Greenhouse gases are being released into the atmosphere 30 times

Energy from the Sun that makes its way to Earth can have trouble finding its way back out to space. The greenhouse effect causes some of this energy to be waylaid in the atmosphere, absorbed and released by greenhouse gases.

Without the greenhouse effect, Earth’s temperature would be below freezing. It is, in part, a natural process. However, Earth’s greenhouse effect is getting stronger as we add greenhouse gases to the atmosphere. That is warming the climate of our planet.

How Does the Greenhouse Effect Work?

Solar energy absorbed at Earth’s surface is radiated back into the atmosphere as heat. As the heat makes its way through the atmosphere and back out to space, greenhouse gases absorb much of it. Why do greenhouse gases absorb heat? Greenhouse gases are more complex than other gas molecules in the atmosphere, with a structure that can absorb heat. They radiate the heat back to the Earth's surface, to another greenhouse gas molecule, or out to space.

There are several different types of greenhouse gases. The major ones are carbon dioxide, water vapor, methane, and nitrous oxide. These gas molecules all are made of three or more atoms. The atoms are held together loosely enough that they vibrate when they absorb heat. Eventually, the vibrating molecules release the radiation, which will likely be absorbed by another greenhouse gas molecule. This process keeps heat near the Earth’s surface. Most of the gas in the atmosphere is nitrogen and oxygen, which cannot absorb heat and contribute to the greenhouse effect.

A Couple of Common Greenhouse Gases

• Carbon dioxide: Made of one carbon atom and two oxygen atoms, carbon dioxide molecules make up a small fraction of the atmosphere, but have a large effect on climate. There was about 270 parts per million (ppm) of carbon dioxide in the atmosphere in the mid-19th Century at the start of the Industrial Revolution. The amount is growing as burning fossil fuels releases carbon dioxide into the atmosphere. The concentration has been over 400 ppm since 2015. (Check NOAA Global Monitoring Laboratory for the latest measurements).

• Methane: A powerful greenhouse gas, able to absorb far more heat than carbon dioxide, methane is made of one carbon and four hydrogen atoms. It is found in very small quantities in the atmosphere but is able to make a big impact on warming. Methane gas is also used as a fuel. When burned, it releases carbon dioxide greenhouse gas into the atmosphere.

Above: (Left) The Earth’s surface, warmed by the Sun, radiates heat into the atmosphere. Some heat is absorbed by greenhouse gases like carbon dioxide and then radiated to space (A). Some heat makes its way to space directly (B). Some heat is absorbed by greenhouse gases and then radiated back towards the Earth’s surface (C). (Right) With more carbon dioxide in the atmosphere later this Century, more heat will be stopped by greenhouse gases, warming the planet. (Image: L.S.Gardiner/UCAR)

More Greenhouse Gases = A Warmer Earth

Even though only a tiny amount of the gases in Earth’s atmosphere are greenhouse gases, they have a huge effect on climate. Sometime during this century, the amount of the greenhouse gas carbon dioxide in the atmosphere is expected to double. Other greenhouse gases like methane and nitrous oxide are increasing as well. The quantity of greenhouse gases is increasing as fossil fuels are burned, releasing the gases and other air pollutants into the atmosphere. Greenhouse gases also make their way to the atmosphere from other sources. Farm animals, for example, release methane gas as they digest food. As cement is made from limestone, it releases carbon dioxide.

With more greenhouse gases in the air, heat passing through on its way out of the atmosphere is more likely to be stopped. The added greenhouse gases absorb the heat. They then radiate this heat. Some of the heat will head away from the Earth, some of it will be absorbed by another greenhouse gas molecule, and some of it will wind up back at the planet’s surface again. With more greenhouse gases, heat will stick around, warming the planet.

In a comment on my Global Warming Crib Sheet, johnmashey asked if I could say something about the lifetimes of greenhouse gases – that is, how long different greenhouse gases stay in the atmosphere. Great idea! Here’s a whole post on it.

Many people don’t realize that the greenhouse gases we emit can stay in the atmosphere for decades, centuries or even millennia. That’s why it’s so important that we cap emissions as soon as possible.

Here’s a table showing a selection of greenhouse gases, their global warming potential (GWP), and their lifetimes:

Source: Table 2.14 in the IPCC AR4 WG-I Report. Original table lists many more gases.

Notice that the carbon dioxide lifetime is “hundreds of years”, rather than a specific number.

The IPCC Third Assessment Report defines a gas’s lifetime as the amount of the gas in the atmosphere divided by the rate at which it is removed from the atmosphere. That sounds simple enough, except that not all gases are removed by just one (or mainly one) process.

Ironically, the gas that accounts for the greatest proportion of global warming, carbon dioxide (CO2), is the hardest to pin down. When CO2 is released into the atmosphere, about three-quarters of it dissolves into the ocean over a few decades. The rest is neutralized by a variety of longer-term geological processes, which can take thousands of years.

From IPCC Fourth Assessment Report, Working Group I (AR4, WG-I) Executive Summary of Chapter 7:

About 50% of a CO2 increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years.

From U.S Greenhouse Gas Inventory Reports:

Atmospheric lifetime: 50-200 years. No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.

From RealClimate post "How long will global warming last?":

My model indicates that about 7% of carbon released today will still be in the atmosphere in 100,000 years. I calculate a mean lifetime, from the sum of all the processes, of about 30,000 years. That’s a deceptive number, because it is so strongly influenced by the immense longevity of that long tail. If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes.

For other gases, a meaningful lifetime is easier to calculate because one process dominates their removal from the atmosphere:

• Methane is mostly scrubbed from the atmosphere by hydroxyl radicals (a chemical reaction).
• Nitrous oxide is destroyed by photolytic reactions (chemical reactions involving photons or light) in the stratosphere.

As you can see from the chart, some gases have extraordinarily long lifetimes. Because emission rates are vastly higher than removal rates, greenhouse gases are accumulating in the atmosphere and will affect climate for generations to come.

Are there any other climate statistics you’d like to better understand? Post a comment and let us know!

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