Global Warming is the observed increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.
Global average air temperature near Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the last century. The Intergovernmental Panel on Climate Change (IPCC) concludes, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations," which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. These conclusions have been endorsed by more than 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized states, however a small number of individual scientists disagree.
Models referenced by the IPCC predict that global temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. The range of values reflects the use of differing scenarios of future greenhouse gas emissions as well as uncertainties regarding climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if no further greenhouse gases are released after this date. This reflects the long average atmospheric lifetime of carbon dioxide (CO2).
An increase in global temperatures can in turn cause other changes, including a flooding of coastal cities, an increase in the frequency and intensity of extreme weather events, reduction in agricutural yields due to changes in the amount and pattern of precipitation, increases in the ranges of disease vectors, reduced summer streamflows, further melting of glaciers, and species extinctions.
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and especially how changes will vary from region to region across the globe. Efforts are underway to reduce or reverse future warming and, in some instances, to allocate future generations the responsibility of bearing the brunt of the expected consequences. Most national governments have signed and ratified the Kyoto Protocol aimed at combating greenhouse gas emissions........
The term global warming is a specific example of the broader term climate change, which can also refer to global cooling. In principle, global warming is neutral as to the period or causes, but in both common and scientific usage the term generally refers to recent warming and implies a human influence.
The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. The term "anthropogenic climate change" is sometimes used when focusing on human-induced changes.
Carbon dioxide during the last 400,000 years and the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.The climate system varies through natural, internal processes and in response to variations in external "forcing" factors including solar activity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. The detailed causes of the recent warming remain an active field of research, but the scientific consensus identifies increased levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available.
Greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, so that Earth would be uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the greenhouse effect as such. Rather, the debate concerns the net effect of the addition of greenhouse gases when allowing for positive or negative feedback.
The primary greenhouse gases are water vapor, carbon dioxide (CO2), and methane (CH4). Water is the most abundant in the atmosphere by concentration, but it is a short-term greenhouse gas and in a dynamic equilibrium in the atmosphere. Great quantities of water can be added to the atmosphere by evaporation or subtracted by precipitation in a period of weeks. Methane is an intermediate-term greenhouse gas and in the atmosphere is converted to CO2 in a period of months to years. CO2 is a long-term greenhouse gas and, once added to the atmosphere can remain in the atmosphere for hundreds of years.
Adding CO2 or CH4 to Earth's atmosphere, with no other changes, will make the planet's surface warmer. The concentration of CO2 in the atmosphere, currently 380 parts per million (ppm), might be naïvely taken to be too low to have much effect. However, the importance of CO2 arises from a feedback effect: a little of the long-term CO2 injected into the atmosphere causes a little warming, which causes a little more of the potent short-term water vapor to be evaporated into the atmosphere, which causes still more warming, which causes more of the potent water vapor to be evaporated, and so forth, until a new dynamic equilibrium concentration of water vapor is reached at a slightly higher humidity and with a much larger greenhouse effect than that due to CO2 alone.
This feedback effect is reversed only as the CO2 is slowly removed from the atmosphere.
Another important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.
None of the effects of greenhouse gases are instantaneous. Due to the thermal inertia of the Earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of about 0.5 °C (0.9 °F) would still occur.
Contrasting with the consensus view, a tiny minority of scientists have proposed alternative hypotheses to explain all or most of the observed increase in global temperatures, including: the warming is within the range of natural variation; the warming is a consequence of coming out of a prior cool period, namely the Little Ice Age; the warming is primarily a result of variances in solar radiation; or the warming is primarily the result of increased activity of the solar magnetic field, which increases shielding of the Earth from cosmic rays which would otherwise cause raindrop nucleation in clouds, which would remove greenhouse-gas water vapor from the atmosphere.
The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the northern hemisphere's late spring, and declines during the northern hemisphere growing season as plants remove some CO2 from the atmosphere.The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption of infrared radiation by atmospheric gases warms a planet's atmosphere and surface.
On Earth, the major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9-26%; methane (CH4), which causes 4-9%; and ozone, which causes 3-7%.
The atmospheric concentrations of CO2 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 CO2 values this high were last attained 20 million years ago. About three-quarters of the anthropogenic (man-made) emissions of CO2 to the atmosphere during the past 20 years are due to fossil fuel burning. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation.
Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 parts per million by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.
Carbon dioxide sink ecosystems (forests and oceans) are being degraded by pollutants. Degradation of major carbon sinks results in higher atmospheric CO2 levels.
Positive feedback effects such as the expected release of CH4 from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions not included in IPCC's climate models.
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