GREEN HOUSE GASES?? WHAT ARE THEY......

What Are Greenhouse Gases?Many chemical compounds found in the Earth’s atmosphere act as “greenhouse gases.” These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth’s surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth’s surface roughly constant.
Many gases exhibit these “greenhouse” properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-made (like gases used for aerosols).

Why Are Atmospheric Levels Increasing?
Levels of several important greenhouse gases have increased by about 25 percent since large-scale industrialization began around 150 years ago (Figure 1). During the past 20 years, about three-quarters of human-made carbon dioxide emissions were from burning fossil fuels.

Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively known as the “carbon cycle” (Figure 2). The movement (“flux”) of carbon between the atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. While these natural processes can absorb some of the net 6.1 billion metric tons of anthropogenic carbon dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 3.2 billion metric tons is added to the atmosphere annually. The Earth’s positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.


What Effect Do Greenhouse Gases Have on Climate Change?
Given the natural variability of the Earth’s climate, it is difficult to determine the extent of change that humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather, sea levels, and land use patterns, commonly referred to as “climate change.”
Assessments generally suggest that the Earth’s climate has warmed over the past century and that human activity affecting the atmosphere is likely an important driving factor. A National Research Council study dated May 2001 stated, “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.”
However, there is uncertainty in how the climate system varies naturally and reacts to emissions of greenhouse gases. Making progress in reducing uncertainties in projections of future climate will require better awareness and understanding of the buildup of greenhouse gases in the atmosphere and the behavior of the climate system.
What Are the Sources of Greenhouse Gases?
In the U.S., our greenhouse gas emissions come mostly from energy use. These are driven largely by economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 percent of total U.S. human-made greenhouse gas emissions

Another greenhouse gas, methane, comes from landfills, coal mines, oil and gas operations, and agriculture; it represents 9 percent of total emissions. Nitrous oxide (5 percent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. Human-made gases (2 percent of total emissions) are released as byproducts of industrial processes and through leakage.
What Is the Prospect for Future Emissions?
World carbon dioxide emissions are expected to increase by 1.9 percent annually between 2001 and 2025 (Figure 5). Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries’ emissions are expected to grow above the world average at 2.7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018.

The U.S. produces about 25 percent of global carbon dioxide emissions from burning fossil fuels; primarily because our economy is the largest in the world and we meet 85 percent of our energy needs through burning fossil fuels. The U.S. is projected to lower its carbon intensity by 25 percent from 2001 to 2025, and remain below the world average (Figure 6).

Newspaper articles about global warming tell the story of the Earth's climate and the diverse opinions and scientific discoveries surrounding the theory of global warming. From the Industrial Revolution to the Kyoto treaty and the advent of hybrid technology, the topic of global climate change has enthralled readers and sparked debate for centuries. Though many people argue over the theory's validity, global warming is a subject that affects us all and newspapers chronicle its discovery and the debate surrounding the issue. Since ancient times, people have believed that human activity could affect the environment. The discovery of past ice ages shows that Earth's climate is in constant flux and that throughout history, scientists have searched for the cause of these changes. Though scientists discovered the greenhouse effect in the late 19th century, the theory of global warming wasn't accepted as a scientifically proven fact until 1992 when the United Nations held a Conference on Environment and Development. Today, global warming is a widely accepted reality and speculation about its effects range from the hysteria to the acceptance. Newspapers chronicle the slowly changing climate and the actions that have affected that change. The Global Warming Archive provides access to thousands of articles on the environment and the scientists who documented its change. From developing nations to industrial countries, global climate affects everyone and newspaper articles tell the story of nature's dramatic impact on history. NewspaperARCHIVE.com, the largest newspaper database available online, has provided a free archive on the history of global warming granting access to thousands of original newspaper articles. The archive includes articles on the early discoveries of scientists, the development of technology, pollution, the greenhouse effect and global summits and treaties dedicated to the topic of global warming.

HELP WORLD BY FOLLOWING THESE...

Share.
Didn’t your mother always tell you to share? If you only use your tent, ladder, or video player once in a while, consider lending it to others. Some communities have a shared tool shed. Workplaces have book exchanges. Or, you and a friend can team up to buy rarely used items. Sharing decreases the energy and pollution from mining, manufacturing, packaging, and transporting new goods.* Climate Leader
Get a new A/C filter.
Cleaning or replacing your air conditioner filters increases efficiency and makes it run in peak condition. Filters can be found along the length of the return duct in walls, ceilings, furnaces, or in the air conditioning unit itself. In window units, filters may lie inside of the air conditioner or they may slide out. See this list of helpful tips on buying an efficient air conditioner.


Go on a tree spree.
Planting trees removes carbon from the atmosphere, filters air, and prevents soil erosion. It’s best to plant trees native to your area that don’t require heavy irrigation.* Climate Hero
Lose the heavy stuff.
Each 100 lbs. in your car increases gas consumption by 1-2%. Another great reason to leave all bricks and rocks at home!* Climate Friend
Practice gas station etiquette.
Handle the pump with care and avoid topping off. Spilled fuel evaporates and causes air pollution. Also, try to buy gas during cooler times in the day or during evening hours when there is less evaporation

WHY WE SHOULD BE CONCERNED???

A weakening in the Earth's ability to absorb carbon dioxide from the atmosphere means that global warming is happening faster than we thought, scientists said yesterday.

If the oceans soak up less greenhouse gas there are fears climate change will worsen
Scientists thought that concentrations of carbon dioxide, the most important greenhouse gas, in the atmosphere would grow in line with the world economy.
The latest figures show, however, that over the past seven years CO2 concentrations have grown 35 per cent faster, partly because the ability of the Southern Ocean and other carbon "sinks" such as vegetation and forests to take it up has been reduced.
It is a development which has alarmed scientists from the Global Carbon Project, the University of East Anglia (UEA) and the British Antarctic Survey (BAS) who compared the period 1970-2000 with the past seven years.
They found that increasing use of coal-fired power stations rather than cleaner alternatives, had increased levels of CO2 in the atmosphere by 17 per cent above anticipated levels, based on economic projection me time there had been a decline in the ability of ocean and land 'sinks' to absorb CO2 from the atmosphere which resulted in an 18 per cent increase.
Over half the decline of the carbon sink efficiency was the result of intensifying winds in Antarctica's Southern Ocean disrupting the sea's ability to store carbon, the scientists said.
If the oceans soak up less of the greenhouse gas there are fears that global warming leading to climate change will get worse.
By 2006, CO2 emissions, the most important greenhouse gas, were up to 9.9 billion tons of carbon, 35 per cent above emissions in 1990, the year of the Kyoto Protocol. (Coincidentally that 35 per cent increase is the same as the increase in concentrations in the atmosphere over the past seven years, but there is no connection.)
The findings, produced by more than 90,000 measurements from merchant ships equipped with automatic instruments, indicated that 30 years of improvements in global emissions, caused by improvements in technology, had now stalled. They are published in the journal Proceedings of the National Academy of Sciences (PNAS).
The study's author Dr Corinne Le Quere, of UEA and the British Antarctic Survey, said the results had come as a shock.
"We expected that emissions would grow because of the expansion in the world economy but not because of a weakening in the sinks. Only the most extreme climate models predicted this. We didn't think it would happen until the second half of the century," she said.
The findings come just days after other research at UEA revealed that the levels of CO2 in the north Atlantic had reduced by about 50 per cent from the mid-1990s to 2005.
Dr Le Quere said it was difficult to pinpoint where the sinks had weakened apart from the Southern Ocean where winds had increased because of climate change and the depletion of the ozone layer.
The stronger winds were causing more "mixing" of the waters, bringing carbon up from the deep seas where it was stored and raising the carbon concentration of the surface water, which allowed less CO2 to dissolve into the ocean from the atmosphere.
A decrease in fossil fuel efficiency had also accounted for speeding up the increase in atmospheric carbon dioxide levels, she said.
" For 30 years we had technical improvements in carbon intensity but this has stalled since 2000 and this has had a major effect.
"There's been a slow change from oil and gas to coal which is more CO2 intensive. As developing countries grow so does their use of energy and coal is easier to access and cheaper."
"Developed countries have not been providing massive investment in technology to counteract that.
"We had anticipated that the growth in CO2 would follow the world economy but we had not anticipated that we would not be as efficient as we were being and the sinks would not respond."
Dr Le Quere warned: "The decline in global sink efficiency suggests that stabilisation of atmospheric CO2 is even more difficult to achieve than previously thought."
But she said there was still time to take action and that technical improvements could have a huge impact.
"The study shows we can control the growth of CO2 but we have to be more aggressive on a global scale."
The study's lead author and executive director of the Global Carbon Project, Dr Pep Canadell, said: "In addition to the growth of global population and wealth, we now know that significant contributions to the growth of atmospheric CO2 arise from the slowdown of natural sinks and the halt to improvements in the carbon intensity of wealth production

Global Warming: What Should We Do About It?
There is little doubt that the Earth is warming. But there is considerable controversy over global warming's future impact on the world's climate and what (if anything) we should do about it.
Researchers at the University of Alaska reported in 2002 that most Alaskan glaciers are melting at twice the rate of previous estimates. An increasing majority of the world's scientists have concluded that changes in the environment like this one provide convincing evidence of a gradual heating up of the Earth's surface. Scientists refer to this as "global warming."
For over 100 years, scientists have known about the physical mechanism that causes the Earth to warm. Today, they call it the "greenhouse effect." Generally, it works like this:
Radiation from the Sun in short wavelengths easily passes through the Earth's atmosphere and strikes the surface, which reflects much of it back as longer wavelengths.
Instead of going back into space, the longer wavelengths are absorbed by gases in the atmosphere.
The atmosphere reflects back to the Earth's surface a significant amount of the trapped radiation, which becomes heat.
Thus, the Earth warms much like a greenhouse or automobile does when the Sun's rays penetrate the glass, but are trapped inside as heat.
Water vapor and other gases in the atmosphere capture and return to Earth about 50 percent of the Sun's incoming radiation. The warming that results is necessary to prevent our planet from becoming extremely cold and hostile to life. But over the past few centuries, human activities on Earth have increased the concentration of some gases in the atmosphere that intensify heating. These gases include carbon dioxide (CO2), methane, nitrous oxide, and others, the so-called "greenhouse gases."
The Evidence of Climate Change
In 1896, a Swedish chemist, Svante A. Arrhenius, became the first scientist to hypothesize that burning fossil fuels (mainly coal, oil, and natural gas) releases CO2 into the atmosphere, which leads to a warming of the Earth's surface. Later, scientists discovered that the rise of CO2 and other greenhouse gas concentrations seemed to begin with the Industrial Revolution and speeded up in the 20th century.
To be sure, there are a number of ways that the Earth can become warmer naturally. Periods of global warming in the past were caused by changes in the Earth's orbit, volcanic eruptions, and variations in the Sun's radiation output. But natural causes apparently cannot explain the current warming of the Earth.
In 1988, the United Nations established the Intergovernmental Panel on Climate Change (IPCC). The purpose of the IPCC is to review the work of scientists around the world to assess the evidence of climate change.
In 2001, the IPCC issued its third report, assessing the evidence of climate change. The IPCC found that during the 20th century, the Earth warmed by about 1degree Fahrenheit. One degree does not seem like a lot. But scientists know that at various times in Earth's history, shifts of just a few degrees had a dramatic impact on the planet's climate and environment. Here are some other major 20th century climate changes that the IPCC reported:
While some areas of the world experienced worsening droughts, others had greater rainfall and flooding.
Most of the world's glaciers were melting.
The average sea level rose several inches.
Plant and animal habitats are moving.
The IPCC also found that the concentration of CO2 in the atmosphere rose by about 30 percent during the last 200 years, the period of the Industrial Revolution. CO2 is the most important greenhouse gas that traps heat from the Sun.
In addition, the IPCC discovered "new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities." About 75 percent of CO2 emissions come from burning fossil fuels. Americans produce more than their share of these emissions and are responsible for 35 percent of all greenhouse gases ever produced by humans.
Most of the remaining CO2 emissions result from the destruction of forests. Since 1855, humans have destroyed up to 20 percent of the world's rain forests in places like Brazil. Burning forests to clear land for farming, roads, and settlement injects large amounts of CO2 into the atmosphere.
Trees take in CO2 in order to grow. They convert CO2 to oxygen as part of their metabolic process. With fewer trees, less CO2 is converted. The destruction of trees hinders nature's way of removing this greenhouse gas from the atmosphere. Scientists refer to forests, farmlands, and the oceans as "carbon sinks," because they remove CO2 from the atmosphere. The problem is that widespread deforestation is decreasing a major sink.
The Persistent Minority
A persistent minority of the world's scientists disagree with the findings of the IPCC. In his book, Hot Talk, Cold Science: Global Warming's Unfinished Debate, environmental science scholar S. Fred Singer points out that there are many things scientists do not yet know. For example, they are not sure how muchCO2 is absorbed by the ocean carbon sink.
Rather than assuming an environmental disaster will result from global warming, Singer identifies its potential benefits. He foresees more food from longer growing seasons, an increase in timber, more water in some dry regions, and a decrease in the use of fossil fuels for heating as winters become more moderate.
Perhaps the greatest uncertainly identified by scientists like Singer involves clouds. As the Earth warms, these scientists predict, ocean evaporation will increase, causing more high cirrus clouds to form. A greater global cloud cover will reflect more of the Sun's radiation back into space and actually cool the planet.
What If We Do Nothing?
Humans have changed the environment since prehistoric times. But environmental disasters in the past were limited to local regions. Today, human activities on a worldwide scale appear to be changing the global climate. Moreover, most scientists now agree that, on balance, global warming is likely to have a negative impact on the planet.
What is likely to happen over the next 100 years if we do nothing about global warming? The IPCC's third assessment report includes the best available projections of likely impacts on the world's environment.
According to the IPCC's report,CO2 concentrations in the atmosphere will double by 2100. This will cause an average increase in the global surface temperature between 3.5F and 10F. The rate of temperature increase during the century will very likely be greater than at any time in the last 10,000 years.
Increasing temperatures will mean more droughts in many areas of the world, including parts of the United States such as the Southwest. In these areas, crop yields will decline and more forest fires will occur. The decreased food supply in poor countries experiencing drought will often lead to famine.
Insects will thrive in a warming world. Many insect-borne diseases like malaria will expand into new regions where the people have little natural resistance.
While some parts of the world will suffer from heat and dryness, other regions will experience increased rainfall along with floods, landslides, and soil erosion. Violent storms will threaten human life, health, and property, driving up insurance rates.
Throughout the 21st century, glacier and icecap melting will accelerate in the Northern Hemisphere. It is possible that the entire Greenland ice cap could melt away, adding to the projected three-foot rise in sea level by 2100.
The rising seas will cause major flooding and loss of land in the coastal regions in the world, affecting tens of millions of people. Low-lying small Pacific islands will likely disappear beneath the waves. A side effect of the warming seas may be the shifting of ocean currents, which could have a major influence on weather over landmasses and commercial fishing.
Ecosystems unable to cope with the climate changes will be at risk. Up to 50 percent of the world's wetlands may be lost before the end of the century. While some animal, bird, and fish species will successfully expand their ranges, those unable to adapt will become extinct. The good news for humans is that even through the worst of the global warming Homo sapiens will survive.
A recent study by a large insurance company estimated the economic impact of global warming if CO2 concentrations double in the 21st century. The study concluded that weather damage, crop losses, and other expenses will cost the world $300 billion per year.
Poor countries in Africa, Asia, and Latin America, which have historically introduced the least amount of greenhouse gases into the atmosphere, will suffer the greatest economic hardships. But when the Earth's surface temperature increases more than a few degrees, even industrialized countries like the United States will experience economic hardships.
There will be some positive benefits from global warming, such as longer crop growing periods. But these benefits will probably not be enough to overcome significant damage to the environment.
What Should We Do About Global Warming?
In 1992, the United States and the other industrialized nations agreed to reduce their greenhouse gas emissions to 1990 levels. This agreement, however, was not legally binding.
In 1997, more than 160 nations met at Kyoto, Japan, to work out a treaty requiring reductions of greenhouse gas emissions. A proposed exemption of all economically developing countries from any mandatory limits on their emissions proved to be a major obstacle. These countries argued that such limits would severely weaken their economic development.
Despite opposition from the United States and other industrialized nations, the developing countries exemption was included in the final treaty. The industrialized countries agreed to reduce their greenhouse gas emissions up to 8 percent below 1990 levels by 2015. This should stabilize the greenhouse effect and begin to slow damage to the global environment.
The Kyoto Treaty included no specific methods that nations had to use to reduce their emissions. Nations would probably have to consider options such as limiting deforestation, requiring more fuel-efficient automobiles, or imposing a "carbon tax" on gasoline and other fossil fuels to discourage usage. Relying more on solar, wind, and nuclear power would also reduce greenhouse gas emissions.
President Bill Clinton signed the Kyoto Treaty, but the U.S. Senate refused to ratify it because of the developing countries' exemption and possible threats to the American economy. In 2001, President George W. Bush withdrew the United States from the Kyoto Treaty. He argued that its percentage requirements for greenhouse gas reductions would cost Americans millions of jobs. A few months later, 180 nations met without the United States to implement the treaty.
In 2002, President Bush came up with his own plan for reducing U.S. greenhouse gas emissions. He proposed a mix of alternative fuel research and tax credits to encourage companies to reduce their emissions voluntarily over a 10-year period. This approach, Bush said, would cut greenhouse gas emissions to levels comparable to those required by the Kyoto Treaty without damaging the American economy.
Critics of President Bush's plan faulted his heavy reliance on voluntary action by companies and claimed that U.S. emissions would grow substantially. Sen. John Kerry (D-Mass.) argued that Congress should set higher fuel-efficiency standards for cars and SUVs. President Bush opposed this because it may force manufacturers to make these vehicles smaller and more expensive.
* * * * *
Global warming is real. The debate centers on what to do about it. The dilemma is how to reduce greenhouse gas emissions without damaging the world economy.

WHAT IS THIS GLOBAL WARMING????

ABOUT GLOBAL WARMING!!!!!

Global mean surface temperature anomaly 1850 to 2006 relative to 1961–1990

Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980
Global warming refers to the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.
The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last 100 years. 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"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. However, a few individual scientists disagree with some of the main conclusions of the IPCC.[4]
Climate models referenced by the IPCC project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing 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 greenhouse gas levels are stabilized.[1] This reflects the large heat capacity of the oceans.
An increase in global temperatures is expected to cause other changes, including sea level rise, increased intensity of extreme weather events,[5] and changes in the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, glacier retreat, species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.

Causes

Carbon dioxide during the last 400,000 years and (inset above) 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.
Main articles: Attribution of recent climate change and scientific opinion on climate change
Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[8][9][10] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[11] identifies elevated 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. In contrast to the scientific consensus that recent warming is mainly attributable to elevated levels of greenhouse gases, other hypotheses have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of increased solar radiation associated with greater numbers of sunspots.[12]
None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[13]

Greenhouse gases in the atmosphere
Main article: Greenhouse effect

Recent increases in atmospheric carbon dioxide (CO2). 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.

Per capita GHG emissions in 2000, including land-use change

Per capita responsibility for current anthropogenic atmospheric CO2
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 and emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface.
Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 30 °C (54 °F), without which Earth would be uninhabitable.[14] The debate centers on how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.
On Earth, the major 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%.[15][16] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and methane have increased by 31% and 149% respectively above pre-industrial levels since 1750. These levels are 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.[17] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[18]
The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[19] 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, and 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 ppm by the year 2100.[20] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[21]
Positive (reinforcing) feedback effects such as the expected release of methane 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[22] not included in climate models cited by the IPCC.[1]

Feedbacks
Main article: Effects of global warming#Further global warming (positive feedback)
The effects of forcing agents on the climate are complicated by various feedback processes.
One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. (Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.)[23] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.
Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect. Seen from above, the same clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models (about 125 to 500 km for models used in the IPCC Fourth Assessment Report). Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[23]
Another important feedback process is ice-albedo feedback.[24] When global temperatures increase, ice near the poles melts at an increasing rate. 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.
Positive feedback due to release of CO2 and CH4 from thawing permafrost is an additional mechanism contributing to warming. Possible positive feedback due to CH4 release from melting seabed ices is a further mechanism to be considered.
The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favour of smaller phytoplankton that are poorer biological pumps of carbon.[25]

Solar variation
Solar variation over the last 30 years.
Main article: Solar variation
Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming.[26] A difference between this mechanism and greenhouse warming is that an increase in solar activity should warm the stratosphere while greenhouse warming should cool the stratosphere. Cooling in the lower stratosphere has been observed since at least 1960,[27] which would not be expected if solar activity were the main contributor to recent warming. (Reduction of stratospheric ozone also has a cooling influence but substantial ozone depletion did not occur until the late 1970s.) Phenomena such as solar variation combined with volcanoes have probably had a warming effect from pre-industrial times to 1950, but a cooling effect since 1950.[1]
A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University have estimated that the Sun may have contributed about 40–50% of the global surface warming over the period 1900–2000, and about 25–35% between 1980 and 2000.[28] Stott and coauthors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[29] Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.
In 2006, a team of scientists from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too minute to contribute significantly to global warming.[30][31] A 2007 paper by Lockwood and Fröhlich further confirms the lack of a correlation between solar output and global warming for the time since 1985.[32]

Temperature changes

Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Main article: Temperature record

Recent
Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[33] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.
Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land [1]. Since the northern hemisphere has more land mass than the southern it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the northern than southern hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.
Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[34] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[35][36]
Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[37] though the cooling may also be due in part to natural variability.
Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[38] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[39]

Pre-human climate variations

Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Further information: Paleoclimatology
See also: Snowball Earth
Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[40]
A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[41][42]
Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models

The projected temperature increase for a range of stabilization scenarios (the coloured bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4, 2007.

Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.

The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
Main article: Global climate model
Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with some simplifications being necessary because of limitations in computer power. These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[43] However, even when the same assumptions of future GHG levels are used, there still remains a considerable range of climate sensitivity.
Including uncertainties in future greenhouse gas concentrations and climate modelling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human derived causes.
Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[44] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.
Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[45][46][47]
The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[48] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects
Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Though it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause other changes, including glacial retreat and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, reduced summer streamflows, species extinctions and increases in the range of disease vectors.
Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[49] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[50]
Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[51] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[49] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]
Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[52] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[53] Two populations of Bay checkerspot butterfly are being threatened by changes in precipitation, though few mechanistic studies have documented extinctions due to recent climate change.[54]