Global warming crib sheet
By Lisa Moore
This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.
The science behind global warming is, well, science, and it can get pretty technical.
By how many degrees has the globe already warmed? How much more can it warm before we’re in trouble? How much carbon dioxide is in the air now, and how much more can we afford to emit before risking climate catastrophe? Which are the most important greenhouse gases? And what do all those funny abbreviations mean?
Below you will find a handy crib sheet that gives you all these numbers and more.
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 or Celsius?
And then there’s the issue of "warming in the pipeline." Temperature may have warmed by only a certain amount today, but some additional warming is certain because warming lags behind greenhouse gas emissions.
First greenhouse gas concentrations increase, and then some time later warming occurs. The amount of future warming that is certain due to existing greenhouse gas concentrations is called "warming in the pipeline."
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, which would lead to a 20-foot rise in sea levels, so this is what is shown in the table below. (For more, see last week’s post on "tipping elements.")
| Warming | Temperature above Pre-industrial (Celsius) | Temperature above Pre-industrial (Fahrenheit) |
|---|---|---|
| Since pre-industrial | 0.7 | 1.3 |
| In the pipeline | 0.6 | 1.0 |
| Total commitment | 1.3 | 2.3 |
| Likely tipping point for Greenland Ice Sheet | 2.0 | 3.6 |
Greenhouse gas notation
For the rest, let’s start with vocabulary. When you hit an abbreviation you don’t understand, it’s hard to follow the rest of the discussion.
| ppm | parts per million -- to describe atmospheric concentrations of greenhouse gases. |
|---|---|
| MMT or Mt | million metric tons or megatons -- to describe the amount of greenhouse gas emissions spewed into the atmosphere. |
| Gt | billion metric tons or gigatons -- equivalent to 1000 Mt |
| 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.
Greenhouse gas percents by volume
| Greenhouse Gas (GHG) | Percent of Long-Lived GHGs | Warming Potential of Gas vs. CO2 | Percent Warming Caused by Gas |
|---|---|---|---|
| carbon dioxide | 99.4% | 1 | 62.9% |
| methane | 0.5% | 25 | 18.2% |
| nitrous oxide | 0.08% | 298 | 6.1% |
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.
| GHG Concentration (CO2 or CO2e) | Year | Notes |
|---|---|---|
| 280 ppm CO2 | 1750 (Preindustrial) | |
| 383 ppm CO2 or CO2e | 2008 | It’s approximately the same in CO2 or CO2e because of the cooling effects of "aerosols" (for example, the sulfate particles Bill discussed in his post on geoengineering).
|
| 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, which is the most recent data 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
Emissions reductions
What emissions reductions are necessary to keep greenhouse gas concentrations below 450 ppm? This table summarizes the cuts we need to make in the next few decades, though of course our efforts will have to continue beyond 2050.
Emissions cuts needed to stop warming
| Year | Developed Countries | Global Emissions |
|---|---|---|
| 2020 | 20 percent below 2005 | Peak and begin to decline |
| 2050 | 80 percent below 2005 | 50 percent below 2005 |
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