Consider this argument often made by climate skeptics:

Water vapor is the most important gas, contributing 97 percent of the greenhouse effect. Carbon dioxide is only small percentage. Therefore, regulating carbon dioxide will have no impact on our climate.

WhileEven if these numbers are generally correct, there are lots of problems with this argument. For example, it disregards the fact that climate forcing by water is really a feedback, and that changes in carbon dioxide are amplified by the water vapor feedback.

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Then there’s this problem: the argument includes an implicit assumption that a small fractional change of any quantity is intrinsically unimportant. It might make intuitive sense: carbon dioxide is only 3 percent of total forcing, and how can 3 percent of anything be important?

This is, of course, nonsense. Consider the following example. You make \$2,000 per month and your expenses are also \$2,000 per month. Now let’s assume that a new expense of \$10 per month suddenly arises. You might argue that the \$10 expense is insignificant compared to the existing expense of \$2,000 per month. But on long time scales, regardless of how much money you have saved up, this small imbalance will eventually bankrupt you.

This analogy is quite apt when considering carbon dioxide and climate. Our relatively small additions of carbon dioxide each year trap a correspondingly small amount of energy. Over the course of a century, however, these small increments of energy combine to greatly warm the planet — just like the \$10 per month increment in expenses adds up over many years to completely drain your account.

Here’s a more concrete example. The average depth of the ocean is 4,000 m (about 12,000 feet). Consider a 0.1 percent increase in ocean depth. That’s a small amount. How could a 0.1 percent increase in anything be important?

Here’s how: a 0.1 percent increase is 4 meters (about 12 feet). Such a sea level rise would be a catastrophe of unimaginable scale. Over 300 million people live within 12 feet of sea level (that’s today; many more will do so in a future world). Not only will such a sea level rise result in the relocation of people and loss of land, it will also destroy trillions of dollars of infrastructure.

While changes in carbon dioxide only change the total forcing by a few percentage points, the total forcing is so large that this change is really significant. It’s enough to increase our surface temperature by a few degrees in a century, a rate of change that has few, if any, precedents in the history of the world. And certainly no precedents since the rise of modern human society a few hundred years ago. Let’s hope we act to head off this very bad future.