The Greenhouse Effect

A Science explainer

A Sloan Stump original illustration

I’ve been making a lot of assertions in this newsletter about the effect that greenhouse gases have on climate change. Presumably, I’m mostly preaching to the choir here because of the self-selection process. This newsletter was previously a chronicle of my travels in Europe and dealt primarily with the (less controversial?) science of evolution. There have been more than a couple of unsubscribes since I started writing about climate change and the influence of greenhouse gases thereon.

So those of you who are left accept the scientific consensus that our burning of fossil fuels is causing the climate to change. But do you understand that connection well enough to explain it to someone else? I’m not sure I did until a couple of years ago, and I’m certainly no expert now. But I’ve continued my jack-of-many-scientific-trades pursuit in order to have some credibility when I talk about such things. Let me see if I can explain it.

First: the data about which there is no real controversy. (Ideology certainly influences the interpretation of science, but the closer you get to the empirical data, the less influence it has.) For the last 65 years there have been precise measurements of the amount of carbon dioxide (CO₂) in the atmosphere. Prior to that, there are very good estimates from sampling tree rings, ice cores, and other seasonal layers. Using these methods, scientists can go back 800,000 years with actual data. Here’s what the graph looks like for the parts per million of CO₂ over that time period:

source: climate.nasa.gov

You can see there are some natural variations over time, but toward the end of that graph, there is something unnatural going on. That coincides with the industrial revolution and beginning of large amounts of coal being burned to power machinery. Zooming in on the last couple hundred years, we get this graph of CO₂ concentrations:

Source: CO2levels.org

Starting in 1958 the measurement of atmospheric CO₂ has occurred at Mauna Loa in Hawaii. Remember from high school chemistry class, that a gas dissipates to fill the container it’s in. But that takes some time. For example, if you sampled the air in a major industrial area, you’d get higher concentrations of CO₂ than you would out in the middle of a wilderness. Mauna Loa was chosen, because it is very remote from any major sources of CO₂ and because it is 11,155 feet above sea level. The measurements there are representative of atmosphere as a whole, not just some local concentrations.

So, there is no doubt that the concentrations of CO₂ have dramatically risen in the last 150 years, and very dramatically in the last 50 years. The next set of data is the average temperature on Earth over the same time period. This is a little more complicated to determine, because you can’t just use one location. Scientists combine measurements from lots of different locations and elevations. There are different models used to combine these and calculate the average global temperature, but these converge on the following trend over the last couple hundred years.

Source: CO2levels.org

So we have a very strong correlation between rising CO₂ levels and rising temperatures. That much is uncontroversial and hardly contested by scientists. But you should have learned in a critical thinking course at some point that correlation is not causation. Just because two variables consistently go up and down together, that doesn’t mean they are causally related. My favorite example of this is the rate of ice cream consumption and homicide rate in New York City: whenever people eat more ice cream there, more people are killed. Are we to suppose that eating ice cream makes you go out and kill someone?!? No, it’s another variable that the two are independently correlated to: the season.

Could there be something similar going on with temperature and CO₂ levels? A good scientist shouldn’t dismiss that as a possibility, and should continue to look for more data and theoretical models that can be tested. It turns out that relevant science has been confirming the causal relationship between CO₂ and temperature for 170 years.

Eunice Foote, an American scientist and women's rights activist, conducted experiments in the 1850s that significantly contributed to our understanding of the greenhouse effect of CO₂ (yes, a female scientist in the 1850s!). In 1856, she presented a paper at the American Association for the Advancement of Science, titled “Circumstances Affecting the Heat of the Sun's Rays.” In it she described the results of her experiments in which she used an air pump, four thermometers, and two glass cylinders to test the impact of sunlight on different gases. She filled one cylinder with moist air and the other with carbon dioxide, and exposed them to sunlight. She observed that the cylinder with carbon dioxide trapped more heat and stayed hot longer. This led her to conclude that if the concentrations of CO₂ in the Earth's atmosphere were to increase, the Earth would become warmer.

This is good evidence evidence of the causal connection — not just correlation — and similar experiments showing this have been repeated many, many times. But to really understand the greenhouse effect, we’d like to know why more CO₂ leads to increased temperatures. For that, we need some chemistry and physics.

The warmth of our planet comes primarily from the sun’s energy. That energy comes across a wide spectrum of electromagnetic frequencies: from the short ultraviolet, through the visible spectrum, and into the long wavelengths of infrared. But a lot of it is in shorter wavelengths. Here’s why: the sun is really hot, and that confers more energy to the photons escaping from it, and photons with higher energy have shorter wavelengths. When those photons hit Earth, it warms up the surface, and then it radiates heat back out toward space. But the heat radiating off Earth isn’t nearly as hot as the sun, so those photons have less energy and are in longer wavelengths.

OK, here’s the problem with putting more CO₂ in the atmosphere (and the theoretical underpinning of the causal connection between higher CO₂ concentrations and higher temperatures): CO₂ is better at absorbing radiation at longer wavelengths than shorter ones. The reason for this is due to its molecular structure and it takes quantum physics to explain. The last time I took a quantum physics course was in 1995… I’m afraid I can no longer rehearse the arguments. But the takeaway is this: the higher energy photons from sun don’t get blocked by the extra CO₂ in the atmosphere, but the heat radiating from Earth that had naturally gone back out into space is absorbed by CO₂. Then that heat is radiated in all directions from those CO₂ molecules — some of which is back to Earth.

So, the more CO₂ molecules in the atmosphere, the more heat that had previously escaped into space is going to be reflected back to Earth.

If you’re not so much into science, think of it in similes: CO₂ is like one-way glass where the reflective side is facing toward Earth. Heat comes from the sun and passes right through the atmosphere to heat Earth. But then when that heat emanates back off Earth, it is trapped inside the atmosphere because it won’t pass back through all that CO₂.

That is why more CO₂ (and other greenhouse gases like methane) in the atmosphere cause Earth to get warmer, and why it is so important for us to stop pumping them into the atmosphere.