How is it possible to predict temperatures on Earth years into the future when my local weather forecast can’t tell me if it will be raining a month from now?
Last week, Bill Nye stopped by to read an excerpt from his new book Unstoppable: Harnessing Science to Change the World, which tackles the issue of climate change and how we as individuals can do our part to slow it down. Now let’s get into more of the scientific details. What exactly about the climate is changing? How do we know? How can we predict temperatures on Earth years into the future when my local weather forecast can’t tell me if it will be raining a month from now?
Greenhouse Gases: Can’t Live with ‘Em, Can’t Live Without ‘Em
As humans, we have a fairly limited range of temperatures at which we are comfortable, astronomically speaking. Specifically, when judging the habitability of a planet, our first check is whether or not the temperature is right for water to be in its liquid form. A planet too far from its star will face temperatures too cold for liquid water and will instead be covered in ice. On the other hand, a planet that is too close to its star will see any liquid water boiled off into its vapor form.
However, our distance to the Sun is not the whole story behind what makes the Earth just the right temperature. We sit in this so-called Goldilocks zone where temperatures are not too hot and not too cold, but without our atmosphere, the swings in temperature between night and day would be pretty inhospitable.
Throughout the day the Earth is warmed by the Sun’s rays—the surface absorbs heat from the incident radiation and then re-emits about 70% of that heat back outward. Without our atmosphere, this heat would be mostly lost, leaving us shivering once the Sun set. Luckily we have an atmosphere, a layer of gases covering the Earth that are trapped here by our planet’s gravity. This gaseous blanket absorbs the heat emitted by the Earth before it can escape back out to space and re-emits it in all directions thus keeping us warm here on the surface.
Mars is not so lucky. The red planet is also in the habitable zone within which temperatures could be just right for hosting liquid water. However, without an atmosphere to make the nightly temperatures of -133 degrees Celsius more temperate, most of this water is trapped instead in polar ice caps. The Moon is also, of course, roughly just as far from the Sun as the Earth is, but without an atmosphere, temperatures there are also far too cold for comfort.
The gases that do most of the heat trapping are called greenhouse gases and include water vapor, methane, nitrous oxide, and carbon dioxide. The increase in the surface temperature of the Earth due to these gases is called the greenhouse effect. So what happens if you have too much of a good thing? To answer that, we can look to Venus.
Astronomers think that in the early days of the formation of the solar system a kind of runaway greenhouse effect occurred on Venus. Water vapor and carbon dioxide in the atmosphere caused temperatures to rise, which in turn worked to evaporate some of the oceans. Evaporation of the oceans adds more water vapor into atmosphere, which begins the process again.
This positive feedback loop is only part of a regular greenhouse effect, however, until a critical temperature is reached: the boiling point of water. Once temperatures are high enough, the oceans on Venus are thought to have boiled off sending large amounts of water vapor into the atmosphere. Once the oceans eventually evaporated entirely, the atmosphere stabilized in a much hotter, denser state.
Some scientists debate whether such a runaway effect could occur here on Earth, although most studies point to “no.” However, there is no doubt that humans are producing greenhouse gases in record amounts, through processes like the burning of fossil fuels, and temperatures are rising as a result.