You can find more posts on climate change science, policy, and news on Climate 411.
Climate change science is, well, science, so it can get pretty technical.
How much warming have we had, and how much more can we expect? How much global warming pollution is in the air right now, and how long will it stay there? What do all those funny abbreviations mean?
Follow me over the fold for a handy global warming crib sheet.
Temperatures
Two factors frequently confuse discussions of temperature:
- What is the baseline temperature we’re comparing to, preindustrial or current?
- Is the temperature scale Fahrenheit (F) or Celsius (C)?
And then there’s the issue of "warming in the pipeline". Some additional warming is certain because warming lags behind greenhouse gas emissions.
Discussions of global temperature often center around "tipping points" - the points after which qualitative climate change become inevitable. The most commonly cited of these is the melting of the Greenland Ice Sheet (GIS), which would lead to a 20-foot rise in sea levels, so this is what is shown in the table below as "common GIS point". (For more, see my diary on "tipping elements".)
Warming | Temp above 1750 (C) | Temp above 1750 (F) |
So far | 0.7 | 1.3 |
In the pipeline | 0.6 | 1.0 |
Total commitment | 1.3 | 2.3 |
Common GIS point | 2.0 | 3.6 |
Greenhouse Gas Notation
Before we go any further, let’s cover some vocabulary.
ppm | Parts-per-million, a measurement of atmospheric concentrations of greenhouse gases |
MMT or Mt | Million metric tons or megatons, to describe amounts of greenhouse gas emissions |
Gt | Billion metric tons or gigatons, equivalent to 1,000 Mt (the notation is the same as for bytes) |
CO2 | Carbon dioxide, an important greenhouse gas |
CO2e | Carbon dioxide equivalents, to describe all greenhouse gases in terms of the warming potential of carbon dioxide (some greenhouse gases cause more warming, ton for ton, though there are less of them) |
Greenhouse Gas Warming Potentials
There are dozens of human-produced greenhouse gases, but the three that account for the most warming are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Although CO2 causes the most warming, that’s because it’s the most prevalent, not the most potent. Methane and nitrous oxide have much higher warming potentials: 25 and 298 times more powerful, respectively. (Note that this is over a 100-year time horizon; the warming potential changes depending on the length of time you're talking about.)
Greenhouse Gas Concentrations
The concentration of greenhouse gases in the atmosphere drives global warming. Here’s a table showing where we’ve been, where we are, and where we’re headed if we continue with business-as-usual.
Concentration | Year | Notes |
280 ppm CO2 | 1750 (pre-industrial) | |
383 ppm CO2 or CO2e | 2007 | It’s approximately the same in CO2 or CO2e because of the cooling effects of aerosols, which can reflect sunlight |
450 ppm CO2e | 2040, in one business-as-usual scenario* | Gives a 50 percent chance of exceeding +2°C, a commonly cited tipping point for the Greenland Ice Sheet (see above) |
1000 ppm CO2 | 2100, in one business-as-usual scenario* | |
* See figures 10.20 and 10.21 in
IPCC’s 4th Assessment Report. Note that these graphs show CO2 only. Business-as-usual scenarios project that CO2e could increase even more quickly than CO2 alone.
Greenhouse Gas Emissions
These statistics are from 2005, the most recent year for which data are available.
Global Emissions
CO2 emissions from fossil fuels | 28.2 Gt CO2 |
Emissions of non-CO2 greenhouse gases | 10.2 Gt CO2e |
Emissions from deforestation (17% of total) | 7.9 Gt CO2 |
Total greenhouse gas emissions | 46.3 Gt CO2e |
U.S. Emissions
Total emissions: 7.2 Gt CO2e
Increase since 1990: 16 percent
Greenhouse Gas Lifetimes
The
IPCC 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 process.
Ironically, the most important greenhouse gas, carbon dioxide (CO2), is the hardest to pin down as far as lifetime goes, which is why people say that CO2 stays in the atmosphere for centuries, instead of a specific number of years. For example, the IPCC says that "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."
This complexity reflects all the different processes that affect CO2 levels. For example, when CO2 is released to the atmosphere, some of it will dissolve into the ocean fairly quickly. However, the ocean's surface layer will saturate and it takes much longer (many centuries to millennia) for the surface layer to mix with the deep ocean.
As the RealClimate blog concludes, "If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose."
For other gases, it’s easier to calculate a meaningful lifetime, because one process dominates their removal from the atmosphere. For example, most methane is scrubbed from the atmosphere by hydroxyl radicals, and nitrous oxide is destroyed by photolytic reactions in the stratosphere.
Table 2.14 in IPCC’s Chapter 2 shows the lifetimes of dozens of greenhouse gases. Here’s a condensed version.
Greenhouse Gas | Lifetime (years) |
Carbon dioxide (CO2) | Hundreds |
Methane (CH4) | 12 |
Nitrous oxide (N2O) | 114 |
"Montreal Protocol gases" | 0.7 – 1,700 |
Hydrofluorocarbons | 1.4 - 270 |
Perfluorinated compounds | 740 – 50,000 |
Because emissions are vastly higher than removal rates, greenhouse gases are accumulating in the atmosphere and will affect climate for generations to come. These numbers are part of the reason why we need to reduce emissions as soon as possible.
Further Reading
All things RealClimate
The mother of all climate stuff: the IPCC Fourth Assessment Report
You can find more posts on climate change science, policy, and news on Climate 411