Welcome to the Greenhouse
Loyal readers will have noticed that I skipped last week’s blog entry as my family and I were on summer vacation. I don’t normally post on Fridays, but when I woke up this morning and heard that the forecasted high temperatures for today were 101 oF in Chapel Hill and 104 oF in Raleigh, I figured a Common Science post on the Greenhouse Effect was in order. This will be a short one, so I won’t be late for work today (at least not too late!). Also, please forgive any editing errors.
Before we used to talk about Climate Change or Global Warming, the term Greenhouse Effect had center stage. The engineer in me misses those days. The US political landscape (at least a portion of it) is littered with science-adverse representatives and candidates who say things like “global warming is hoax”. I invite you to Google “Michele Bachmann global warming
” for some examples. If we had stuck with the term Greenhouse Effect, the job of the science-denying politician would have been made much more difficult. Let me try to explain.
The sun bathes the earth in electromagnetic radiation (See my column “Is Your Cell Phone Trying to Kill You
” for more information on electromagnetic radiation) with most of the energy arriving in the ultraviolet, visible, and near-infrared energy range. Some of this electromagnetic radiation is immediately reflected by the earth’s atmosphere. Of that which passes through, approximately 50% is absorbed by warming the surface of the earth (think hot pavement on a summer day), and the other 50% of this energy is reflected back into the atmosphere as infrared radiation.
99% of the atmosphere is made up of nitrogen, oxygen, and argon, which are transparent to infrared radiation. What this means is that if there were nothing else in the air, the reflected infrared radiation from the earth’s surface would just pass right out through our atmosphere back into space. If this were the case, the average temperature on earth would be approximately 0 oF.
The remaining 1% of the atmosphere is made up primarily of water and carbon dioxide, which both are very effective absorbers of the infrared radiation. They absorb the energy which has been reflected by the earth and re-radiate it in all directions. This is the Greenhouse Effect, in that the water and the carbon dioxide in the air trap in the heat in a similar way that the glass does in a green house. The Greenhouse Effect raises the global average temperature from 0 to 57 oF. Don’t underestimate the importance of this difference, at a global average temperature of 0 oF, life –it is existed at all– would never have evolved into humanity.
The point I am trying to make is that the Greenhouse Effect is a vital and easy-to-explain part of nature and completely and totally undeniable. So the next step is to talk about water and carbon dioxide.
Politicians whose policy positions are inconvenienced by global warming like to say things like “water is a more important part of warming than carbon dioxide”. Now water is an effective absorber of infrared radiation and has higher concentration in the air than carbon dioxide, but here is the rub. The amount of water in the atmosphere cannot increase over time. As the water concentration in the air rises, we feel the humidity, but when the concentration reached the saturation limit (100% relative humidity) the water vapor coalesces into droplets and falls to the earth. Therefore, the concentration of water in the atmosphere stays within a relatively limited band.
Carbon dioxide does not become saturated in the atmosphere, so its concentration can, and has, built up over time. Prior to the beginning of the Industrial Revolution, the concentration of carbon dioxide in the atmosphere was 280 parts per million (or 0.28%). Today, as a result of burning 150 years of petroleum and coal, it is 380 parts per million (or 0.38%). These numbers may not seem like much, and their apparent smallness is utilized by climate science deniers, but a change in 0.1% carbon dioxide is a really big deal. First of all, the atmosphere is staggeringly big, so you’ve got to add a tremendous amount of carbon dioxide to alter its composition by 0.1%. Also, and here is the important part, since only 1% of the atmosphere is absorbing the infrared radiation, as change of 0.1 is a 10% increase in the ability of the air to absorb the heat!
The correlation between carbon dioxide concentration and temperature is undeniable. See below a graph showing the trend from 1880 to present.
You will sometimes hear climate-science-denying politicians try to construe this data in reverse, claiming that higher temperatures are causing the higher carbon dioxide concentration. Taking this position requires a complete dismissal of the entire analysis above as well as pretty much everything we have learned about physics in the history of the world.
If there are any reporters out there reading this, I have the following request. The next time you hear that someone does not believe in Global Warming, ask them if they believe in the Greenhouse Effect. The answer is likely to be amusing.