Friday, March 21, 2014
Climate Change: Part 3 Earth Energy Balance
Note: This program first aired on March 15, 2014.
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.
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/