Gases That Contribute to the Greenhouse Effect Include

Taken from: http://climate.nasa.gov/causes

Most scientists agree the main cause of the current global warming trend is human expansion of the "greenhouse effect" -- warming that results when the atmosphere traps heat radiating from Earth toward space.

Certain gases in the atmosphere behave like the glass on a greenhouse, allowing sunlight to enter, but blocking heat from escaping. Long-lived gases, remaining semi-permanently in the atmosphere, which do not respond physically or chemically to changes in temperature are described as "forcing" climate change whereas gases, such as water, which respond physically or chemically to changes in temperature are seen as "feedbacks."

Gases that contribute to the greenhouse effect include:

·  Water vapor. The most abundant greenhouse gas, but importantly, it acts as a feedback to the climate. Water vapor increases as the Earth's atmosphere warms, but so does the possibility of clouds and precipitation, making these some of the most important feedback mechanisms to the greenhouse effect.

·  Carbon dioxide (CO2). A minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO2 concentration by a third since the Industrial Revolution began. This is the most important long-lived "forcing" of climate change.

·  Methane. A hydrocarbon gas produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as ruminant digestion and manure management associated with domestic livestock. On a molecule-for-molecule basis, methane is a far more active greenhouse gas than carbon dioxide, but also one which is much less abundant in the atmosphere.

·  Nitrous oxide. A powerful greenhouse gas produced by soil cultivation practices, especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning.

·  Chlorofluorocarbons (CFCs). Synthetic compounds of entirely of industrial origin used in a number of applications, but now largely regulated in production and release to the atmosphere by international agreement for their ability to contribute to destruction of the ozone layer. They are also greenhouse gases .

Not enough greenhouse effect: The planet Mars has a very thin atmosphere, nearly all carbon dioxide. Because of the low atmospheric pressure, and with little to no methane or water vapor to reinforce the weak greenhouse effect, Mars has a largely frozen surface that shows no evidence of life.

Too much greenhouse effect: The atmosphere of Venus, like Mars, is nearly all carbon dioxide. But Venus has about 300 times as much carbon dioxide in its atmosphere as Earth and Mars do, producing a runaway greenhouse effect and a surface temperature hot enough to melt lead.

On Earth, human activities are changing the natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2. To a lesser extent, the clearing of land for agriculture, industry, and other human activities have increased concentrations of greenhouse gases.

The consequences of changing the natural atmospheric greenhouse are difficult to predict, but certain effects seem likely:

·  On average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not.

·  Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer.

·  A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise.

·  Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities.

The role of human activity

In its recently released Fourth Assessment Report, the Intergovernmental Panel on Climate Change, a group of 1,300 independent scientific experts from countries all over the world under the auspices of the United Nations, concluded there's a more than 90 percent probability that human activities over the past 250 years have warmed our planet.

The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 379 parts per million in the last 150 years. The panel also concluded there's a better than 90 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in Earth's temperatures over the past 50 years.

They said the rate of increase in global warming due to these gases is very likely to be unprecedented within the past 10,000 years or more. The panel's full Summary for Policymakers report is online at http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf.

Solar irradiance

It's reasonable to assume that changes in the sun's energy output would cause the climate to change, since the sun is the fundamental source of energy that drives our climate system.

Indeed, studies show that solar variability has played a role in past climate changes. For example, a decrease in solar activity is thought to have triggered the Little Ice Age between approximately 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the 1720s and glaciers advanced in the Alps.

But several lines of evidence show that current global warming cannot be explained by changes in energy from the sun:

·  Since 1750, the average amount of energy coming from the Sun either remained constant or increased slightly.

·  If the warming were caused by a more active sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere. That's because greenhouse gasses are trapping heat in the lower atmosphere.

·  Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.

Taken from: http://climate.nasa.gov/uncertainties

This website presents a data-rich view of climate and a discussion of how that data fits together into the scientists' current picture of our changing climate. But there's a great deal that we don't know about the future of Earth's climate and how climate change will affect humans.
For convenience and clarity, climate scientists separate things that affect climate change into two categories: forcings and feedbacks (see sidebar at right).
Also, climate scientists often discuss "abrupt climate change," which includes the possibility of "tipping points" in the Earth's climate. Climate appears to have several states in which it is relatively stable over long periods of time. But when climate moves between those states, it can do so quickly (geologically speaking), in hundreds of years and even, in a handful of cases, in only a few decades. These rapid 'state changes' are what scientists mean by abrupt climate change. They are much more common at regional scales than at the global scale, but can be global. State changes have triggers, or "tipping points," that are related to feedback processes. In what's probably the single largest uncertainty in climate science, scientists don't have much confidence that they know what those triggers are.
Below is an explanation of just a few other important uncertainties about climate change, organized according to the categories forcing and feedback. This list isn't exhaustive. It is intended to illustrate the kinds of questions that scientists still ask about climate.
Forcings
1.  Solar Irradiance. The sun has a well-known eleven-year irradiance cycle that produces a .08% variation in output.1 Solar irradiance has been measured by satellite daily since the late 1970s, and this known solar cycle is incorporated into climate models. There is some evidence from proxy measurements-sunspot counts going back centuries, measurements from ancient trees, and others-that solar output varies over longer periods of time, too. While there is currently no evidence of a trend in solar output over the past half century, because there are no direct observations of solar output prior to the 1970s, climate scientists do not have much confidence that they understand longer-term solar changes. A number of U.S. and international spacecraft study the sun.
2.  Aerosols, dust, smoke, and soot. These come from both human and natural sources. They also have very different effects on climate. Sulfate aerosols, which result from burning coal, biomass, and volcanic eruptions, tend to cool the Earth. Increasing industrial emissions of sulfates is believed to have caused a cooling trend in the Northern Hemisphere from the 1940s to the 1970s. But other kinds of particles have the opposite effect. The global distribution of aerosols has only been tracked for about a decade from the ground and from satellites, but those measurements cannot yet reliably distinguish between types of particulates. So aerosol forcing is another substantial uncertainty in predictions of future climate.
Feedbacks
3.  Clouds. Clouds have an enormous impact on Earth's climate, reflecting back into space about one third of the total amount of sunlight that hits the Earth's atmosphere. As the atmosphere warms, cloud patterns may change, altering the amount of sunlight absorbed by the Earth. Because clouds are such powerful climate actors, even small changes in average cloud amounts, locations, and type could speed warming, slow it, or even reverse it. Current climate models do not represent cloud physics well, so the Intergovernmental Panel on Climate Change has consistently rated clouds among its highest research priorities. NASA and its research partners in industry, academia, and other nations have a small flotilla of spacecraft and aircraft studying clouds and the closely related phenomenon of aerosols.
4.  Carbon cycle. Currently, natural processes remove about half of each year's human carbon dioxide emissions from the atmosphere, although this varies a bit year to year. It isn't well understood where this carbon dioxide goes, with some evidence that the oceans are the major repository and other evidence that land biota absorbs the majority. There is also some evidence that the ability of the Earth system to continue absorbing it may decline as the world warms, leading to faster accumulation in the atmosphere. But this possibility isn't well understood either. The planned Orbiting Carbon Observatory mission will mark NASA's first attempt to answer some of these questions via space observations.
5.  Ocean circulation. One very popular hypothesis about climate change is that as the Earth as a whole warms, ocean circulation in the Atlantic will change to produce cooling in Western Europe. In its most extreme form, this hypothesis has advancing European ice sheets triggering a new ice age. A global-warming induced ice age is not considered very likely among climate scientists. But the idea highlights the importance of ocean circulation in maintaining regional climates. Global ocean data sets only extend back to the early 1990s, so there are large uncertainties in predictions of future ocean changes.
6.  Precipitation. Human civilization is dependent upon where and when rain and snow fall. We need it for drinking water and for growing our food. Global climate models show that precipitation will generally increase, but not in all regions. Some regions will dry instead. Scientists and policymakers would like to use climate models to assess regional changes, but the models currently show wide variation in their results. For just one example, some models forecast less precipitation in the American southwest, where JPL is, while others foresee more precipitation. This lack of agreement on even the direction of change makes planning very difficult. There's much research to be done on this question.
7.  Sea level rise. In its 2007 Fourth Assessment Report, the Intergovernmental Panel on Climate Change used new satellite data to conclude that shrinkage of ice sheets may contribute more to sea level rise than it had thought as recently as 2001. The panel concluded that it could not "provide a best estimate or an upper bound for sea level rise" over the next century due to their lack of knowledge about Earth's ice. There are 5-6 meters worth of sea level in the Greenland ice sheet, and 6-7 meters in the West Antarctic Ice Sheet, while the much larger East Antarctic Ice Sheet is probably not vulnerable to widespread melting in the next century. Many hundreds of millions of people live within that range of sea level increase, so our inability to predict what sea level rise is likely over the next century has substantial human and economic ramifications. / / / Forcings and Feedbacks
Climate forcings are the initial drivers of a climate shift. Solar irradiance is one example of a forcing. If the sun generates more light, the Earth will warm.
Climate feedbacks are processes that change as a result of a change in forcing, and cause additional climate change. An example of this is the "ice-albedo feedback." As the atmosphere warms, sea ice will melt. Ice is highly reflective, while the underlying ocean surface is far less reflective. The darker ocean will absorb more heat, getting warmer and making the Earth warmer overall. A feedback that increases an initial warming is called a "positive feedback." A feedback that reduces an initial warming is a "negative feedback." The ice-albedo feedback is a very strong positive feedback that has been included in climate models since the 1970s.
/ Resources
For answers to frequently asked questions about global climate change, visit the FAQ section of this website.