discreet quantum
states of climate. The Earth system is capable of
abrupt jumps between these states. These jumps have
profound impacts on not only the distribution of tem-
perature and precipitation, but also on the contents of
carbon dioxide and methane in the atmosphere.
Example 3. By comparison with the last glacial
period, climate during the present interglacial has
been remarkably well behaved. Only once (8200
years ago) did the Atlantic’s conveyor circulation shut
down (Alley et al., 1997). But rather than remaining
off for many centuries, only a few decades elapsed
before it popped back into action. Of interest in
connection with our quest to evaluate the predictions
made by computer-generated simulations is a series of
small cycles in temperature (Bond et al., 2001) well
recorded at high northern latitudes. Each full cycle
involved a temperature swing of about 1◦C and lasted
about 1500 years. The Medieval Warm (800 to 1350
AD) – Little Ice (1350 to 1850) pair was the most
recent of this series. Interest in these cycles stems
from the fact that their timing closely matches that
for cycles in the production rate in our atmosphere
of two radioisotopes, carbon fourteen and beryllium
ten (Bond et al., 2001). These changes in produc-
tion rate reflect the modulation of the inflow of ener-
getic galactic cosmic rays into our atmosphere. This
modulation is the result of variations in the strength
of the magnetic field created by ions which stream
JÖKULL No. 55, 2005 9