Ice Ages Only thanks to Feedback
Science struggled to explain fully why an ice age occurs every 100,000
years. As researchers now demonstrate based on a computer simulation, not only
do variations in insolation play a key role, but also the mutual influence of
glaciated continents and climate.
Ice ages and warm periods have alternated fairly
regularly in the Earth’s history: the Earth’s climate cools roughly every
100,000 years, with vast areas of North America, Europe and Asia being buried
under thick ice sheets. Eventually, the pendulum swings back: it gets warmer
and the ice masses melt. While geologists and climate physicists found solid
evidence of this 100,000-year cycle in glacial moraines, marine sediments and
arctic ice, until now they were unable to find a plausible explanation for it.
Using computer simulations, a Japanese, Swiss and
American team including Heinz Blatter, an emeritus professor of physical
climatology at ETH Zurich, has now managed to demonstrate that the
ice-age/warm-period interchange depends heavily on the alternating influence of
continental ice sheets and climate.
“If an entire continent is covered in a layer of
ice that is 2,000 to 3,000 metres thick, the topography is completely
different,” says Blatter, explaining this feedback effect. “This and the
different albedo of glacial ice compared to ice-free earth lead to considerable
changes in the surface temperature and the air circulation in the atmosphere.”
Moreover, large-scale glaciation also alters the sea level and therefore the
ocean currents, which also affects the climate.
Weak effect with a strong impact
As the scientists from Tokyo University, ETH Zurich
and Columbia University demonstrated in their paper published in the journal
Nature, these feedback effects between the Earth and the climate occur on top of
other known mechanisms. It has long been clear that the climate is greatly
influenced by insolation on long-term time scales. Because the Earth’s rotation
and its orbit around the sun periodically change slightly, the insolation also
varies. If you examine this variation in detail, different overlapping cycles
of around 20,000, 40,000 and 100,000 years are recognisable (see box).
Given the fact that the 100,000-year insolation
cycle is comparatively weak, scientists could not easily explain the prominent
100,000-year-cycle of the ice ages with this information alone. With the aid of
the feedback effects, however, this is now possible.
Simulating the ice and climate
The researchers obtained their results from a
comprehensive computer model, where they combined an ice-sheet simulation with
an existing climate model, which enabled them to calculate the glaciation of
the northern hemisphere for the last 400,000 years. The model not only takes
the astronomical parameter values, ground topography and the physical flow
properties of glacial ice into account but also especially the climate and
feedback effects. “It’s the first time that the glaciation of the entire
northern hemisphere has been simulated with a climate model that includes all
the major aspects,” says Blatter.
Using the model, the researchers were also able to
explain why ice ages always begin slowly and end relatively quickly. The
ice-age ice masses accumulate over tens of thousands of years and recede within
the space of a few thousand years. Now we know why: it is not only the surface
temperature and precipitation that determine whether an ice sheet grows or
shrinks. Due to the aforementioned feedback effects, its fate also depends on
its size. “The larger the ice sheet, the colder the climate has to be to
preserve it,” says Blatter. In the case of smaller continental ice sheets that
are still forming, periods with a warmer climate are less likely to melt them.
It is a different story with a large ice sheet that stretches into lower
geographic latitudes: a comparatively brief warm spell of a few thousand years
can be enough to cause an ice sheet to melt and herald the end of an ice age.
The Milankovitch cycles
The explanation for the cyclical alternation of ice
and warm periods stems from Serbian mathematician Milutin Milankovitch
(1879-1958), who calculated the changes in the Earth’s orbit and the resulting
insolation on Earth, thus becoming the first to describe that the cyclical
changes in insolation are the result of an overlapping of a whole series of
cycles: the tilt of the Earth’s axis fluctuates by around two degrees in a
41,000-year cycle. Moreover, the Earth’s axis gyrates in a cycle of 26,000
years, much like a spinning top. Finally, the Earth’s elliptical orbit around
the sun changes in a cycle of around 100,000 years in two respects: on the one
hand, it changes from a weaker elliptical (circular) form into a stronger one.
On the other hand, the axis of this ellipsis turns in the plane of the Earth’s
orbit. The spinning of the Earth’s axis and the elliptical rotation of the axes
cause the day on which the Earth is closest to the sun (perihelion) to migrate
through the calendar year in a cycle of around 20,000 years: currently, it is
at the beginning of January; in around 10,000 years, however, it will be at the
beginning of July.
Based on his calculations, in 1941 Milankovitch
postulated that insolation in the summer characterises the ice and warm periods
at sixty-five degrees north, a theory that was rejected by the science
community during his lifetime. From the 1970s, however, it gradually became
clearer that it essentially coincides with the climate archives in marine
sediments and ice cores. Nowadays, Milankovitch’s theory is widely accepted.
“Milankovitch’s idea that insolation determines the ice ages was right in
principle,” says Blatter. “However, science soon recognised that additional
feedback effects in the climate system were necessary to explain ice ages. We
are now able to name and identify these effects accurately.”
Literature reference
Abe-Ouchi A, Saito F, Kawamura K, Raymo ME, Okuno
J, Takahashi K, Blatter H: Insolation-driven 100,000-year glacial cycles and
hysteresis of ice-sheet volume. Nature, 2013, 500: 190-193, doi:
10.1038/nature12374
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