“All science is either physics or stamp collecting.”
- Ernest Rutherford, Nobel Prize-winning physicist (1871-1937)
I’m always amazed that so many climate change skeptics and deniers have a hard time believing the concept that as we add our emissions to the atmosphere every year, we can’t be at least part of the cause for the global warming and climate change we’ve been observing over the last half-century. We’re adding greenhouse gases such as carbon dioxide, methane and nitrous oxide, and although methane is twenty times more potent a greenhouse gas than carbon dioxide, it’s the sheer volume of carbon dioxide that makes it the bigger problem.
I’m a firm believer that if people properly undertand the science, then they’ll understand why so many of the world’s scientists are concerned about this issue, and perhaps even become concerned themselves. I find the physics behind greenhouse gases fascinating. What is it about these particular molecules that give them their property of trapping heat in our atmosphere?
First, let’s quickly review which gases make up the bulk of our atmosphere:
Nitrogen (N2) - 78%
Oxygen (O2) - 21%
Argon - 0.9%
Carbon Dioxide (CO2) - 0.04%
Methane (CH4) - 0.0002%
Water vapour (H2O) - variable, on average 0.4% throughout the atmosphere
Notice that nearly 100 percent of the atmosphere is made up of gases that aren’t greenhouse gases. Neither nitrogen nor oxygen contribute to the greenhouse effect.
The main greenhouse gases in our atmosphere are: water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), and ozone (O3). All of these are trace gases in much smaller amounts than nitrogen or oxygen.
Although water vapour is the largest component, and also a more potent greenhouse gas than carbon dioxide, it’s generally a fixed amount. We can’t add more water vapour to the atmosphere because the excess will simply precipitate out. (Unless, of course, global warming continues; warmer air can hold onto more water vapour than colder air—one of the reasons we experience more extreme weather phenomena such as hurricanes as part of climate change). And don’t misunderstand me, we need these gases, just not in excessive amounts. It’s estimated that without them our planet’s average temperature would be about 33 degrees Celsius (59 degrees Fahrenheit) colder than it is now.
Even if you know nothing about physics or chemistry, you might notice something different about the greenhouse gases compared with oxygen and nitrogen. For one thing, they all have more than two atoms, and all of them other than ozone have combinations of different types of atoms. These larger molecules have a property that nitrogen and oxygen don’t share: they can absorb infrared radiation, also known as heat energy. They function as insulators in our atmsophere, helping to trap all of the radiating heat from our planet’s surface, reflecting it back rather than letting it escape into space.
It’s the same thing as a greenhouse which helps plants grow in a warmer environment than the outside temperature would normally allow. Visible light gets in, some of it is reflected back—green plants look green because that component of visible light is reflected back to your eyes—but most of it is absorbed. Much of the absorbed energy from light is converted into infrared radiation and that’s the heat you feel in a greenhouse.
Our atmosphere functions the same way. Visible light from the sun gets through the atmosphere and some is reflected back (blue from oceans, green from forests, yellow from deserts), but much is aborbed and then reflected back as infrared radiation. Just like the glass in a greenhouse, greenhouse gases allow visible light through, but trap heat in the infrared.
How these gases do that is a bit complicated, but a simple way to think about it is they start to vibrate more when exposed to infrared radiation. If you think of the bonds that hold atoms together inside a molecule as springs, then as greenhouse gas molecules get exposed to infrared radiation, they abosrb that energy and the springs start to vibrate more quickly, squeezing together and pulling apart more vigourously, even bending the bonds if they absorb enough energy.
Since all of these greenhouse gas molecules are surrounded by nitrogen and oxygen molecules, it’s not hard for them to transfer some of that energy to the non-greenhouse gases which in turn start to vibrate more energetically themselves. In that way, the whole atmosphere ultimately becomes warmer thanks to the physical properties of the trace amounts of these few molecules in our atmosphere which contain more than two atoms.
Since our human activities contribute so much added greenhouse gases—30 billion tons of carbon dioxide added each year alone—it’s not surprising that we’re playing a part in climate change. Most climate scientists think we’re playing the biggest part. But even if deniers and skeptics want to hold onto the belief that climate change is part of a natural cycle, wouldn’t it make sense that we should avoid aggravating it even further and do our best to curtail our emissions by looking to renewable sources of energy?
It all comes down to simple physics.