Climate Change Part 3. The gases in air

The gases in the air important to climate change are called “greenhouse gases.” To understand what a greenhouse gas does, you first have to understand a little about radiation. Not the science-fiction stuff that changes cockroaches into giant monsters that destroy cities, but the radiation we deal with every day – the sunlight that sustains life; the heat given off from a light bulb or a fire or even your friend. The radiation from the light bulb, a fire, or your friend, is called “infrared” radiation, or “long-wave” radiation. This is the radiation we measure with the infra-red instrument in the GLOBE Surface Temperature Protocol.

Most of the atmosphere is nitrogen and oxygen. These gases don’t interact significantly with infrared radiation. But the greenhouse gases do. A greenhouse gas molecule absorbs infrared radiation coming from one direction, gets energetic, and then re-radiates energy in all directions. Water vapor, which can account for up to 4% of the volume of air near the surface, is a greenhouse gas. There are many other important greenhouse gases, listed next to the pie chart in Figure 4. These gases all have molecules made up of three or more atoms. Nitrogen and oxygen are made up of only two atoms.

In the figure, we assume water vapor is 3% by volume. This is fairly humid even for surface observations. As you know from weather broadcasts, the water vapor content of air changes from day to day. And some parts of Earth are more humid than other parts. The 3%-by-volume value is reached near the surface in the Tropics, and in the mid-latitudes on very hot and humid days. In other areas, like the Sahara desert, the air is much drier. Cold air has less water vapor than warm air.

Figure 4. Gases in the Air (by volume) near Earth’s surface, for 3% water vapor content (corresponds to 68% relative humidity at 29°C or 19 gm water vapor per 1 kg dry air). CFCs are Chlorofluorocarbons, which have been shown to destroy ozone in the stratosphere.

How does a greenhouse gas make Earth warmer?

Imagine the sun heating Earth. The sunlight mostly goes right through the lower atmosphere and heats the ground. The ground warms up, and heats the air near the ground, mostly by warm air currents rising from the ground. These air currents cause distant objects appear to waver on hot summer days.

This warm ground and air give off infrared radiation. Greenhouse gas molecules overhead intercept this energy before it escapes to space, and re-radiate it in all directions. This means that some radiation still goes up – but some goes down as well. The net effect is less radiation – and therefore less energy – escaping to space. Too see an animation of how a greenhouse gas works, see LEARN: Atmospheric Science Explorers and look for the greenhouse effect.

The major greenhouse gases in Earth’s atmosphere are carbon dioxide and water vapor, and both occur naturally. If it weren’t for these gases, the Earth would have been much cooler. The amount of water vapor in Earth’s atmosphere probably hasn’t changed much for a long time. Water cycles through the air quickly, largely because it so easily condenses or freezes and turns into rain or snow. But gases like carbon dioxide cycle through the atmosphere slowly. This means the fraction of air that is carbon dioxide also changes slowly.

The problem today is that human activity is increasing the carbon dioxide content in the air. There is also more methane and nitrous oxide than there used to be.