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Global warming crib sheet

By Lisa Moore

Posted Tue Feb 19, 2008 12:44pm PST

Related topics: Science, Researching global warming basics

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