Thursday, September 22, 2016

So What are the Greenhouse Gases...and What are NOT Greenhouse Gases

In my last post I talked about the greenhouse effect, which is a normal part of our atmosphere and the phenomenon that keeps the planet at a temperature that allows life as we know it. This is scientifically established, like gravity. Keep in mind that we are not talking about anything humans have done at this point, just what nature itself does. The greenhouse effect is basic physics. And it is natural.

Today I'll take a closer look at the gases that make up the atmosphere and which ones actually create the greenhouse effect. This seemed to be a sticking point for some folks who, whether intentionally or unintentionally, exhibited confusion over the basic atmospheric processes that make our planet livable.  

Take a look at the pie charts below.

As you can see in the top pie (purple, blue, and yellow circle), the dry atmosphere is about 78% nitrogen (N2), 21% oxygen (O2), and about 0.9% argon (Ar). In total, these three simple elements make up about 99.96% of all of the gases in the atmosphere. And their combined impact on the greenhouse effect is - drum roll please - zero, zilch, nada, nothing. Nitrogen and oxygen are not greenhouse gases and thus do not contribute to the greenhouse effect. 

Now look at the bottom pie above (the mostly light blue one).

Only about 0.04% of the atmosphere controls the temperature balance of the earth. And most of that 0.04% is carbon dioxide (CO2), which is indeed a greenhouse gas. Other greenhouse gases in trace amounts including methane (CH4), nitrous oxide (N2O), and ozone (O3). Finally, you can add in water vapor (H2O), which is not included in the dry atmosphere totals above because it varies, but it on average is roughly 1% of the atmosphere.

So to reiterate, this approximately 0.04% of the atmosphere controls the entire natural greenhouse effect that keeps this planet at temperatures conducive to animal, plant, and human life as we know it.

But all greenhouse gases are not created equal. They vary in how efficient they are in absorbing long-wave radiation coming back up from the Earth. When you rank them based on their relative contribution to the greenhouse effect you get:
  1. water vapor
  2. carbon dioxide
  3. methane
  4. ozone
Oops. Isn't carbon dioxide (CO2) supposed to be THE most important greenhouse gas? Well, in a way it is, but it actually isn't the biggest contributor to the greenhouse effect. Water vapor actually generates more greenhouse effect than any other single gas. Estimates vary, but the combined effects of water, both as a gas (water vapor) and as a liquid (droplets in clouds), contributes between 66% and 85% of the greenhouse effect.

So why don't we talk more about water vapor? Well, besides some really technical stuff that I won't go into here, water vapor is naturally cycled into and out of the atmosphere on a relatively short time cycle (think, rain and snow). This means that while it has powerful short term impacts on temperature (a cloud passing in front of the sun will immediately make it feel cooler), water vapor is not a major driver of long-term climate change. It's more of a feedback, whereas CO2 is a forcing. (I'll talk about what that means in the next post) To find out more about the role of water vapor, check out this entertaining but informative video (notice the repeated cameo by Carl Sagan). For different reasons, ozone also has a pretty limited effect (and in fact actually contributes a small net cooling). Therefore, the concentrations of CO2 and methane are the main drivers of greenhouse gas induced climate change.

Remember, we have not said anything yet about human activity. Everything about the normal greenhouse effect is basic physics and natural. It is what keeps our planet "just right" (as opposed to too hot like Venus or too cold like Mars).

In the next post I'll go into more detail on CO2 and methane and their contributions to the greenhouse effect. I'll also look at why they are so important to future climate changes.

 [This is part of a series of posts explaining the basic science of climate change. More posts will be added weekly.]