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Unit 12: Earth's Changing Climate // Section 2: Tipping Earth's Energy Balance

Earth's climate is a dynamic system that is driven by energy from the sun and constantly impacted by physical, biological, and chemical interactions between the atmosphere, global water supplies, and ecosystems (Fig. 2).

Components and interactions of the global climate system

Figure 2. Components and interactions of the global climate system
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Source: © Intergovernmental Panel on Climate Change 2001: Synthesis Report, SYR Figure 2-4.

As discussed in Unit 2, "Atmosphere," energy reaches Earth in the form of solar radiation from the sun. Water vapor, clouds, and other heat-trapping gases create a natural greenhouse effect by holding heat in the atmosphere and preventing its release back to space. In response, the planet's surface warms, increasing the heat emitted so that the energy released back from Earth into space balances what the Earth receives as visible light from the sun (Fig. 3). Today, with human activities boosting atmospheric GHG concentrations, the atmosphere is retaining an increasing fraction of energy from the sun, raising earth's surface temperature. This extra impact from human activities is referred to as anthropogenic climate change.

Earth's energy balance

Figure 3. Earth's energy balance
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Source: Courtesy Jared T. Williams. © Dan Schrag, Harvard University.

Many GHGs, including water vapor, ozone, CO2, methane (CH4), and nitrous oxide (N2O), are present naturally. Others are synthetic chemicals that are emitted only as a result of human activity, such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Important human activities that are raising atmospheric GHG concentrations include:

Measuring CO2 levels at Mauna Loa, Hawaii, and other pristine air locations, climate scientist Charles David Keeling traced a steady rise in CO2 concentrations from less than 320 parts per million (ppm) in the late 1950s to 380 ppm in 2005 (Fig. 4). Yearly oscillations in the curve reflect seasonal cycles in the northern hemisphere, which contains most of Earth's land area. Plants take up CO2 during the growing season in spring and summer and then release it as they decay in fall and winter.

Atmospheric CO2 concentrations, 1958–2005

Figure 4. Atmospheric CO2 concentrations, 1958–2005
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Source: © 2005. National Aeronautics and Space Administration. Earth Observatory.

Global CO2 concentrations have increased by one-third from their pre-industrial levels, rising from 280 parts per million before the year 1750 to 377 ppm today. Levels of methane and N2O, the most influential GHGs after CO2, also increased sharply in the same time period (see Table 1 below).

If there are so many GHGs, why does CO2 get most of the attention? The answer is a combination of CO2's abundance and its residence time in the atmosphere. CO2 accounts for about 0.1 percent of the atmosphere, substantially more than all other GHGs except for water vapor, which may comprise up to 7 percent depending on local conditions. However, water vapor levels vary constantly because so much of the Earth's surface is covered by water and water vapor cycles into and out of the atmosphere very quickly—usually in less than 10 days. Therefore, water vapor can be considered a feedback that responds to the levels of other greenhouse gases, rather than an independent climate forcing (footnote 1).

Other GHGs contribute more to global climate change than CO2 on a per-unit basis, although their relative impacts vary with time. The global warming potential (GWP) of a given GHG expresses its estimated climate impact over a specific period of time compared to an equivalent amount by weight of carbon dioxide. For example, the current 100-year GWP for N2O is 296, which indicates that one ton of N2O will have the same global warming effect over 100 years as 296 tons of CO2. Internationally-agreed GWP values are periodically adjusted to reflect current research on GHGs' behavior and impacts in the atmosphere.

However, CO2 is still the most important greenhouse gas because it is emitted in far larger quantities than other GHGs. Atmospheric concentrations of CO2 are measured in parts per million, compared to parts per billion or per trillion of other gases, and CO2's atmospheric lifetime is 50 to 200 years, significantly longer than most GHGs. As illustrated in Table 1, the total extent to which CO2 has raised global temperature (referred to as radiative forcing and measured in watts per square meter) since 1750 is significantly larger than forcing from other gases.

Table 1. Current greenhouse gas concentrations. Source: T.J. Blasing and Karmen Smith, "Current Greenhouse Gas Concentrations," Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, July 2006,
Gas Pre-1750 concentration Current tropospheric concentration 100-year GWP Atmospheric lifetime (years) Increased radiative forcing (watts/meter2)
Carbon dioxide 280 parts per million 377.3 parts per million 1 Variable (up to 200 years) 1.66
Methane 688-730 parts per billion 1,730-1,847 parts per billion 23 12 0.5
Nitrous oxide 270 parts per billion 318-319 parts per billion 296 114 0.16
Tropospheric ozone 25 34 Not applicable due to short residence time Hours to days 0.35
Industrial gases (HFCs, PFCs, halons) 0 Up to 545 parts per trillion Ranges from 140 to 12,000 Primarily between 5 and 260 years 0.34 for all halocarbons collectively
Sulfur hexafluoride 0 5.22 parts per trillion 22,200 3,200 0.002

A look at current emissions underlines the importance of CO2. In 2003 developed countries emitted 11.6 billion metric tons of CO2, nearly 83 percent of their total GHG emissions. Developing countries' reported emissions were smaller in absolute terms, but CO2 accounted for a similarly large share of their total GHG output (footnote 2). In 2004, CO2 accounted for 85 percent of total U.S. GHG emissions, compared to 7.8 percent from methane, 5.4 percent from N2O, and 2 percent from industrial GHGs (footnote 3).

These emissions from human activities may reshape the global carbon cycle. As discussed in Units 2 ("Atmosphere") and 3 ("Oceans"), roughly 60 percent of CO2 emissions from fossil fuel burning remain in the atmosphere, with about half of the remaining 40 percent absorbed by the oceans and half by terrestrial ecosystems. However, there are limits to the amount of anthropogenic carbon that these sinks can take up. Oceans are constrained by the rate of mixing between upper and lower layers, and there are physical bounds on plants' ability to increase their photosynthesis rates as atmospheric CO2 levels rise and the world warms.

Scientists are still trying to estimate how much carbon these sinks can absorb, but it appears clear that oceans and land sinks cannot be relied on to absorb all of the extra CO2 emissions that are projected in the coming century. This issue is central to projecting future impacts of climate change because emissions that end up in the atmosphere, rather than being absorbed by land or ocean sinks, warm the earth.

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