EPA chief Scott Pruitt says carbon dioxide is not a primary contributor to global warming

Environmental Protection Agency Administrator Scott Pruitt said Thursday he does not believe carbon dioxide is a primary contributor to global warming.

“I think that measuring with precision human activity on the climate is something very challenging to do and there's tremendous disagreement about the degree of impact, so no, I would not agree that it's a primary contributor to the global warming that we see,” he told CNBC's “Squawk Box.”

The carbon dioxide we exhale does not contribute to global warming for the simple reason that we also take up an equivalent amount of carbon dioxide from the air, albeit indirectly. Everything we eat can be traced back to photosynthesis, the process by which plants take up carbon dioxide from the air and use it to produce the vast array of organic compounds needed for life. Our bodies can be regarded as living engines that require fuel and oxygen to produce the energy needed to sustain life. In that sense we are not all that different from a car. Both for us and for the car the source of oxygen is the air, roughly 20% of which is made up of oxygen. An internal combustion engine burns gasoline and spews out water, carbon dioxide and a few combustion byproducts. We, instead of gasoline, burn the carbohydrates, fats and proteins in food. Like gasoline, these organic compounds are converted to carbon dioxide and water, which we then exhale.

How is it then that we don't worry about the massive amounts of carbon dioxide that are released with every breath taken by the billions and billions of people and animals that inhabit the world? Because every atom of carbon in the exhaled carbon dioxide comes from food that was recently produced by photosynthesis. Everything we eat, save for a few inorganic components like salt, was in some way produced by photosynthesis. This is obvious when we eat plant products such as grains, fruits and vegetables, but of course it is also the case for meat. The animals that we eat were raised on plant products. Indeed, a growing animal is basically a machine that converts plants into flesh. So, since all the carbon dioxide we exhale originated in carbon dioxide captured by plants during photosynthesis, we are not disturbing the carbon dioxide content of the atmosphere by breathing.

Why do we blame climate change on carbon dioxide, when water vapor is a much more common greenhouse gas?

Extra water vapor we put in the atmosphere doesn't last long enough to change the long-term temperature of our planet. But water does play a major supporting role in climate change.

With all the attention given to humans' climate-warming carbon dioxide (CO2) emissions, you might be surprised to learn that CO2 is not the most important greenhouse gas affecting the Earth's temperature. That distinction belongs to water.

We can thank water vapor for about half of the “greenhouse effect” keeping heat from the sun inside our atmosphere.1 “It's the most important greenhouse gas in our climate system, because of its relatively high concentrations,” says Kerry Emanuel, professor emeritus of atmospheric science at MIT. “It can vary from almost nothing to as much as 3% of a volume of air.”

Compare that to CO2, which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system.

So why do we never hear climate scientists raising the alarm about our “water emissions”? It's not because humans don't put water into the atmosphere. Even the exhaust coming from a coal power plant—the classic example of a climate-warming greenhouse gas emission—contains almost as much water vapor as CO2.2 It's why that exhaust forms a visible cloud.

But water vapor differs in one crucial way from other greenhouse gases like CO2, methane, and nitrous oxide. Those greenhouse gases are always gases (at least when they're in our atmosphere). Water isn't. It can turn from a gas to a liquid at temperatures and pressures very common in our atmosphere, and so it frequently does. When it's colder it falls from the air as rain or snow; when it's hotter it evaporates and rises up as a gas again.

“This process is so rapid that, on average, a molecule of water resides in the atmosphere for only about two weeks,” says Emanuel.

This means extra water we put into the atmosphere simply doesn't stick around long enough to alter the climate; you don't have to worry about warming the Earth every time you boil a kettle. And there's really no amount of water vapor we could emit that would change this. “If we were to magically double the amount of water vapor in the atmosphere, in roughly two weeks the excess water would rain and snow back into oceans, ice sheets, rivers, lakes, and groundwater,” Emanuel says.

Nonetheless, water vapor is an important part of the climate change story—just in a slightly roundabout way.

At any given temperature, this is a theoretical upper limit to the amount of water vapor the air can hold. The warmer the air, the higher that upper limit. And while the air rarely holds as much water as it could—thanks to rain and snow—Emanuel says that over the long term, rising temperatures steadily raise the average amount of water vapor in the atmosphere at any given time.

And of course, temperatures today are rising, thanks to humans' emissions of longer-lasting greenhouse gases like CO2. Water vapor amplifies that effect. “If the temperature rises, the amount of water vapor rises with it,” says Emanuel. “But since water vapor is itself a greenhouse gas, rising water vapor causes yet higher temperatures. We refer to this process as a positive feedback, and it is thought to be the most important positive feedback in the climate system.”

In short, it's true that water vapor is in some sense the “biggest” greenhouse gas involved in climate change, but it's not in the driver's seat. CO2 is still the main culprit of the global warming we're experiencing today. Water vapor is just one of the features of our climate that our CO2 emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.

In short, it's true that water vapor is in some sense the “biggest” greenhouse gas involved in climate change, but it's not in the driver's seat. CO2 is still the main culprit of the global warming we're experiencing today. Water vapor is just one of the features of our climate that our CO2 emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.

What about this scientists