This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.
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:
|Greenhouse Gas||Lifetime (Years)||100-Year GWP|
|Carbon Dioxide (CO2)||hundreds||1|
|Nitrous Oxide (N2O)||114||298|
|Sulphur hexafluoride (SF6)||3,200||22,800|
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.
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!