Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 103

single flood to raise the level of the global ocean by as much as a
metre. The meltwater, being fresh, was light and so formed a low-
density lid over the North Atlantic. Being light it could not sink, even
when very cold, and so North Atlantic Deep Water formation could
be suddenly halted. This cut off the Gulf Stream at its roots and
brought it to a halt, spreading a chill across the surrounding land. The
chill would not be removed until this part of the ocean conveyor belt
could—once the salinity was restored—suddenly lurch back into
action to spread warmth once more around the north.
This oceanic switch is a remarkable part of the Earth machine, and
Wally Broecker's ideas—long fiercely contested—are now widely
accepted. 66 This mechanism shows how easy it is to click enormous
parts of the Earth machine into different configurations, where
patterns of heat and cold, of rainy areas and dry areas, can be more
or less instantly readjusted. There really are tipping points in the
Earth system: the evidence in the strata is clear, and the strata do not
lie (they might be coy about revealing the truth, but that's another
matter entirely). When these tipping points are crossed they can
transform entire regions, of both land and sea: as the sea is such
an enormous store of heat—it can hold much more than the
atmosphere—any changes to it also immediately affect the surround-
ing landscapes.
Could the system be brought to a halt again, to plunge northern
Europe into another deep freeze even as the rest of the world warms?
Well, there are no more great masses of ice on the North American
continent, but there is still a great ice cap on Greenland, and rivers
flowing into the Arctic from Siberia. Lately, the Arctic Ocean has
begun to freshen slightly as rivers have tended to flow more strongly
and Greenland ice has begun to melt at its edges. For the time being
this is not enough to disrupt global current patterns. It is a system to
watch very closely, though, in coming decades.