Climate Change: Vital Signs of the Planet: Causes Vital Signs of the Planet Questions (FAQ) A blanket around the Earth A layer of greenhouse gases – primarily water vapor, and including much smaller amounts of carbon dioxide, methane and nitrous oxide – acts as a thermal blanket for the Earth, absorbing heat and warming the surface to a life-supporting average of 59 degrees Fahrenheit (15 degrees Celsius). Most climate scientists agree the main cause of the current global warming trend is human expansion of the "greenhouse effect" 1 — warming that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere block heat from escaping. Long-lived gases that remain semi-permanently in the atmosphere and do not respond physically or chemically to changes in temperature are described as "forcing" climate change. Gases, such as water vapor, 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 more than 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 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 has 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 Fifth 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 95 percent probability that human activities over the past 50 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 400 parts per million in the last 150 years. The panel also concluded there's a better than 95 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. The panel's full Summary for Policymakers report is online at . 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 gases 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. References NASA - Methane's Impacts on Climate Change May Be Twice Previous Estimates Methane's Impacts on Climate Change May Be Twice Previous Estimates 07.18.05 Even on a cold winter day, standing inside a greenhouse can be downright balmy if the sun is shining outside. The glass lets the sun's warming rays in, but doesn't let as much of that warmth escape outdoors.

Our Earth is much like that greenhouse, where a mixture of gases in our atmosphere acts together like a pane of glass, letting the sun's rays in, and without letting as much warmth escape out to space. Image to right -- Rice Paddies in China: This image shows Chinese farmers transplanting rice in paddy fields in Yunnan Province, China, July 1999. Fossil fuels, cattle, landfills and rice paddies are the main human-related sources. Previous studies have shown that new rice harvesting techniques can significantly reduce methane emissions and increase yields. Click on image to enlarge Credit: Changsheng Li Singling out how much each greenhouse gas (GHG) contributes overall to climate warming can be a tricky task. When it comes to measuring the impacts of greenhouse gases on our climate, scientists typically look at how much of each gas exists in the atmosphere. However, Drew Shindell, a climatologist at NASA's Goddard Institute for Space Studies, New York, NY, believes we need to look at the GHGs when they are emitted at Earth's surface, instead of looking at the GHGs themselves after they have been mixed into the atmosphere.

"The gas molecules undergo chemical changes and once they do, looking at them after they've mixed and changed in the atmosphere doesn't give an accurate picture of their effect," Shindell said. "For example, the amount of methane in the atmosphere is affected by pollutants that change methane's chemistry, and it doesn't reflect the effects of methane on other greenhouse gases," said Shindell, "so it's not directly related to emissions, which are what we set policies for." Once GHGs like methane and the molecules that create ozone are released into the air, these gases mix and react together, which transforms their compositions. When gases are altered, their contribution to the greenhouse warming effect also shifts. So, the true effect of a single GHG emission on climate becomes very hard to single out. Image to left -- Inland Wetlands: Sources of methane include natural sources like wetlands, gas hydrates in the ocean floor, permafrost, termites, oceans, freshwater bodies, and non-wetland soils. Click on image to enlarge. Credit: U.S. EPA, Leo Kenney, Region 1 The leading greenhouse gases include carbon dioxide, methane, nitrous oxide, and halocarbons.

These gases are called ‘well mixed’ greenhouse gases because of their long lifetimes of a decade or more, which allows them to disperse evenly around the atmosphere. They are emitted from both man-made and natural sources. Ozone in the lower atmosphere, called tropospheric ozone or smog, also has greenhouse warming effects. In the upper atmosphere, ozone protects life on Earth from the sun’s harmful ultraviolet rays. Some of the major investigations into the state of our warming planet come from a series of reports from the Intergovernmental Panel on Climate Change (IPCC) Assessment. These reports involved the work of hundreds of climate experts. The reports rely on measurements of greenhouse gases as they exist in the atmosphere, after they may have mixed with other gases. Shindell finds there are advantages to measuring emissions of greenhouse gases and isolating their impacts, as opposed to analyzing them after they have mixed in the atmosphere. His study on the subject was just published in the journal Geophysical Research Letters. According to new calculations, methane's effect on warming the world's climate may be double what is currently thought.

The new interpretations reveal methane emissions may account for a whopping third of the climate warming from well-mixed greenhouse gases between the 1750s and today. The IPCC report states that methane increases in our atmosphere account for only about one sixth of the total effect of well-mixed greenhouse gases on warming. Part of the reason the new calculations give a larger effect is that they include the effect methane has on air pollution. A major component of air pollution is near-surface-level or tropospheric ozone, which is not directly emitted, but is instead formed chemically from methane other hydrocarbons, carbon monoxide and nitrogen oxides. The IPCC report includes the effects of tropospheric ozone increases on climate, but it is not attributed to particular sources. By categorizing the climate effects according to emissions, Shindell and colleagues found the total effects of methane emissions are substantially larger. In other words, the true source of some of the warming that is normally attributed to smog is really due to methane that leads to increased smog. "If we control methane, which is viable, then we are likely to soften global warming more than one would have thought, so that's a very positive outcome," Shindell said.

Sources of methane include natural sources like wetlands, gas hydrates in the ocean floor, permafrost, termites, oceans, freshwater bodies, and non-wetland soils. Fossil fuels, cattle, landfills and rice paddies are the main human-related sources. Greenhouse Effect: Background Material The Greenhouse Effect This section provides an overview of the earth's atmospheric "greenhouse effect" by briefly exploring the atmospheres of nearby planets and discussing our atmosphere's greenhouse gases. The general concepts found in this section include the following: The earth's "greenhouse effect" is what makes this planet suitable for life as we know it. The earth's atmosphere contains trace gases, some of which absorb heat. These gases (water vapor, carbon dioxide, methane, ozone, and nitrous oxide) are referred to as "greenhouse gases." Albedo has an important influence on the earth's temperature. Greenhouses are structures designed to retain heat. The heat-trapping ability of a greenhouse is influenced by a number of factors including the transparency of the greenhouse cover, color of the surfaces inside the greenhouse, and type of surfaces inside. This section includes two classroom activities.

Introduction The Goldilocks Principle can be summed up neatly as "Venus is too hot, Mars is too cold, and Earth is just right." The fact that Earth has an average surface temperature comfortably between the boiling point and freezing point of water, and thus is suitable for our sort of life, cannot be explained by simply suggesting that our planet orbits at just the right distance from the sun to absorb just the right amount of solar radiation. Our moderate temperatures are also the result of having just the right kind of atmosphere. A Venus-type atmosphere would produce hellish, Venus-like conditions on our planet; a Mars atmosphere would leave us shivering in a Martian-type deep freeze. Instead, parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a pleasant range. The Martian blanket is too thin, and the Venusian blanket is way too thick! The 'blanket' here is a collection of atmospheric gases called 'greenhouse gases' based on the idea that the gases also 'trap' heat like the glass walls of a greenhouse do. These gases, mainly water vapor ( ), carbon dioxide ( ), methane ( ), and nitrous oxide ( ), all act as effective global insulators.

To understand why, it's important to understand a few basic facts about solar radiation and the structure of atmospheric gases. Solar Radiation The sun radiates vast quantities of energy into space, across a wide spectrum of wavelengths. Most of the radiant energy from the sun is concentrated in the visible and near-visible parts of the spectrum. The narrow band of visible light, between 400 and 700 nm, represents 43% of the total radiant energy emitted. Wavelengths shorter than the visible account for 7 to 8% of the total, but are extremely important because of their high energy per photon. The shorter the wavelength of light, the more energy it contains. Thus, ultraviolet light is very energetic (capable of breaking apart stable biological molecules and causing sunburn and skin cancers). The remaining 49 - 50% of the radiant energy is spread over the wavelengths longer than those of visible light. These lie in the near infrared range from 700 to 1000 nm; the thermal infrared, between 5 and 20 microns; and the far infrared regions.

Various components of earth's atmosphere absorb ultraviolet and infrared solar radiation before it penetrates to the surface, but the atmosphere is quite transparent to visible light. Absorbed by land, oceans, and vegetation at the surface, the visible light is transformed into heat and re-radiates in the form of invisible infrared radiation. If that was all there was to the story, then during the day earth would heat up, but at night, all the accumulated energy would radiate back into space and the planet's surface temperature would fall far below zero very rapidly. The reason this doesn't happen is that earth's atmosphere contains molecules that absorb the heat and re-radiate the heat in all directions. This reduces the heat radiated out to space. Called 'greenhouse gases' because they serve to hold heat in like the glass walls of a greenhouse, these molecules are responsible for the fact that the earth enjoys temperatures suitable for our active and complex biosphere. Greenhouse Gases Carbon dioxide ( ) is one of the greenhouse gases. It consists of one carbon atom with an oxygen atom bonded to each side. When its atoms are bonded tightly together, the carbon dioxide molecule can absorb infrared radiation and the molecule starts to vibrate.

Eventually, the vibrating molecule will emit the radiation again, and it will likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface from the cold of space. Carbon dioxide, water vapor ( ), methane ( ), nitorus oxide ( ), and a few other gases are greenhouse gases. They all are molecules composed of more than two component atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The major components of the atmosphere ( and ) are two-atom molecules too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect. Greenhouse Effect Atmospheric scientists first used the term 'greenhouse effect' in the early 1800s. At that time, it was used to describe the naturally occurring functions of trace gases in the atmosphere and did not have any negative connotations. It was not until the mid-1950s that the term greenhouse effect was coupled with concern over climate change. And in recent decades, we often hear about the greenhouse effect in somewhat negative terms. The negative concerns are related to the possible impacts of an enhanced greenhouse effect.

This is covered in more detail in the Global Climate Change section of this Web site. It is important to remember that without the greenhouse effect, life on earth as we know it would not be possible. While the earth's temperature is dependent upon the greenhouse-like action of the atmosphere, the amount of heating and cooling are strongly influenced by several factors just as greenhouses are affected by various factors. In the atmospheric greenhouse effect, the type of surface that sunlight first encounters is the most important factor. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate radiation differently. Sunlight falling on a white glacier surface strongly reflects back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark desert soil is strongly absorbed, on the other hand, and contributes to significant heating of the surface and lower atmosphere. Cloud cover also affects greenhouse warming by both reducing the amount of solar radiation reaching the earth's surface and by reducing the amount of radiation energy emitted into space. Scientists use the term albedo to define the percentage of solar energy reflected back by a surface. Understanding local, regional, and global albedo effects is critical to predicting global climate change.

Concluding Thoughts The ability of certain trace gases to be relatively transparent to incoming visible light from the sun, yet opaque to the energy radiated from the earth is one of the best understood processes in the atmospheric sciences. This phenomenon, the greenhouse effect, is what makes the earth habitable for life. For students to truly understand the nature and importance of the greenhouse effect, they should understand the answers to these questions: What is a greenhouse and how does it work? How is the earth's atmosphere similar to a greenhouse? What factors influence the function of a greenhouse? What is albedo and how is it related to understanding global climate change? What are the limitations in comparing the earth's atmosphere to a greenhouse? Activities Atmosphere A planet's climate is decided by its mass, its distance from the sun and the composition of its atmosphere. Earth's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%. Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space.

These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effect of a greenhouse. This blanketing is known as the natural greenhouse effect. Without the greenhouse gases, Earth's average temperature would be roughly -20°C. T The atmosphere is a cloud of gas and suspended solids extending from the Earth's surface out many thousands of miles, becoming increasingly thinner with distance but always held by the Earth's gravitational pull. The atmosphere is made up of layers. it surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble we live in. This protective bubble consists of several gases (listed in the table to the right) with the top four making up 99.998% of all gases. Of the dry composition of the atmosphere nitrogen, by far, is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins. Oxygen is used by all living things and is essential for respiration.

It is also necessary for combustion or burning. Argon is used in light bulbs, in double-pane windows, and to preserve the original Declaration of Independence and the Constitution. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket that prevents the escape of heat into outer space. These percentages of atmospheric gases are for a completely dry atmosphere. The atmosphere is rarely, if ever, dry. Water vapor (water in a 'gas' state) is nearly always present up to about 4% of the total volume. In the Earth's desert regions (30°N/S) when dry winds are blowing, the water vapor contribution to the composition of the atmosphere will be near zero. Water vapor contribution climbs to near 3% on extremely hot/humid days. The upper limit, approaching 4%, is found in tropical climates. The table below shows the changes in atmospheric composition with the inclusion of different amounts of water vapor. The earth's atmosphere is a very thin layer wrapped around a very large planet. Two gases make up the bulk of the earth's atmosphere: nitrogen ( ), which comprises 78% of the atmosphere, and oxygen ( ), which accounts for 21%. Various trace gases make up the remainder.

Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere. Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. Cambridge University Graphic The atmosphere is divided into five layers. It is thickest near the surface and thins out with height until it eventually merges with space. 1) The troposphere is the first layer above the surface and contains 70 to 80 per cent of the Earth's atmosphere. Weather occurs in this layer. 2) Many jet aircrafts fly in the stratosphere because it is very stable. Also, the ozone layer absorbs harmful rays from the Sun. 3) Meteors or rock fragments burn up in the mesosphere. 4) The thermosphere is a layer with auroras. It is also where the space shuttle orbits. 5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our atmosphere. National Geographic Video U.S.

Weather Service Graphic The inhabitants of our planet live in the Troposphere. Earth's atmosphere varies in density and composition as the altitude increases above the surface. The lowest part of the atmosphere is called the troposphere (and it extends from the surface up to about 10 km (6 miles). The gases in this region are predominantly molecular Oxygen ( O2 ) and molecular Nitrogen ( N2 ). All weather is confined to this lower region and it contains 70 to 80 per cent of the Earth's atmosphere and 99% of the water vapor. The highest mountains are still within the troposphere and all of our normal day-to-day activities occur here. The high altitude jet stream is found near the tropopause at the the upper end of this region. The layer above this is the Stratosphere, this is where the Ozone Layer is formed. The atmosphere above 10 km is called the stratosphere. The gas is still dense enough that hot air balloons can ascend to altitudes of 15 - 20 km and Helium balloons to nearly 35 km, but the air thins rapidly and the gas composition changes slightly as the altitude increases.

Within the stratosphere, incoming solar radiation at wavelengths below 240 nm. is able to break up (or dissociate) molecular Oxygen ( O2 ) into individual Oxygen atoms, each of which, in turn, may combine with an Oxygen molecule ( O2 ), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms ( O3 ). This gas reaches a peak density of a few parts per million at an altitude of about 25 km (16 miles). The Ozone Layer absorbs ultra-violet radiation from the Sun. Without the Ozone Layer life as we know would cease to exist on our planet. Ozone is important because it is the only atmospheric gas which absorbs light in the B region of UVB rays. The Ozone layer extends from a height of 20 kilometers to 60 kilometers above the Earth's surface. The air is very thin at these altitudes. If all of the Ozone in the Earth's atmosphere were compressed into a single layer at the Earth's surface, it would only be 3 millimeters thick-basically two stacked pennies! The gas becomes increasingly rarefied at higher altitudes.

At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The existence of charged particles at this altitude and above, signals the beginning of the ionosphere a region having the properties of a gas and of a plasma. What are greenhouse gases and how do they affect the climate? - FAQ - U.S. Energy Information Administration (EIA) Frequently Asked Questions What are greenhouse gases and how do they affect the climate? The major greenhouse gases that the United States emitted as a result of human activity and their share of total U.S. greenhouse gas emissions in 2014:1 Carbon dioxide (CO2)—80.9% Nitrous oxide (N2O)—5.9% Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen triflouride (NF3)—2.6% There are other greenhouse gases that are not counted in United States or international greenhouse gas inventories: Water vapor is the most abundant greenhouse gas.

Most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere. Therefore, the U.S. Energy Information Administration (EIA) does not estimate emissions of water vapor. Research conducted by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations in the atmosphere. Ozone is technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, ozone is needed to block harmful ultraviolet light. At lower elevations of the atmosphere (troposphere), ozone is harmful to human health and is a pollutant regulated independently of its warming effects. Greenhouse gases are transparent to incoming (short-wave) radiation from the sun but block infrared (long-wave) radiation from leaving the earth's atmosphere. This greenhouse effect traps radiation from the sun and warms the planet's surface. As concentrations of these gases increase, more warming occurs than would happen naturally. 1 Based on global warming potential . Source: U.S. Environmental Protection Agency (EPA), U.S.

Greenhouse Gas Inventory Report: 1990-2014 , Executive Summary (April 2016) Learn more: NASA GISS: Research Features: Ozone and Climate Change By Jeannie Allen, NASA Earth Observatory , February 2004 "Ozone chemistry is at the heart of atmospheric chemistry." — Bill Stockwell, Desert Research Institute Ozone affects climate, and climate affects ozone. Temperature, humidity, winds, and the presence of other chemicals in the atmosphere influence ozone formation, and the presence of ozone, in turn, affects those atmospheric constituents. Interactions between ozone and climate have been subjects of discussion ever since the early 1970s when scientists first suggested that human-produced chemicals could destroy our ozone shield in the upper atmosphere. The discussion intensified in 1985 when atmospheric scientists discovered an ozone "hole" in the upper atmosphere (stratosphere) over Antarctica. Today, some scientists are predicting the stratospheric ozone layer will recover to 1980 ozone levels by the year 2050.

These scientists say we can expect recovery by that time because most nations have been abiding by international agreements to phase out production of ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and halons. But the atmosphere continues to surprise us, and some atmospheric scientists recently demonstrated a new spin on the ozone recovery story that may change its ending. Well before the expected stratospheric ozone layer recovery date of 2050, ozone's effects on climate may become the main driver of ozone loss in the stratosphere. As a result, ozone recovery may not be complete until 2060 or 2070. Ozone's impact on climate consists primarily of changes in temperature. The more ozone in a given parcel of air, the more heat it retains. Ozone generates heat in the stratosphere, both by absorbing the sun's ultraviolet radiation and by absorbing upwelling infrared radiation from the lower atmosphere (troposphere). Consequently, decreased ozone in the stratosphere results in lower temperatures.

Observations show that over recent decades, the mid to upper stratosphere (from 30 to 50 km above the Earth's surface) has cooled by 1° to 6° C (2° to 11° F). This stratospheric cooling has taken place at the same time that greenhouse gas amounts in the lower atmosphere (troposphere) have risen. The two phenomena may be linked. Says Dr. Drew Shindell of the NASA Goddard Institute for Space Studies (GISS), "I've long been aware that chemistry and climate influence one another strongly. I started to ask how cold the stratosphere might get because of increasing amounts of greenhouse gases. I was wondering whether or not the cooling in the stratosphere would be rapid enough that more ozone depletion would take place than we had previously calculated. Would the cooling be so fast that even more ozone depletion would occur before the impact of international agreements to limit ozone had time to take effect?" This would create a possible feedback loop. The more ozone destruction in the stratosphere, the colder it would get just because there was less ozone. And the colder it would get, the more ozone depletion would occur.

A midnight sun shines across the frigid Antarctic landscape, and makes dark triangles of researchers' tents. Low temperatures in the stratosphere above both the Antarctic and the Arctic contribute to ozone depletion. Since ozone depletion itself contributes to decreasing temperatures, a feedback loop may be in operation. (Photograph courtesy Alberto Behar, NASA JPL/National Science Foundation) Thin clouds made of mixtures of ice, nitric acid, and sulphuric acid form in the upper atmosphere (stratosphere) over the poles when temperatures drop below -78°C (-109°F). Ozone depletion occurs in such polar stratospheric clouds. This particular cloud formation appeared over Iceland at an altitude of about 22 km on February 4, 2003. Its beautiful colors result from the very small size of the ice crystals it contains. These tiny particles bend and scatter the sun's rays much like a prism. (Photograph courtesy Mark R. Schoeberl, NASA GSFC) The deepest ozone losses over both the Arctic and the Antarctic result from special conditions that occur in the winter and early spring. As winter arrives, a vortex of winds develops around the pole and isolates the polar stratosphere.

When temperatures drop below -78°C (-109°F), thin clouds form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the surfaces of ice crystals in the clouds release active forms of CFCs. Ozone depletion begins, and the ozone "hole" appears. In spring, temperatures begin to rise, the ice evaporates, and the ozone layer starts to recover. Reflections on a Possible Delay The concept that stratospheric cooling due to ozone loss may lead to a delay in recovery of the ozone layer has fallen on fertile ground. Scientists running different kinds of global models are finding similar results. "That gives us confidence," says Dr. Venkatachalam Ramaswamy, at NOAA's Geophysical Fluid Dynamic Laboratory. "We're confident in our assessment, because the models can help us to understand the observed ozone and temperature changes on a global scale." Stratospheric cooling may have been taking place over recent decades for a number of reasons. One reason may be that the presence of ozone itself generates heat, and ozone depletion cools the stratosphere.

Another contributing factor to the cooling may be that rising amounts of greenhouse gases in the lower atmosphere (troposphere) are retaining heat that would normally warm the stratosphere. However, scientists hold varying degrees of conviction about the nature of the link between tropospheric warming and stratospheric cooling. "The warming of the troposphere and its potential influence upon the stratospheric circulation is an important consideration," points out Ramaswamy, "though the quantitative linkages are uncertain. It is possible that they may be interdependent only in a tenuous manner." The graph above shows total ozone and stratospheric temperatures over the Arctic since 1979. Changes in ozone amounts are closely linked to temperature, with colder temperatures resulting in more polar stratospheric clouds and lower ozone levels. Atmospheric motions drive the year-to-year temperature changes. The Arctic stratosphere cooled slightly since 1979, but scientists are currently unsure of the cause. Future NASA missions, starting with the Aura satellite, will improve our understanding of the links between global climate change and ozone chemistry. (Graph based on data provided by Paul Newman, NASA GSFC) "The problem is that we haven't had adequate data," Ramaswamy continues.

"Observations have been primarily limited to only a very few locations in the stratosphere. We have only 20 years of full global coverage from satellites. Of course radiosonde goes back 40 years but that is not global coverage." GISS's Jim Hansen agrees with Ramaswamy on the need for data. "Climate forcing by ozone is uncertain because ozone change as a function of altitude is not well measured. Especially at the tropopause (where the troposphere meets the stratosphere), we don't know enough. The climate system is highly sensitive, especially to changes in the tropopause region. We need exact temperatures and ozone profiles at different altitudes and around the globe." Hansen and others look forward to the launch of NASA's Aura satellite in 2004. A vital part of NASA's Earth Observing System, Aura will observe the composition, chemistry and dynamics of the Earth's upper and lower atmosphere, including temperatures and ozone amounts. "What Aura will give us is quite exciting. There will be a suite of instruments measuring in regions not well measured before," says Hansen.

NASA's Aura satellite, to launch in 2004, will observe the composition, chemistry and dynamics of the Earth's upper and lower atmosphere, including temperatures and ozone amounts. "What Aura will give us is quite exciting. There will be a suite of instruments measuring in regions not well measured before," says Jim Hansen, at the NASA Goddard Institute for Space Studies (GISS). (Image by Jesse Allen) In spite of large uncertainties that remain, scientists express a sense of accomplishment with their achievements so far. "I think one of the successes has to be the fact that we can now explain the observed temperature trends in the stratosphere reasonably well, states Ramaswamy. "There is actually a very strong indication that the observed changes in radiative and chemical species are responsible for globe-wide cooling of the stratosphere." The Variable Arctic Although many global scale models agree with each other and with observations on the future of ozone recovery, most regional scale models do not agree. Atmospheric models show that the cooling influence of ozone depletion accounts very well for observed cooling winter-time temperature trends in the Antarctic, but not in the Arctic. Differences among regions make predictions about complex atmospheric chemistry problematic.

The Arctic and Antarctic regions, where low stratospheric ozone amounts are of great concern, differ in significant ways. The complex topography of the high latitude Northern Hemisphere, with its distribution of land masses and oceans, makes the Arctic atmosphere more dynamic and variable. The Antarctic is colder than the Arctic. Antarctic winds form a relatively stable vortex for long periods of time, and the vortex allows temperatures of the air trapped within it to get extremely low. Shindell explains, "In the south, air masses just sit over the pole and get colder." Such stability makes the Antarctic somewhat more predictable than the Arctic. Shindell says, "It's so variable in the Arctic that we have to have better data to figure out what we should believe and what we can have confidence in for the future." These coastal mountains in southeast Alaska are representative of the rugged terrain of the Northern Hemisphere's high latitudes. High mountains and the contrast between large continental landmasses and open ocean in the Northern Hemisphere disturb the air over the Arctic, preventing the formation of a stable circulation pattern. In part, it is the lack of a stable "polar vortex" that prevents the Arctic from experiencing the extremely cold temperatures and dramatic ozone loss seen above Antarctica.

In spite of this, large ozone losses occurred in the Arctic during the last several years. (Photograph courtesy NOAA Photo Library ) Although dramatic ozone depletion did not occur in the Arctic in the 1980s when it occurred in the Antarctic, times are changing. Very large ozone losses have occurred in the Arctic recently, especially in the late 1990s. Ozone chemistry is very sensitive to temperature changes. Since temperatures in the Arctic stratosphere often come within a few degrees of the threshold for forming polar stratospheric clouds, further cooling of the stratosphere could cause these clouds to form more frequently and increase the severity of ozone losses. The Arctic may be changing in another way that differs from the Antarctic. With stratospheric cooling, the differences in temperature between the stratosphere and the troposphere are increasing. Differences in temperature creates winds, so stratospheric wind speeds have been increasing. (The Antarctic isn't affected by increasing greenhouse gases like the Arctic is because it's colder, and the polar wind circulation over the Antarctic is already very strong.) Shindell says that from both observations and models, he has found increasing wind speeds not only at high altitudes but also near the surface.

"That's a large effect on climate," he points out. "Changes in stratospheric ozone and winds affect the flow of energy at altitudes just below, which then affect the next lower altitudes, and so on all the way to the ground. That would be the most intriguing aspect of all this, though it's still controversial." Ozone and Climate at the Surface Interactions between ozone and climate naturally occur not only in the stratosphere, but also at the Earth's surface (troposphere). There are known chemical and physical aspects of ozone formation we can watch carefully as climate changes. Ozone forms in the troposphere by the action of sunlight on certain chemicals (photochemistry). Chemicals participating in ozone formation include two groups of compounds: nitrogen oxides (NOx) and volatile organic compounds (VOCs). In general, an increase in temperature accelerates photochemical reaction rates. Scientists find a strong correlation between higher ozone levels and warmer days. With higher temperatures, we can expect a larger number of "bad ozone" days, when exercising regularly outdoors harms the lungs. However, ozone levels do not always increase with increases in temperature, such as when the ratio of VOCs to NOx is low.

As the troposphere warms on a global scale, we can expect changes in ozone air quality. Generally speaking, warming temperatures will modify some but not all of the complex chemical reactions involved in ozone production in the troposphere (such as those involving methane). Because of the short-lived nature of these chemical constituents and variations across space and time, the uncertainty is too large to make predictions. Scientists can only speculate about specific kinds of change, about the direction of change in a particular location, or about the magnitude of change in ozone amounts that they can attribute to climate. Some speculation involves VOC emissions from natural biological processes. Certain kinds of plants such as oak, citrus, cottonwood, and almost all fast-growing agriforest species emit significant quantities of VOCs. Higher temperatures of a warming climate encourage more plant growth, and therefore higher levels of VOCs in areas where VOC-emitting plants grow abundantly. Soil microbes also produce NOx. Soil microbial activity may also increase with warmer temperatures, leading to an increase in NOx emissions and a consequent increase in ozone amounts. A warming climate can lead to more water vapor in the lower atmosphere, which would tend to produce more ozone.

But cloud cover can also diminish chemical reaction rates because of reduced sunlight and therefore lower rates of ozone formation. Monitoring and analyzing such interactions is the best way we can improve our predictive capabilities. (Photograph courtesy Jeannie Allen, NASA GSFC/SSAI) Another impact of climate on ozone pollution in the troposphere arises from the probability that higher temperatures will lead to greater demand for air conditioning and greater demand for electricity in summer. Most of our electric power plants emit NOx. As energy demand and production rises, we can expect amounts of NOx emissions to increase, and consequently levels of ozone pollution to rise as well. Water vapor is also involved in climate change. A warmer atmosphere holds more water vapor, and more water vapor increases the potential for greater ozone formation. But more cloud cover, especially in the morning hours, could diminish reaction rates and thus lower rates of ozone formation. Understanding the interactions between ozone and climate change, and predicting the consequences of change requires enormous computing power, reliable observations, and robust diagnostic abilities. The science community's capabilities have evolved rapidly over the last decades, yet some fundamental mechanisms at work in the atmosphere are still not clear. The success of future research depends on an integrated strategy, with more interactions between scientists' observations and mathematical models.

Related Links (J.E. Hansen, Ed.). NASA Goddard Institute for Space Studies. New York. Network for the Detection of Stratospheric Change (NDSC) 2003. Connections between Climate Change and Ozone Depletion . Ramaswamy, V., et al. 2001. Stratospheric temperature trends: Observations and model simulations. Rev. Geophys. J. Geophys. Res. 107, no. D12, ACL-3, doi:10.1029/2001JD000420. Stockwell, W.R., et al. 1996. Ozone formation, destruction and exposure in Europe and the United States. In Forest Decline and Ozone: A Comparison of Controlled Chamber and Field Experiments (H. Sandermann, A. Wellburn, and R.L. Heath, Eds.), pp. 1-38. Springer-Verlag. New York. Thompson, A.M., et al. 2002. Strategies for Observing and Modeling Pollution. EOS, Trans. Amer. Geophys.

Union, 83, 575, 581.

Question: What contributes to the greenhouse effect at lower atmospheric levels, but in the upper atmosphere protects life on Earth?
Answer: Ozone