Michael Fickes
Research Project
12/14/2012
Climate Cycles
Humans. With our population over seven billion, we have the potential to
radically alter our planet, for better or worse. We pour millions of tons of carbon dioxide
into the atmosphere every day. We have seen temperatures rising dramatically over the
past century: much faster than our research indicates that they have in the past. The
question we must now face is whether or not we are to blame for this change. We must
bear in mind that Earth has cycles in temperature that have nothing to do with the
human factor. These cycles can be caused by such factors as solar variability, ocean
currents, and Earth’s orbit.
The temperature data from the
past one-hundred fifty years, when
graphed on the right axes, looks like
humans must be changing climate.
Around
the
time
humans
started
releasing large amounts of carbon
dioxide, temperature begins shooting
up. However, there are problems with
this explanation, such as the drop in temperature between 1940 and 1960. There must
be other factors that influence the temperature besides humans.
Scientists have been able to look back at temperatures from millions of years ago
to help us understand what we are seeing today. We can see that we are in fact in an
overall warming period, but also note that global temperatures now are much lower
than they have been in the past. Of course, this does not mean that the climate change
of today is insignificant. We are seeing vast increases in temperature. These increases
will cause great change in how our world looks. We can see, however, that Earth has
undergone much greater temperature changes in her history.
Observable in the image above are some of Earth’s longest climate cycles. There
are many temperature cycles of varying lengths, ranging from the yearly cycle of the
seasons to lengthy patterns of change in Earth’s orbit, known as Milankovitch cycles. In
fact, the climate is so complex; there may even be cycles we have not yet observed.
The most obvious climate cycle is the change of the seasons. While it seems like
such a basic element of life to those who do not live near the equator, the seasons are
actually very dramatic climate cycles. This yearly oscillation is due to the fact that the
north-south axis of Earth is not orthogonal to its
orbital plane, that is to say that it is tilted (1). As
Earth moves around the Sun in its orbit, the angle
at which the energy strikes the surface changes.
University of Michigan Astronomy Department
During
June,
the
Northern
Hemisphere
experiences the summer solstice as the Southern Hemisphere observes the winter
solstice, and vice versa in December. If a hemisphere is tilted towards the Sun, it is
exposed more directly to the Sun’s rays and experiences longer days, which contribute
to a warmer season. The seasons are most dramatic at the poles (and have the least
effect near the equator) since the angle of incidence of the Sun’s energy fluctuates by a
greater amount the further from the equator one travels.
Another cycle which has dramatic effects on parts of the world is the El Niño/La
Niña cycle. El Niño is characterized by unusually warm ocean temperatures in the
Equatorial Pacific, as opposed to La Niña, which is characterized by unusually cold ocean
temperatures in this region. In normal conditions, the prevailing winds blow west. This
pushes the warm surface water towards Asia and away from South America. Due to the
upwelling of colder water on the South American coast as a result of these winds, there
is a great difference in water temperature between the two sides of the Pacific. In fact, in
normal conditions, there is a disparity of about 8 degrees C (2). During El Niño events,
the trade winds weaken, causing colder
temperatures in the west and warmer
temperatures in the east. The upwelling
near the South American coast lessons,
cutting off what had been a supply of
nutrient-rich water. This results in a
decline in primary productivity, adversely
affecting higher levels of the food chain,
including commercial fisheries (2). Of
course, precipitation follows the shift of
the warmer water eastward. This results in
drought in Indonesia while causing flooding in South America.
The Sun undergoes cycles in its energy output of roughly 11 years, with longer
cycles of about 85 years. These cycles can be seen in the graph below. While these
fluctuations result in a 0.1% increase in the amount of energy that strikes Earth, they can
have a profound effect on our climate (3). According to the National Aeronautics and
Space Administration (NASA), solar heating accounts for .15 degrees C (25%) of the
increase in temperature over the past century. This increase in solar activity is a result of
magnetic fields on the Sun. These concentrated fields inhibit convection and create what
are known as sunspots. These sunspots, at 4,600 K are about 25% cooler than the
surrounding surface temperature of 6,000 K (4). A sunspot’s magnetic field sometimes
gets twisted due to the Sun’s
plasma state of matter. If the
field is warped in certain
ways, the tension it creates
can
snap,
releasing
large
amount of plasma into space
in a phenomena known as a
coronal mass ejection. This
energy striking Earth results
in the observed fluctuations in the cyclical increase in solar energy absorbed by the
planet.
Some of the longest cycles are those with causes originating in Earth’s orbit.
These so-called Milankovitch cycles have no practical effect on climate change in the
short-term, but over tens of thousands of years they can cause large amounts of
variability in temperature. One such cycle is the change in the axial tilt of Earth. While
the axis is currently tilted 23 degrees (6), over 41,000 years this value cycles between 22
and 25 degrees (7). The change in the axial tilt affects the severity of the seasons; a
higher degree of obliquity means more variability between summer and winter. Another
cycle is the 22,000-year-long precession of the equinoxes. Due to the elliptical nature of
Earth’s orbit, its distance from the Sun is variable. The time of the year when that
distance is smallest is known as the perihelion. The procession of the equinoxes is the
change in timing of the perihelion. Currently the perihelion occurs in January, resulting
in milder seasons in the Northern Hemisphere and more severe seasons in the Southern
Hemisphere. A final Milankovich cycle is the change in the eccentricity (roundness) of
Earth’s orbit. Over periods of 100,000 and 400,000 years, Earth’s orbit undergoes a cycle
that changes the shape due to the gravitational pull of the other planets, particularly
Saturn and Jupiter. A greater eccentricity means that there are higher temperatures at
some parts of the year. Depending on which seasons these changes take place in,
seasons can either be stronger or weaker. For example, when the eccentricity is high, if
the perihelion occurs during summer in the Northern Hemisphere, there will be much
stronger seasons that year in the Northern Hemisphere. All of these Milankovich cycles
are relatively weak on their own and their effects often counteract each other, but at the
right times, their effects compound each other resulting in large changes in Earth’s
climate. This causes slow but powerful changes in the temperature.
Natural variability in the climate plays a much larger role in climate change than
humans have. On the other hand, the rate of climate change is higher now than we have
ever observed it to be. In a study analyzing the changes in the energy entering and
leaving Earth, two Swiss scientists report that humans are responsible for 74% of
observed warming since 1950 years (8). Huge amounts of warming linked to
anthropogenic sources coupled with already powerful natural cycles will lead to a
different world in the very near future. If we want to prevent that change, we need to act
now before Earth’s climate cycles take the warming we have started and increase it to
levels not seen in millions of years.
Works Cited
1) http://www.astronomy.org/astronomy-survival/seasons.html
2) http://www.pmel.noaa.gov/tao/elnino/
3) http://www.nasa.gov/topics/solarsystem/features/solar_variability.html
4) http://www-das.uwyo.edu/~geerts/cwx/notes/chap02/sunspots.html
5) http://www.space.com/11506-space-weather-sunspots-solar-flares-coronalmass-ejections.html
6) http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725
7) http://www.ncdc.noaa.gov/paleo/milankovitch.html
8) Huber and Knutti, 2011: Anthropogenic and natural warming inferred from
changes in Earth’s energy balance. Nature Geoscience.
http://www.iac.ethz.ch/people/knuttir/papers/huber11natgeo.pdf
Additional sources
http://www.ncdc.noaa.gov/paleo/ctl/clisci100ka.html
http://www.universetoday.com/55043/earths-temperature/
https://dept.astro.lsa.umich.edu/ugactivities/Labs/seasons/SeasonsIntrotroppics.html
Philander, S.G.H., 1990: El Niño, La Niña and the Southern Oscillation. Academic Press,
San Diego, CA, 289 pp.
Hayes, S.P., L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi, 1991: Bulletin if the
American Meteorological Society, 72, 339-347.
McPhaden, M.J., 1993: www.pmel.noaa.gov/pubs/docs/mcph1401.html. Oceanography,
6, 36-44.