Earth's orbital variations and sea ice synch glacial periods

Earth's orbital variations and sea ice synch glacial periods

by Kevin Stacey,
Brown University

New research shows how sea ice growth in the Southern
Hemisphere during certain orbital periods could control the pace of ice
ages on Earth. Credit: Brown University

Earth is currently in what climatologists call an interglacial
period, a warm pulse between long, cold ice ages when glaciers dominate
our planet's higher latitudes. For the past million years, these
glacial-interglacial cycles have repeated roughly on a 100,000-year
cycle. Now a team of Brown University researchers has a new explanation
for that timing and why the cycle was different before a million years
ago.

Using
a set of computer simulations, the researchers show that two periodic
variations in Earth's orbit combine on a 100,000-year cycle
to cause an expansion of sea ice in the Southern Hemisphere. Compared
to open ocean waters, that ice reflects more of the sun's rays back into
space, substantially reducing the amount of solar energy the planet
absorbs. As a result, global temperature cools.


"The 100,000-year pace of glacial-interglacial periods has been
difficult to explain," said Jung-Eun Lee, an assistant professor in
Brown's Department of Earth, Environmental and Planetary Studies and the
study's lead author. "What we were able to show is the importance of
sea ice in the Southern Hemisphere along with orbital forcings in
setting the pace for the glacial-interglacial cycle."


The research is published in the journal Geophysical Research Letters.


Orbit and climate


In the 1930s, Serbian scientist Milutin Milankovitch identified three
different recurring changes in Earth's orbital pattern. Each of these
Milankovitch Cycles can influence the amount of sunlight the planet
receives, which in turn can influence climate. The changes cycle through
every 100,000, 41,000 and 21,000 years.


The problem is that the 100,000-year cycle alone is the weakest of
the three in the degree to which it affects solar radiation. So why that
cycle would be the one that sets the pace of glacial cycle is a
mystery. But this new study shows the mechanism through which the
100,000-year cycle and the 21,000-year cycle work together to drive
Earth's glacial cycle.


The 21,000-year cycle deals with precession—the change in orientation
of Earth's tilted rotational axis, which creates Earth's changing
seasons. When the Northern Hemisphere is tilted toward the sun, it gets
more sunlight and experiences summer. At the same time, the Southern
Hemisphere is tilted away, so it gets less sunlight and experiences
winter. But the direction that the axis points slowly changes—or
precesses—with respect to Earth's orbit. As a result, the position in
the orbit where the seasons change migrates slightly from year to year.
Earth's orbit is elliptical, which means the distance between the planet
and the sun changes depending on where we are in the orbital ellipse.
So precession basically means that the seasons can occur when the planet
is closest or farthest from the sun, or somewhere in between, which
alters the seasons' intensity.

In other words, precession causes a period during the 21,000-year
cycle when Northern Hemisphere summer happens around the time when the
Earth is closest to the sun, which would make those summers slightly
warmer. Six months later, when the Southern Hemisphere has its summer,
the Earth would be at its furthest point from the sun, making the
Southern Hemisphere summers a little cooler. Every 10,500 years, the
scenario is the opposite.


In terms of average global temperature, one might not expect
precession to matter much. Whichever hemisphere is closer to the sun in
its summer, the other hemisphere will be farther away during its summer,
so the effects would just wash themselves out. However, this study
shows that there can indeed be an effect on global temperature if
there's a difference in the way the two hemispheres absorb solar
energy—which there is.


That difference has to do with each hemisphere's capacity to grow sea
ice. Because of the arrangement of the continents, there's much more
room for sea ice to grow in the Southern Hemisphere. The oceans of the
Northern Hemisphere are interrupted by continents, which limits the
extent to which ice can grow. So when the precessional cycle causes a
series of cooler summers in the Southern Hemisphere, sea ice can expand
dramatically because there's less summer melting.


Lee's climate models rely on the simple idea that sea ice reflects a
significant amount of solar radiation back into space that would
normally be absorbed into the ocean. That reflection of radiation can
lower global temperature.

The Southern Hemisphere has a higher capacity to grow
sea ice than the Northern Hemisphere, where continents block growth. New
research shows that the expansion of Southern Hemisphere sea ice during
certain periods in Earth's orbital cycles can control the pace of the
planet's ice ages. Credit: Jung-Eun Lee / Brown University

"What we show is that even if the total incoming energy is the same
throughout the whole precession cycle, the amount of energy the Earth
actually absorbs does change with precession," Lee said. "The large
Southern Hemispheric sea ice that forms when summers are cooler reduces
the energy absorbed."


But that leaves the question of why the precession cycle, which
repeats every 21,000 years, would cause a 100,000-year glacial cycle.
The answer is that the 100,000-year orbital cycle modulates the effects
of the precession cycle.


The 100,000-year cycle deals with the eccentricity of Earth's
orbit—meaning the extent to which it deviates from a circle. Over a
period of 100,000 years, the orbital shape goes from almost circular to
more elongated and back again. It's only when eccentricity is
high—meaning the orbit is more elliptical—that there's a significant
difference between the Earth's furthest point from the sun and its
closest. As a result, there's only a large difference in the intensity
of seasons due to precession when eccentricity is large.


"When eccentricity is small, precession doesn't matter," Lee said.
"Precession only matters when eccentricity is large. That's why we see a
stronger 100,000-year pace than a 21,000-year pace."


Lee's models show that, aided by high eccentricity, cool Southern
Hemisphere summers can decrease by as much as 17 percent the amount of
summer solar radiation absorbed by the planet over the latitude where
the difference in sea ice distribution is largest—enough to cause
significant global cooling and potentially creating the right conditions
for an ice age.


Aside from radiation reflection, there may be additional cooling
feedbacks started by an increase in southern sea ice, Lee and her
colleagues say. Much of the carbon dioxide—a key greenhouse gas—exhaled
into the atmosphere from the oceans comes from the southern polar
region. If that region is largely covered in ice, it may hold that
carbon dioxide in like a cap on a soda bottle. In addition, energy
normally flows from the ocean to warm the atmosphere in winter as well,
but sea ice insulates and reduces this exchange. So having less carbon
and less energy transferred between the atmosphere and the ocean add to
the cooling effect.


Explaining a shift


The findings may also help explain a puzzling shift in the Earth's
glacial cycle. For the past million years or so, the 100,000-year
glacial cycle has been the most prominent. But before a million years
ago, paleoclimate data suggest that pace of the glacial cycle was closer
to about 40,000 years. That suggests that the third Milankovitch Cycle,
which repeats every 41,000 years, was dominant then.


While the precession cycle deals with which direction the Earth's
axis is pointing, the 41,000-year cycle deals with how much the axis is
tilted. The tilt—or obliquity—changes from a minimum of about 22 degrees
to a maximum of around 25 degrees. (It's at 23 degrees at the moment.)
When obliquity is higher, each of the poles gets more sunlight, which
tends to warm the planet.


So why would the obliquity cycle be the most important one before a million years ago, but become less important more recently?


According to Lee's models, it has to do with the fact that the planet
has been generally cooler over the past million years than it was prior
to that. The models show that, when the Earth was generally warmer than
today, precession-related sea ice expansion in the Southern Hemisphere
is less likely to occur. That allows the obliquity cycle to dominate the
global temperature
signature. After a million years ago, when Earth became a bit cooler on
average, the obliquity signal starts to take a back seat to the
precession/eccentricity signal.


Lee and her colleagues believe their models present a strong new
explanation for the history of Earth's glacial cycle—explaining both the
more recent pace and the puzzling transition a million years ago.


As for the future of the glacial cycle, that remains unclear, Lee
says. It's difficult at this point to predict how human contributions to
Earth's greenhouse gas concentrations might alter the future of Earth's
ice ages.