The major greenhouse gases, carbon dioxide, methane, nitrous oxide and water vapor, occur naturally in the atmosphere. Without them, the Earth would be too cold to support life as we know it. The basic science of the greenhouse effect is not controversial.  Scientists understand the greenhouse effect and can easily reproduce it in the laboratory. There is no disagreement about the fact that these gases are transparent to incoming short-wave solar radiation, and that they tend to absorb outgoing long-wave radiation and re-emit part of that radiation back down to the Earth’s surface. In effect, they act as a blanket to warm the surface of the Earth.

Concern about climate change arises from the fact that human activities are releasing large quantities of these substances − and other even more powerful manufactured greenhouse gases such as halocarbons − into the atmosphere (Table 1). Because carbon dioxide and many of the halocarbons have very long atmospheric lifetimes, the increased concentrations are likely to result in an enhanced greenhouse effect for centuries to come.

Table 1 Selected chemically reactive greenhouse gases and their precursors: abundances, trends, budgets, lifetimes, and GWPs.

Sources: Data from IPCC WGI 2001; Blasing and Jones 2003.

^aSpecies with chemical feedback that affect the duration of atmospheric response – value are perturbation lifetimes

^b Regulated under Protocol

^c Global Warming Potential (GWP) is an index describing the relative effectiveness of well-mixed greenhouse gases in absorbing outgoing infrared radiation.  The index approximates the time-integrated warming effect of a unit mass of a given greenhouse gas relative to that of carbon dioxide.

^d As measured at (Blasing and Jones 2003)

We are also loading the atmosphere with other types of pollutants. Some of these tend to produce cooling by reflecting incoming sunlight. Dust from disturbed soil surfaces and other tiny particles from combustion, especially sulphate aerosols, act in this way. Unlike carbon dioxide and many other greenhouse gases, however, these aerosols only stay in the atmosphere a very short time. So, although they may temporarily mask the warming effects of the greenhouse gases, warming will eventually dominate. Figure 1 depicts the estimated relative impacts of greenhouse gases, aerosols, and other factors on global temperatures from pre-industrial times (circa 1750) to the present (circa 2000).

Figure 1. Many external factors force climate change. The error bars show ranges of uncertainty in radiative forcing.  Source: IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the IPCC. Cambridge, UK: Cambridge University Press.
Figure 9, page 37 

Over the past 400,000 years, atmospheric carbon dioxide concentrations varied from about 180 parts per million (ppmv) at the height of each glaciation to about 310 ppmv at the peak of each warming. Similarly, methane concentrations varied from approximately 350 to 800 parts per billion (ppbv). Since the beginning of the Industrial Revolution, burning of fossil fuels, deforestation, expanding agriculture, and other human activities have contributed to rapid increases in CO₂ and methane concentrations. In the mid-eighteenth century, the estimated atmospheric concentration of CO₂ stood at 280 ppmv. As of the year 2002, it had risen to approximately 372 ppmv. Similarly, methane concentrations increased from approximately 700 ppbv at the beginning of the Industrial Revolution to current levels between 1,729−1,843 ppbv, as measured at different locations. These modern levels are, thus, well above the range of natural variability in the recent geologic past. Future emissions are expected to further increase these concentrations.