Another worry is that Arctic warming will influence ocean circulation in ways that
are not fully understood. One link in the chain is the salinity of seawater,
which is decreasing in the north Atlantic thanks to an increase in glacial melt waters.
“Because fresh water and salt water have different densities, this ‘freshening’
of the ocean could change circulation patterns.” said Dr. Thomson, a British climate expert.
“The most celebrated risk is to the mid-Atlantic Conveyor Belt, a current which brings warm
water from the tropics to north-western Europe, and which is responsible for that region’s
unusually mild winters,” he added. Some of the ACIA’s experts are fretting over evidence
of reduced density and salinity in waters near the Arctic that could adversely affect this current.
The biggest popular worry, though, is that melting Arctic ice could lead to a
dramatic rise in sea level. Here, a few caveats are needed. For a start,
much of the ice in the Arctic is floating in the sea already. Archimedes’s principle
shows that the melting of this ice will make no immediate difference to the sea’s level,
although it would change its albedo. Second, if land ice, such as that covering Greenland,
does melt in large quantities, the process will take centuries. And third, although
the experts are indeed worried that global warming might cause the oceans to rise,
the main way they believe this will happen is by thermal expansion of the water itself.
Nevertheless, there is some cause for nervousness. As the ACIA researchers document,
there are signs that the massive Greenland ice sheet might be melting more rapidly
than was thought a few years ago. Cracks in the sheet appear to be allowing melt water
to trickle to its base, explains Michael Oppenheimer, a climatologist at
Princeton University who was not one of the report’s authors.
That water may act as a lubricant, speeding up the sheet’s movement into the sea.
If the entire sheet melted, the sea might rise by 6-7 meters. But when will this
kind of disastrous ice disintegration really happen? While acknowledging it
this century is still an unlikely outcome, Dr. Oppenheimer argues that the evidence
of the past few years suggests it is more likely to happen over the next few centuries
if the world does not reduce emissions of greenhouse gases.
He worries that an accelerating Arctic warming trend may yet
push the ice melt beyond an “irreversible on / off switch”.