Greenhouse gases in the atmosphere The atmospheric concentrations of carbon dioxide and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that carbon dioxide values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of carbon dioxide to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation. The longest continuous instrumental measurement of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically from 315 ppmv as shown by the Keeling Curve. The concentration reached 376 ppmv in 2003. South Pole records show similar growth. The monthly measurements display small seasonal oscillations. Methane is produced biologically and released from gas pipelines. Some biological sources are "natural" such as termites and others are attributable to human activity such as agriculture, e.g., rice paddies. Recent evidence suggests that forests may also be a source (RC; BBC). Note that this is a contribution to the natural greenhouse effect, and not to the anthropogenic greenhouse effect (Ealert).

Adding carbon dioxide (CO2) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C lower, and the Earth uninhabitable.

The Natural Greenhouse Effect The earth receives an enormous amount of solar radiation. Just above the atmosphere, the solar power flux density averages about 1366 watts per square meter, or 1.740×1017 W over the entire Earth. This figure greatly exceeds the power generated by human activities. The difference between the natural greenhouse effect and global warming is that- global warming is anthropogenic whereas greenhouse effect is not. The solar power hitting Earth is balanced over time by an equal amount of power radiating from the Earth (as the amount of energy from the Sun that is stored is small). Almost all radiation leaving the Earth takes two forms: reflected solar radiation and thermal black body radiation. Reflected solar radiation accounts for 30% of the Earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface. The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave ultraviolet over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans. This absorbed energy heats the atmosphere, oceans, and land and powers life on the planet. It should be noted that the surface of the Earth is in constant flux with daily, yearly and age long cycles and trends in temperature and other variables for a variety of causes; thus these percentages apply on average only. Like the Sun, the Earth is a thermal radiator. Because the Earth's surface is much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates that Earth radiates its thermal energy at longer wavelengths than the Sun. While the Sun's radiation peaks at a visible wavelength of 500 nanometers, Earth's radiation peak is in the longwave (far) infrared at about 10 micrometres. The Earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres (this is, probably, not entirely coincidental: the transparency to "visible" wavelengths makes eyes adapted to seeing these wavelengths useful; and eye that could see in a strongly-absorbed wavelength would not be so useful). Only about 6% of the Earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a radiator in the far infrared, so it re-radiates this energy. The Earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of Earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor. Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over Earth's surface. Visible and near-visible radiation from the Sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, Earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption. Another, simpler, but essentially equivalent way of looking at this is that the surface is heated by two sources: direct solar radiation, and thermal radiation from the atmosphere; it is thus warmer than if heated by solar radiation alone. The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the Earth's surface temperature were determined solely by the albedo and blackbody properties of the surface.

The greenhouse gases Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%). Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect.

There has been an observed global average temperature increase of about 0.5oC since 1960 (Science 308, 1431, 2005). There is still some public controversy about the role of human activities and that of CO2 and other greenhouse gas increases for global warming.