19
Are disaster scenarios about tipping points like “turning off the Gulf Stream” and release of methane from the Arctic a cause for concern?
Results from
the best available climate models do not predict an abrupt change in (or
collapse of) the Atlantic Meridional Overturning Circulation, which includes
the Gulf Stream, in the near future. However, this and other potential
high-risk abrupt changes, like the release of methane and carbon dioxide from
thawing permafrost, remain active areas of scientific research. Some abrupt
changes are already underway, such as the decrease in Arctic sea ice extent,
and as warming increases, the possibility of other major abrupt changes cannot
be ruled out.
The
composition of the atmosphere is changing towards conditions that have not been
experienced for millions of years, so we are headed for unknown territory, and
uncertainty is large. The climate system involves many competing processes that
could switch the climate into a different state once a threshold has been
exceeded.
A well-known
example is the south-north ocean overturning circulation, which is maintained
by cold salty water sinking in the North Atlantic and involves the transport of
extra heat to the North Atlantic via the Gulf Stream. During the last ice age,
pulses of freshwater from the melting ice sheet over North America led to
slowing down of this overturning circulation. This in turn caused widespread
changes in climate around the Northern Hemisphere. Freshening of the North
Atlantic from the melting of the Greenland ice sheet is gradual, however, and
hence is not expected to cause abrupt changes.
Another
concern relates to the Arctic, where substantial warming could destabilise
methane (a greenhouse gas) trapped in ocean sediments and permafrost,
potentially leading to a rapid release of a large amount of methane. If such a
rapid release occurred, then major, fast climate changes would ensue. Such
high-risk changes are considered unlikely in this century, but are by
definition hard to predict. Scientists are therefore continuing to study the
possibility of exceeding such tipping points, beyond which we risk large and
abrupt changes.
In addition
to abrupt changes in the climate system itself, steady climate change can cross
thresholds that trigger abrupt changes in other systems. In human systems, for
example, infrastructure has typically been built to accommodate the climate
variability at the time of construction. Gradual climate changes can cause
abrupt changes in the utility of the infrastructure—such as when rising sea
levels suddenly surpass sea walls, or when thawing permafrost causes the sudden
collapse of pipelines, buildings, or roads. In natural systems, as air and
water temperatures rise, some species—such as the mountain pika and many ocean
corals—will no longer be able to survive in their current habitats and will be
forced to relocate (if possible) or rapidly adapt. Other species may fare
better in the new conditions, causing abrupt shifts in the balance of
ecosystems; for example, warmer temperatures have allowed more bark beetles to
survive over winter in some regions, where beetle outbreaks have destroyed
forests.
20
If emissions of greenhouse gases were
stopped, would the climate return to the conditions of 200 years ago?
No. Even if
emissions of greenhouse gases were to suddenly stop, Earth’s surface
temperature would require thousands of years to cool and return to the level in
the pre-industrial era.
If emissions
of CO2 stopped altogether, it would take many thousands of years for
atmospheric CO2 to return to “pre-industrial” levels due to its very slow
transfer to the deep ocean and ultimate burial in ocean sediments. Surface
temperatures would stay elevated for at least a thousand years, implying a
long-term commitment to a warmer planet due to past and current emissions. Sea
level would likely continue to rise for many centuries even after temperature
stopped increasing [Figure 9]. Significant cooling would be required to reverse
melting of glaciers and the Greenland ice sheet, which formed during past cold
climates. The current CO2-induced warming of Earth is therefore essentially
irreversible on human timescales. The amount and rate of further warming will
depend almost entirely on how much more CO2 humankind emits.
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