Wednesday, March 28, 2012

Global Warming: A Matter of Balance

Don't cringe, this will not be a political essay. It's about the science.

Global warming is all about energy, since it is energy which determines the temperature, or average heat content, of an object.

Over the next few minutes I will ask you to think about three connected facts about energy which together define the basis for global warming and climate on Earth:

  1. Earth receives incoming energy from the Sun in the form of visible sunlight.
  2. Earth emits or loses outgoing energy to space in the form of invisible infrared radiation.
  3. The balance between this incoming sunlight energy and the outgoing infrared energy determines Earth's average heat content and therefore, climate.
POINT ONE - Earth Receives Energy From Sunlight

So, if the Earth's energy content affects the planet's temperature, understanding global warming requires knowing where and how the Earth gets its energy. Virtually all of the energy that influences the temperature of the Earth's surface comes from the Sun as visible sunlight.

Most of this incoming sunlight energy is absorbed by either the air in the atmosphere, the water in the oceans, or by solid objects on the land - like soil, rocks, and plants. When sunlight energy is absorbed, it warms the objects absorbing it - recall the last time you touched a surface warmed by the sun or felt your skin warmed by sunlight.

However, not all of the sunlight energy headed towards the Earth is absorbed - some of it is reflected away from the planet before it can be absorbed - by light-colored surfaces such as clouds, snow, and ice. Reflected sunlight energy is NOT absorbed, and does not add to Earth's energy content. Thus it's cooler on a cloudy day than it is on a sunny day.


Less well-known is the fact that the Earth loses energy to space about as rapidly as it gains energy from sunlight. This energy lost to space is called infrared radiation, which is just light energy of a different wavelength than visible sunlight. This infrared energy radiates or moves out from solid objects, water, air, even you and me! If you want to feel infrared radiation, put your hand close to, but not touching, your face. Your hand and face both feel warmer, because both feel or sense the infrared energy radiating from your body.

This outbound infrared radiation moves in all directions, eventually escaping through the atmosphere into space.

However, there are invisible gases in the atmosphere which can block the escape of infrared energy from the Earth into space. The most important gases that can do this are water vapor and carbon dioxide.

When there is more water vapor or carbon dioxide in the air, these gases intercept and absorb more of the infrared radiation otherwise on its way to space.

As a result, these gases themselves get warmer, and in turn radiate infrared energy, some of it back towards Earth's surface, some of it out to space. The infrared energy radiated back to the surface of the planet causes an overall increase in Earth's heat content - something called the greenhouse effect - because it resembles what happens on a sunny day inside a greenhouse (or your car with closed windows). Sunlight passes through the clear glass, but the resulting infrared heat energy cannot escape through that same glass, and the energy content - temperature - of the greenhouse, or your car, increases.

Charles Darwin's greenhouse at his family estate in Down

This greenhouse effect is really an example of the THIRD and final POINT - that the balance of incoming energy minus outgoing energy determines Earth's average heat content, and controls the climate.

If the amount of sunlight energy absorbed by the Earth is exactly balanced by the amount of infrared radiation escaping the Earth, the planet is in energy balance, and the overall average energy content of Earth remains constant. In this case, overall climate averages would not change.

But as described above for incoming sunlight and outgoing infrared energy, from time to time things happen in the atmosphere or on the surface of the Earth that change that balance of incoming and outgoing energy. As a result of those changes, Earth's energy balance gets out of whack and average climates can change.

What has all of this got to do with you and me and our children and grandchildren?

Scientists measuring the amount of carbon dioxide in the air, and where it comes from, have determined that our burning of coal, oil, and natural gas, along with cutting down vast areas of forest around the world, has caused the average amount of carbon dioxide in the atmosphere to increase from 265 parts of carbon dioxide per million parts of other air molecules before the industrial revolution, to almost 400 ppm carbon dioxide today. And the energy imbalance and temperature increase caused by that rise in carbon dioxide has, in turn, caused more liquid water to evaporate into the atmosphere, further increasing the greenhouse effect - adding more to the energy imbalance initially triggered by the increase in carbon dioxide.

Of course, as with any measurement, there are uncertainties about how much or how little. How much carbon dioxide will cause how much of an energy imbalance will cause how much global warming and climate change. And subject to even more uncertainty is what we might do about it all.

But if you remember that global warming and climate change arise from an imbalance in the energy coming to the Earth and the energy leaving the Earth, I think you'll be better prepared to evaluate new information as it's discovered and presented, and make informed and intelligent decisions when you have the opportunity.