Note: This program first aired on March 29, 2014.
Last time, we talked about the energy balance, or budget
sheet for Earth. Earth’s climate at any given time results from the equilibrium
of energy coming in and energy going out. When more energy comes in than
leaves, the climate warms, when less energy comes in than leaves, the climate
cools. As we are at the beginning of a projected warming period, we need
to look at the pieces of the climate
system to see what might be causing the changes we are observing.
This begs the question then, what are the things that can
change the energy balance of the Earth’s climate system? The first one doesn’t
have anything to do with us, it originates out in space, that being of course,
the sun. Anything that can change the total amount of energy from the sun that
makes it to Earth can have the potential to change the climate system, if
everything else stays the same. Returning to our household budget analogy,
changes to the amount of solar radiation are like changes to your pay check. If
you get a raise, but nothing else about your budget changes, you end up saving
more. If you take a pay cut but maintain the same budget, your savings decrease.
As a star, the sun has a natural evolution that changes the
amount of energy it emits. Over the past several billion years, the sun has
gotten hotter. It will get hotter, and then cooler as it continues to evolve
over then next several billion years. This isn’t a factor in current climate
change, because this solar evolution takes place over a huge time scale,
billions of years. There is a shorter, 11 year cycle of solar activity, related
to sunspots that some scientists have tried to correlate to changes in solar
radiation, with varying degrees of success. Other ways that solar radiation can
vary are through the Milankovich cycles, periodic changes to the shape of the
Earth’s orbit, called eccentricity, the timing of perihelion and aphelion (the times
when the Earth is closest to and furthest from the Sun), changes to the timing of equinoxes and
solistices called precession and changes to the angle of Earth’s rotational
tilt, which varies from 21.5 to 24.5 degrees. When these cycles come into phase,
say the Earth is at perihelion (or as close as it gets to the sun during its
orbit) during an especially eccentric (or maximally elliptically shaped) orbit,
at the same time as the summer solstice and when the tilt is at its 24.5
degrees, all three factors add up to maximize the amount of sun hitting the
hemisphere experiencing summer. These cycles can amplify or dampen seasonal
effects and in conjunction with other climate feedback systems can induce rapid
climate shifts. For example, they are thought to play a role in the glacial
advances and retreats of the past two million years. On a much, much smaller
time frame, we know that regionally, the amount of sun reaching the Earth
varies, we call this phenomenon the seasons. In the winter less sunlight is reaching
us than in the summer. When we talk about global climate we average these
variations out because in winter here, its summer some where else on Earth, but
seasonal variation can influence climate, particularly when in conjunction with
other climate change drivers. Mostly though we think of these variation as
being averaged out.
So, in conclusion, it is possible for the output of the sun
to vary, but the changes are either too miniscule or operate on too long a time
scale to account for the totality of the observed changes to current global
average temperatures. Next time we will look at the other independent
components of the climate system, albedo, or reflectivity of the Earth, which
influences how much energy is reflected away from Earth and therefore not
available to enter the climate system at all, and the components of the climate
system that absorb light and heat, influencing how that heat either escapes or
recirculates in Earth’s atmosphere and oceans.
References:
Real Climate (website run by climate scientists) is an
excellent source of timely and topical info and perspective by those in the
thick of this research. Here is a post about solar forcing and how relevant it
is (or isn’t): http://www.realclimate.org/index.php/archives/2005/07/the-lure-of-solar-forcing/
Cute little slide show about Milankovich cycles: http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch/
