| The climate system varies through natural, | | | | values this high were last attained 20 |
| internal processes and in response to | | | | million years ago. "About three-quarters of |
| variations in external forcing factors | | | | the anthropogenic [man-made] emissions of CO2 |
| including solar activity, volcanic emissions, | | | | to the atmosphere during the past 20 years |
| variations in the earth's orbit (orbital | | | | are due to fossil fuel burning. The rest of |
| forcing) and greenhouse gases. The detailed | | | | the anthropogenic emissions are predominantly |
| causes of the recent warming remain an active | | | | due to land-use change, especially |
| field of research, but the scientific | | | | deforestation." |
| consensus[6] identifies increased levels of | | | | |
| greenhouse gases due to human activity as the | | | | The present atmospheric concentration of CO2 |
| main influence. This attribution is clearest | | | | is about 383 parts per million (ppm) by |
| for the most recent 50 years, for which the | | | | volume. Future CO2 levels are expected to |
| most detailed data are available. Contrasting | | | | rise due to ongoing burning of fossil fuels |
| with the scientific consensus, other | | | | and land-use change. The rate of rise will |
| hypotheses have been proposed to explain some | | | | depend on uncertain economic, sociological, |
| of the observed increase in global | | | | technological, natural developments, but may |
| temperatures, including: the warming is | | | | be ultimately limited by the availability of |
| within the range of natural variation; the | | | | fossil fuels. The IPCC Special Report on |
| warming is a consequence of coming out of a | | | | Emissions Scenarios gives a wide range of |
| prior cool period, namely the Little Ice Age; | | | | future CO2 scenarios, ranging from 541 to 970 |
| or the warming is primarily a result of | | | | ppm by the year 2100. Fossil fuel reserves |
| variances in solar radiation. | | | | are sufficient to reach this level and |
| | | | continue emissions past 2100, if coal, tar |
| None of the effects of forcing are | | | | sands or methane clathrates are extensively |
| instantaneous. Due to the thermal inertia of | | | | used. |
| the Earth's oceans and slow responses of | | | | |
| other indirect effects, the Earth's current | | | | Positive feedback effects such as the |
| climate is not in equilibrium with the | | | | expected release of CH4 from the melting of |
| forcing imposed. Climate commitment studies | | | | permafrost peat bogs in Siberia (possibly up |
| indicate that even if greenhouse gases were | | | | to 70,000 million tonnes) may lead to |
| stabilized at present day levels, a further | | | | significant additional sources of greenhouse |
| warming of about 0.5 °C (0.9 °F) would | | | | gas emissions not included in climate models |
| still occur | | | | cited by the IPCC. |
| | | | |
| Greenhouse gases in the atmosphere | | | | Feedbacks |
| | | | |
| Recent increases in atmospheric carbon | | | | The effects of forcing agents on the climate |
| dioxide (CO2). The monthly CO2 measurements | | | | are complicated by various feedback |
| display small seasonal oscillations in an | | | | processes. |
| overall yearly uptrend; each year's maximum | | | | |
| is reached during the northern hemisphere's | | | | One of the most pronounced feedback effects |
| late spring, and declines during the northern | | | | relates to the evaporation of water. CO2 |
| hemisphere growing season as plants remove | | | | injected into the atmosphere causes a warming |
| some CO2 from the atmosphere. | | | | of the atmosphere and the earth's surface. |
| | | | The warming causes more water to be |
| Recent increases in atmospheric carbon | | | | evaporated into the atmosphere. Since water |
| dioxide (CO2). The monthly CO2 measurements | | | | vapor itself acts as a greenhouse gas, this |
| display small seasonal oscillations in an | | | | causes still more warming; the warming causes |
| overall yearly uptrend; each year's maximum | | | | more water vapor to be evaporated, and so |
| is reached during the northern hemisphere's | | | | forth until a new dynamic equilibrium |
| late spring, and declines during the northern | | | | concentration of water vapor is reached at a |
| hemisphere growing season as plants remove | | | | slight increase in humidity and with a much |
| some CO2 from the atmosphere. | | | | larger greenhouse effect than that due to CO2 |
| | | | alone. This feedback effect can only be |
| The greenhouse effect was discovered by | | | | reversed slowly as CO2 has a long average |
| Joseph Fourier in 1824 and was first | | | | atmospheric lifetime. |
| investigated quantitatively by Svante | | | | |
| Arrhenius in 1896. It is the process by which | | | | Feedback effects due to clouds are an area of |
| absorption and emission of infrared radiation | | | | ongoing research and debate. Seen from below, |
| by atmospheric gases warms a planet's | | | | clouds absorb infrared radiation and so exert |
| atmosphere and surface. | | | | a warming effect. Seen from above, the same |
| | | | clouds reflect sunlight and so exert a |
| Greenhouse gases create a natural greenhouse | | | | cooling effect. Increased global water vapor |
| effect, without which, mean temperatures on | | | | concentration may or may not cause an |
| Earth would be an estimated 30 °C (54 | | | | increase in global average cloud cover. The |
| °F) lower, so that Earth would be | | | | net effect of clouds thus has not been well |
| uninhabitable. Thus scientists do not | | | | modeled, however, cloud feedback is second |
| "believe in" or "oppose" the greenhouse | | | | only to water vapor feedback and is positive |
| effect as such; rather, the debate concerns | | | | in all the models that contributed to the |
| the net effect of the addition of greenhouse | | | | IPCC Fourth Assessment Report. |
| gases, while allowing for associated positive | | | | |
| and negative feedback mechanisms. | | | | Another important feedback process is |
| | | | ice-albedo feedback. The increased CO2 in the |
| On Earth, the major natural greenhouse gases | | | | atmosphere warms the Earth's surface and |
| are water vapor, which causes about | | | | leads to melting of ice near the poles. As |
| 36–70% of the greenhouse effect (not | | | | the ice melts, land or open water takes its |
| including clouds); carbon dioxide (CO2), | | | | place. Both land and open water are on |
| which causes 9–26%; methane (CH4), | | | | average less reflective than ice, and thus |
| which causes 4–9%; and ozone, which | | | | absorb more solar radiation. This causes more |
| causes 3–7%. The atmospheric | | | | warming, which in turn causes more melting, |
| concentrations of CO2 and CH4 have increased | | | | and this cycle continues. |
| by 31% and 149% respectively above | | | | |
| pre-industrial levels since 1750. These | | | | Positive feedback due to release of CO2 and |
| levels are considerably higher than at any | | | | CH4 from thawing permafrost is an additional |
| time during the last 650,000 years, the | | | | mechanism contributing to warming. Possible |
| period for which reliable data has been | | | | positive feedback due to CH4 release from |
| extracted from ice cores. From less direct | | | | melting seabed ices is a further mechanism to |
| geological evidence it is believed that CO2 | | | | be considered. |