Is it getting warm in here? The Greenhouse Effect

What comes to mind when you think of the “Greenhouse effect”? If you imagine, well, a greenhouse, you’re not alone.

However, the greenhouse effect and the way a greenhouse actually works have little to do with each other. A greenhouse is a closed system, kept warm mainly by the lack of circulating air. The greenhouse effect describes radiative interactions—that is, the interaction of light with matter.

Range of light
Adapted from: Columbia University

The sun emits energy as ultra-violet, visible, and infrared (IR) radiation. Most of this energy is able to pass through the atmosphere without being absorbed or reflected back into space. It is then absorbed into the ground, warming it. The warm ground reemits energy as long-wavelength IR radiation (energy is lost into the ground as heat, AKA thermal radiation, and lower energy means longer wavelength). Unlike the original solar radiation which moseys through the atmosphere undisturbed, this thermal radiation is absorbed by molecules in the atmosphere. This energy absorption means that the air is warmer!

Understanding the greenhouse effect, rays from sun to earth and caught in atmosphere

But what absorbs the energy? This is where greenhouse gases come into play. Certain molecules of gas in the air are “IR-active,” which means that they can absorb energy in the infrared (IR) region, such as the radiation that’s re-emitted by the ground (see here). Such molecules include water (H2O), methane (CH4), and carbon dioxide (CO2), among others: because we find these IR-active molecules in our atmosphere, we call them greenhouse gases. (Being IR-active has to do with the structure of the molecule, which you can read more in our graphic below.) The greenhouse effect is very useful—without it, our Earth wouldn’t be our Earth. Instead, it’d be checking in at a cool [calculated] -18.8 degrees Celsius (-1.8 degrees Fahrenheit) . Greenhouse gases actually keep the Earth habitable.

How Greenhouse Gases absorb heat on molecular level
Sources: California Institute of Technology and UC Davis

However, within the past 150 years the percentage of greenhouse gases in the atmosphere has been rising. Scientists attribute this growth to human effects: take, for example, the growing concentration of IR-active CO2 in the atmosphere. According to ice records (see here), the highest concentration of CO2 in the atmosphere during the last 800,000 years was around 300 ppm (parts per million). In 2013, scientists measured the CO2 concentration in the atmosphere to be a record breaking 400 ppm—the highest level of CO2 in the atmosphere since 10-15 million years ago (see here). CO2 levels oscillate seasonally—they tend to be higher during the winter, when we use more gas to heat our homes, and lower during the summer when there are more plants growing, but there has also been a steady upward trend in recent times (see here).

Unlike the original solar radiation which moseys through the atmosphere undisturbed, thermal radiation is absorbed by molecules in the atmosphere.

This high a level of CO2 is unprecedented, so the exact effects it will have on our environment are still not clear. Increased levels of greenhouse gases in the atmosphere contribute to global warming – you can read more about current research on global warming here. According to a recent Nature paper increased CO2 can also affect plant physiology, because plants metabolize CO2 for energy. One 2008 study cited in the paper suggests that certain crops such as rice have a lower protein content under high CO2 conditions, which could pose a problem for the human diet. Another 2005 study suggests that plants will grow and photosynthesize faster. The exact ramifications of increasing greenhouse gases in the atmosphere is unknown, but the consensus is clear: things are changing and will continue to.

So, what can we do? One course of action is to invest in energies that don’t release as many greenhouse gases: burning fossil fuels releases methane and carbon dioxide, but hydrogen energy for example uses H2, which is not IR active. Although it produces IR-active H2O, this can be recycled into our water system! Another is to be aware of how much energy we are using: check out these two carbon footprint calculators (one from Nature and one from the EPA to figure out how much you might be contributing and where you can decrease your contribution, and feel free to discuss your results in the comments section below!


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