The first thing we need to understand about the Earth’s
climate is that it is the result of a balance, what is called the energy
balance. When the same amount of energy comes in that leaves, the system is in
balance and the average temperature stays the same. If more energy leaves the system than comes
in, the net amount of energy decreases, and the system cools. If more energy
comes into the system than leaves, energy builds up and the average temperature
rises. Its like your household budget. If you spend more money than you make,
your savings account gets smaller. If you save more than you spend, your
savings account gets bigger. If you spend what you make each week, no more, no
less, your savings account will stay the same.
Earth’s climate works the same way. For the past 10,000 years or so
we’ve been in equilibrium, or balance. The energy in has equaled the energy out,
and the overall average climate has been stable. We are in a situation now
however where we are taking in more energy than we are giving out, and the
climate is out of equilibrium. The temperature will shift until the system is
back into equilibrium, in this case, it will warm until the inputs and outputs
are back in balance.
Recall if you will the law of the conservation of energy:
energy is neither created nor destroyed, but it can change form. What is the
source of energy that comes to Earth? The source of the energy that Earth runs
on is the Sun. The sun’s energy comes to us in the form of visible light. About
30% of that light is immediately reflected back away from Earth as light, and
plays no part in the climate system. It bounces off clouds in the atmosphere as
well as brightly reflective surfaces on Earth that have what is called a high
albedo, or reflectivity, things like snow, ice, light colored desert. That
energy that is reflected doesn’t change wavelength, it arrives as visible light
and it leaves as visible light. The other roughly 70% of light however enters
the Earth’s climate system by getting absorbed by the atmosphere and the
surface of the Earth. After getting absorbed, it is reradiated back out as
infrared radiation aka heat. Infrared radiation is a longer lower energy wave
length, and is not visible to us, but we can feel it. For thermal equilibrium,
the Earth would need to reradiate all 70% of that incoming energy back out. The
lag time it takes between the light coming in, and the infrared going back out
is what creates an average global temperature greater than absolute zero. While
that infrared radiation is making its way back into space, its heating up rocks
and water and air and gasses in the atmosphere, which then reradiate the infrared
radiation back out again.
Of that 70% of the Sun’s energy that
makes it into the climate system, 23% is absorbed by the atmosphere. When that
energy is reradiated back out, it goes in all directions, some back into space,
some towards Earth. The Earth’s surface absorbs the other 48% of energy
directly from the Sun (23+48 = 71, or the about 70 % total). A great deal of
that energy actually goes into non sensible heat operations. The first and
largest is evaporation. Water absorbs the light energy, but since water has
such a high specific heat, it takes a lot of energy before the water actually
changes temperature. 25% of that energy hitting Earth goes into simply
evaporating water. Heat is still moved, in fact evaporation is major component
of moving heat around the climate system, but its not like we feel it like we
do when we stand next to the woodstove. The next 5% is moved via convection.
The heat is moved not by radiation but physically by air warmed by contact with
the warm surface of the Earth. The air rises into the atmosphere taking the
heat with it. Its only the last 17% that actually gets reradiated back towards
space as infrared radiation, and this is where the tally sheet doesn’t balance.
Of that 17%, only 12 ultimately makes it back into space. The remaining 5% is
left to build up here in the climate system. And when not everything that comes
in makes it back out, we have a system out of equilibrium. The only way for a
system to get back into equilibrium is for it to shift. That’s what’s going on
now.
Next week we’ll take a look at how the pieces of the climate
system work together, and why heat is building up here on Earth.
References:
Your tax dollars at work: The NASA Earth Observatory website
is a wealth of great earth and sky science. This page on the Energy Budget is
very helpful and full of details I couldn’t cover here: http://earthobservatory.nasa.gov/Features/EnergyBalance/
