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As the sea ice melts in the Arctic, less incoming solar radiation
is reflected and is absorbed by the dark ocean waters, causing
more sea ice to melt (photo: NASA).
It is the coupling of ocean and atmosphere
models that serve as the basis of global
climate models because interactions between
both of these complex systems have one of the
greatest influences on climate. For example,
since the ocean covers more than 70 percent
of the Earth’s surface it stores vast amounts of
heat, most of which is located at the equator.
As the heat rises, it warms the atmosphere and
creates air temperature gradients (layers of hot
and cold air) along with winds. These winds
push against the sea surface, driving ocean
currents that circulate warm and cold ocean
waters to different parts of the planet. In a
sense, the Earth’s ocean and atmosphere
form one complex system that directly
influences climate.
When the ocean and atmosphere interact they
create ‘positive feedbacks’ that influence
one another in astonishing ways. Long says,
“A positive feedback is something which
reinforces the consequences of an initial
change”. He gives the example of a climate
process known as the ‘ice-albedo feedback’.
Sea ice in the Arctic is highly reflective.
Because it’s white it can reflect a lot of
incoming solar radiation back out into space,
maintaining or creating colder temperatures,
which in turn can create more sea ice. On
the other hand, if more sea ice is melting it
exposes more of the dark, low albedo sea,
which absorbs more solar radiation and heats
up the atmosphere, melting more sea ice.
These examples of positive feedbacks are
common in high northern latitudes where
changes in surface albedo of the land or the
oceans can change quickly. “That’s why many
scientists think that in the future or even today
the Arctic is warming much more quickly than
lower latitudes”, said Long.
Positive feedbacks also play a role in what is
known as ‘hysteresis’. In hysteresis, temporary
changes in a system are not only long-term, but
irreversible. If someone is to make sense of the
headline splashed on the cover of the morning
newspaper: ‘We have passed the climate tipping
point’, this could be understood as an extreme
example of hysteresis, where ultimate climate
change disaster is irreversible, but it is also
not that simple. The Greenland ice sheet keeps
retreating further and further due to positive
feedbacks that lead to more melting, however,
it is still uncertain as to whether it is indeed
irreversible. The Greenland Ice Sheet has been
much smaller today than in the past – for
example during the last interglacial, about
130,000 years ago, scientists now think the
ice sheet reduced in size by as much as a third
– but it didn’t melt entirely. In fact, it “re-grew
to larger than its present size during the last
ice age”, says Long. If small changes do
make a big difference then much can be
learnt from the past.
As Long has noted, in the case of the
mid-Holocene cooling event, there appear
to be no obvious external factors that brought
about this important cooling of the climate
in the North Atlantic region. This means that
there may be small internal changes in the
climate system that haven’t been accounted
for. But there is a bigger problem still – if the
changes are internal, and are the result of
positive feedbacks within the system, it may
prove highly difficult for researchers to be able
to detect them. Hysteresis also means that
systems may have a kind of ‘memory’ or
‘lag in time’.
So the changes impacting a system either
internally or externally may not come into
effect until much later. According to Long, in
terms of understanding future changes in the
Earth’s climate: “We can eyeball the data; we
can look at patterns from here and patterns
from there. But a more powerful way of doing
this is to integrate your observations with
climate modelling and that’s why we need our
mathematicians and other colleagues helping
us look at the data we’re developing”.
Whether tipping point describes actual, sudden
or transformative events in the world or is
simply a useful metaphor, it has created an
ongoing global discussion that seems to have
its own positive feedbacks. The more tipping
point is used in and outside of the social and
physical sciences, the more it seems to affect
how people identify spontaneous changes in
the world that we are only beginning to
understand. If we are to become more aware
of these unique changes over time and how
they affect the world we live in, the tipping
point concept may serve as a way to illuminate
pathways of knowledge never before taken.
If it does, we could be at the brink of uniting
the world we experience with the seemingly
unknown, myriad of co