On Facebook last week a friend posted a picture of the Earth’s atmosphere, taken at sunrise from the International Space Station. In the photo, the atmosphere is shown as a thin blue line separating the sphere of the planet from the blackness of space.
The image reminded me of how thin the atmosphere is compared to the size of our planet. I’ve heard it said that if the Earth was the size of a basketball, the thickness of the atmosphere would be like a latex balloon stretched over it. A thin skin to be sure!
How can such an insubstantial wisp of gas be so important to us on Earth? The key lies in the chemical properties of the gases that make up the atmosphere. Each type of gas reacts to a different wavelength of light either entering or leaving the Earth’s system. Most of the gases don’t react to the short wavelengths of incoming solar light, allowing us on the surface to see the full visible spectrum of light that is displayed in a rainbow. Ozone is the major gas that absorbs incoming solar light; that protects us from the harmful ultraviolet rays. That’s why the development of the “ozone hole” was such a cause for concern, since the high-energy incoming solar light can cause all sorts of medical havoc like cancers and cataracts.
Once sunlight hits the surface of the Earth, it warms it up and allows the Earth to send energy back to space in the form of long wavelength radiation to maintain the Earth’s energy balance. This longer-wavelength, lower energy light is absorbed by different gases in the atmosphere that we call “greenhouse gases”, including carbon dioxide, methane and several other gases. Without these gases, the Earth would be much colder than it is now, making it difficult for life on Earth to exist. But recent observations have shown that carbon dioxide is now at a greater concentration than it has been for the last 800,000 years, based on measurements from ice cores and ocean sediments. The graph below shows what is known as the “Keeling Curve” describing concentrations of carbon dioxide in the atmosphere.
Carbon dioxide is a very small portion of the gas in the atmosphere. You’d think it would not be very important for controlling the Earth’s temperature. But because of the shape of the CO2 molecule, it vibrates at the same frequency as the light that is emitted from Earth back to space. That makes it a very good absorber of this light. A little bit goes a long way! You could compare this to putting a bit of arsenic in a stew—it does not take much to poison the pot. Carbon dioxide and other trace greenhouse gases have a huge effect on Earth’s temperature because they keep the terrestrial light from leaving the Earth, trapping the energy near the surface and helping to heat it up. As greenhouse gases are increasing, more and more energy is being trapped in the ocean-earth-atmosphere system, warming it up.
Where has the excess carbon in the atmosphere come from? We can use radiocarbon dating methods to determine that the extra carbon is from sources that are millions of years old. The only sources that provide such old carbon are fossil fuels like oil and coal, which were created millions of years ago when carbon-bearing plants were buried far underground and compressed into the fuels we use today.
That “thin blue line” is indeed thin, but critical to life as we know it.
