Environment

Global warming - What role does water vapor really play?

Scientists say man-made CO2 causes global warming; climate skeptics insist that water vapor is responsible. Here's why both assumptions are true.
Without water vapor average temperatures would be up to 30 degrees Celsius lower/ Credits: Reuters

Here are the perfect ingredients for a conspiracy theory: water vapor is the most important factor influencing the greenhouse effect but doesn’t feature on the UN’s list of greenhouse gases responsible for anthropogenic global warming.

Critics of the idea of man-made global warming love this simple fact and have turned it into one of their most potent arguments to sabotage decisive climate action.

So why doesn’t the UN’s climate body the International Panel on Climate Change (IPCC) list water vapor as a greenhouse gas? It’s because water vapor does not by itself increase temperatures. It amplifies already occurring warming.

Water vapor’s role in the Earth’s climate system is defined by the very short time it remains in the atmosphere and actively traps heat. While additional CO2 from factories or airplanes can remain in the atmosphere for centuries, extra water vapor will only remain a few days before raining down as water.

The concentration of water vapor in the atmosphere is in equilibrium. The atmosphere can only hold more water vapor if overall temperatures increase. So a small warming effect caused by human CO2 emissions will increase the amount of water vapor in the atmosphere.

The added water vapor leads to even more warming, thus amplifying the CO2 warming effect. Water vapor follows temperature changes, it doesn’t cause or, as climatologists say, ‘force’ them. As a feedback effect, water vapor is comparable to a car’s turbo charger that increases a motor’s power.

However, the amount of water vapor in the atmosphere changes regionally. While there is virtually no water vapor above deserts or the Arctic and Antarctic regions, the air above the equator can consist of up to four percent water vapor.

In humid equatorial regions, where there is already a strong natural greenhouse effect, additional CO2 and water vapor have little impact on local climate. The opposite is true in cold, dry places, which is one reason why warming is much more pronounced in Polar regions.

Concentration matters

Regional differences aside, the atmosphere contains on average only 0.4 percent of water vapor and ten times less CO2. This relatively small concentration is another argument often cited to refute the idea of man-made global warming. How can CO2 cause rising temperatures, skeptics demand, if it only accounts for 0.04 percent of the atmosphere?

Again the riddle is solved easily.

Oxygen and nitrogen are the most abundant elements in the Earth’s atmosphere and make up 99 percent of it. But neither of the two gases traps or emits heat.

This is why water vapor is responsible for most of the natural greenhouse effect. Scientists estimate that without water vapor average temperatures would be up to 30 degrees Celsius lower. CO2, on the other hand, is responsible for a much smaller but still substantial amount of the natural warming effect.

If things remain like this, we could continue living on a cozy, warm planet. But too much of a good thing is often bad. CO2 levels have increased from 0.028 percent of the atmosphere to about 0.04 percent since the Industrial Revolution. This has led to a temperature increase of about 0.7 degrees Celsius so far.

About half of this warming could be due to feedback warming from water vapor, estimates the IPCC. But it would not have happened without the added CO2 pumped into the atmosphere. CO2 is the guy robbing the bank, water vapor is just the getaway driver.


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