This is the final installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.
Compared to 100 years ago, the temperature of the atmosphere is warmer. No one disputes that. And no one disputes that an extra source of heat must be causing it – that's a basic law of physics. But how do we know that the source of the heat is increasing levels of greenhouse gases from burning fossil fuels and deforestation?
So far in this series I've described how the relationship between atmospheric carbon dioxide (CO2) and global warming was discovered, how carbon isotopes prove that rising CO2 concentrations are from the burning of fossil fuels, and how the orbital shifts that cause ice ages cannot explain our recent warming.
That certainly suggests that global warming is caused by increased CO2 from burning fossil fuels, but how can we be sure?
Besides the greenhouse effect, there are three other heat sources that conceivably could explain global warming:
- Increased output from the sun.
- Increased absorption of heat from the sun due to a change in the Earth's reflectivity or "albedo".
- An internal variation in the climate system that transfers heat to the atmosphere from another part of the Earth.
We know that the increasing greenhouse gases are warming the atmosphere. What about these other possibilities?
It's not the sun. Scientists realized early on that increases in solar output could be the culprit, so we launched satellites to monitor the sun. But the data has shown no appreciable change in solar output for the past three decades:
Source: Foukal et al. 2006. Variations in solar luminosity and their effect on the Earth's climate. Nature 443: 161-166.
It's not Earth's reflectivity. You know how you feel cooler in summer when you wear light-colored shirts? That's because they reflect light rather than absorb heat. The same principle applies to an entire planet. A change in the planet's "color" or brightness can make it hotter or colder by altering its reflectivity. In fact, this may be occurring on Mars, where dust storms appear to blowing light-colored dust from the surface and darkening the planet.
But it's not happening on Earth. Satellite data reveal that the Earth's reflectivity increased (causing cooling instead of warming) from the 1960s to the early 1980s, and has decreased modestly since. The reasons for this are discussed in my earlier post on global dimming. The recent decrease in reflectivity is much too small to account for the current warming.
It's not internal variations. The ocean is the only source of heat capable of warming the atmosphere on decadal time scales. If this were happening, the ocean would be cooling as the heat from the ocean was transferred to the atmosphere. But in fact the ocean has been warming rather than cooling over the past few decades. Whatever is warming the atmosphere is also warming the ocean.
The Greenhouse Effect is the only explanation left. As I said at the beginning of this post, it's a basic law of physics that if the globe is warming, the additional heat has to be coming from somewhere. So to end this detective story I'll quote from Sir Arthur Conan Doyle, creator of Sherlock Holmes: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
The only possible explanation for the global warming we're currently experiencing is the Greenhouse Effect, from burning fossil fuels and deforestation.