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| The original version of this fact sheet, published in 2002 and written by John Weier, is archived as a PDF. Over the last five years, 600 scientists from the Intergovernmental Panel on Climate Change sifted through thousands of studies about global warming published in forums ranging from scientific journals to industry publications and distilled the world’s accumulated knowledge into this conclusion: “Warming of the climate system is unequivocal.” | |||
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| Far from being some future fear, global warming is happening now, and scientists have evidence that humans are to blame. For decades, cars and factories have spewed billions of tons of greenhouse gases into the atmosphere, and these gases caused temperatures to rise between 0.6°C and 0.9°C (1.08°F to 1.62°F) over the past century. The rate of warming in the last 50 years was double the rate observed over the last 100 years. Temperatures are certain to go up further. | The effects of global warming are already being felt worldwide. The Larsen-B Ice Shelf on the Antarctic Peninsula collapsed over 35 days in early 2002, prompted by 3°C of warming since the 1940s. (NASA image by Jesse Allen, based on MODIS data.) | ||
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 |  But why should we worry about a seemingly small increase in temperature? It turns out that the global average temperature is quite stable over long periods of time, and small changes in that temperature correspond to enormous changes in the environment. For example, during the last ice age, when ice sheets a mile thick covered North America all the way down to the northern states, the world was only 9 to 15 degrees Fahrenheit colder than today. Much of modern human civilization owes its existence to the stability in the average global temperature since the end of the last ice age—a stability that allowed human cultures to transition from roaming, hunter-gatherer societies into more permanent, agriculture-supported communities. Even the temperature change of a degree or two that has occurred over the last century is capable of producing significant changes in our environment and way of life. | Cars, factories, and power plants pump billions of tons of carbon dioxide into the atmosphere every year. Since 1750, carbon dioxide levels have increased 35 percent, while temperatures have gone up between 0.6°C and 0.9°C. Scientists have very high confidence that increased concentrations of greenhouse gases are causing the planet to warm. (Photograph ©2002 Travel Geographer. NASA graphs by Robert Simmon, based on carbon dioxide data from Dr. Pieter Tans, NOAA/ESRL and temperature data from NASA Goddard Institute for Space Studies.) | |
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| In the future, it is very likely that rising temperatures will lead to more frequent heat waves, and virtually certain that the seas will rise, which could leave low-lying nations awash in seawater. Warmer temperatures will alter weather patterns, making it likely that there will be more intense droughts and more intense rain events. Moreover, global warming will last thousands of years. To gain an understanding of how global warming might impact humanity, it is necessary to understand what global warming is, how scientists measure it, and how forecasts for the future are made. | One inevitable consequence of global warming is sea-level rise. In the face of higher sea levels and more intense storms, coastal communities face greater risk of rapid beach erosion and damage from destructive storms like the intense nor’easter of April 2007. (Photograph ©2007 metimbers2000.) | ||
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| bal warmth begins with sunlight. When light from the Sun reaches the Earth, roughly 30 percent of it is reflected back into space by clouds, atmospheric particles, reflective ground surfaces, and even ocean surf. The remaining 70 percent of the light is absorbed by the land, air, and oceans, heating our planet’s surface and atmosphere and making life on Earth possible. Solar energy does not stay bound up in Earth’s environment forever. Instead, as the rocks, the air, and the sea warm, they emit thermal radiation, or infrared heat. Much of this thermal radiation travels directly out to space, allowing Earth to cool. | |||
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| Some of this outgoing radiation, however, is re-absorbed by water vapor, carbon dioxide, and other gases in the atmosphere (called greenhouse gases because of their heat-trapping capacity) and is then re-radiated back toward the Earth’s surface. On the whole, this re-absorption process is good. If there were no greenhouse gases or clouds in the atmosphere, the Earth’s average surface temperature would be a very chilly -18°C (0°F) instead of the comfortable 15°C (59°F) that it is today. What has scientists concerned now is that over the past 250 years humans have been artificially raising the concentration of greenhouse gases in the atmosphere at an ever-increasing rate. By 2004, humans were pumping out over 8 billion tons of carbon dioxide per year. Some of it was absorbed by “sinks” like forests or the ocean, and the rest accumulated in the atmosphere. We produce millions of pounds of methane by allowing our trash to decompose in landfills and by breeding large herds of methane-belching cattle. Nitrogen-based fertilizers and other soil management practices lead to the release of nitrous oxide into the atmosphere. Once these greenhouse gases get into the atmosphere, they stay there for decades or longer. According to the Intergovernmental Panel on Climate Change (IPCC), since the industrial revolution began in about 1750, carbon dioxide levels have increased 35 percent and methane levels have increased 148 percent. Paleoclimate readings taken from ice cores and fossil records show that these gases, two of the most abundant greenhouse gases, are at their highest levels in at least the past 650,000 years. Scientists have very high confidence (a phrase the IPCC translates to “greater than 90 percent certainty”) that the increased concentrations of greenhouse gases have made it more difficult for thermal radiation to leave the Earth, and as a result, Earth has warmed. | Land, air, and oceans absorb most of the energy that comes from the Sun (yellow arrows, left), and the rest is reflected back into space. The land and oceans re-emit the energy they absorb (red arrows, right) in the form of heat, which is then absorbed by greenhouse gases in the atmosphere. In turn, greenhouse gases re-emit the heat, sending it back to the surface. This heat-trapping effect keeps the Earth comfortably warm, but the build-up of greenhouse gases intensifies the effect, driving temperatures up. (Image adapted from Kiel and Trenberth, 1997, by Debbi McLean.) | ||
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| Introduction | |||
| Until about 30 years ago, atmospheric scientists believed that all of the ozone in the lower atmosphere (troposphere) intruded from the upper atmosphere (stratosphere), where it formed by the action of sunlight on oxygen molecules. The work of atmospheric chemists during the 1970s dramatically altered that view. Now we understand that more than half of the ozone in the troposphere comes from chemical interactions within the troposphere itself. In the stratosphere, ozone shields us from the Sun’s deadly ultraviolet radiation. But in the troposphere, this same gas impairs lung capacity and reduces agricultural productivity. Both human activities and natural processes generate the chemical compounds that serve as “precursors” to the formation of ozone. Currently human activities generate about as much ozone as natural processes do (Fishman 2002), creating a public health hazard. To solve the ozone pollution problem, governments have established ozone standards, revising them as new knowledge comes to light. The U.S. Environmental Protection Agency (EPA) asserts that accumulated knowledge from 3,000 new studies of the last 15 years proves that breathing ozone in smaller amounts than previously thought causes significant harm to human lungs. (U.S. Environmental Protection Agency, 1997). National standards of many nations may need adjustment. Governments enact legislation to enforce adherence to pollution standards. They also support ozone-monitoring activities in key parts of the world, and fund research to enhance our ability to forecast ozone levels. But in spite of standards, regulations, monitoring and research, the ozone problem persists. We are not only creating more ozone in some populated areas, but our ozone pollution crosses political boundaries and often reaches across the Earth’s widest oceans. To regulate air quality, we need international cooperation and a strong scientific basis on which to make legally binding agreements. Solving the problem also requires a shift in the ways we get and use energy. Everyone has a role to play in regaining healthy levels of ozone in the air we breathe. Red alder, Alnus rubra, shows a typical symptom of overexposure to ozone: discoloration of small groups of cells between the veins, appearing as uniformly sized red to brown or purple spots (stippling). Mature leaves show more stippling than young ones, usually only on the upper side of the leaf. Several common plant species respond to ambient levels of ozone pollution with visible symptoms that are easy to diagnose (biomonitor). To learn more about biomonitoring go to: Ozone Biomonitoring (Photograph courtesy of Pat Temple, U.S. Forest Service). | |||
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| The Shuttle Columbia (STS 107) carried the Shuttle Ozone Limb Sounding Experiment-2 (SOLSE-2) as a demonstration of new technology that will be used on the next generation of meteorological satellites to monitor ozone change. Current instruments flying on NOAA and NASA satellites look directly downward toward the Earth, which limits their ability to accurately measure ozone in the lower layers of the stratosphere, the layer of atmosphere 10 to 50 kilometers (6.2 to 31 miles) in altitude. New technology called “limb viewing” (depicted below) allows observation of the atmosphere from the side rather than straight down. When viewed from the side, the Earth looks like a flat circle, and the atmosphere appears like a halo around it. This halo is known as Earth’s “limb.” From that side view, the layers of the atmosphere appear like layers in a cake, allowing instruments like SOLSE-2 to see the lower layers of the stratosphere. This is important because most of the recently observed ozone change, like the “ozone hole,” occurs in the lower stratosphere. |
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| To understand how SOLSE-2 is able to measure ozone in the stratosphere, remember that sunlight is composed of numerous different wavelengths of electromagnetic energy, and that the individual molecules in the atmosphere can either scatter that incoming light or absorb it. The way each component of the Earth’s atmosphere interacts with sunlight is called the “absorption spectrum.” Like fingerprints reveal the identities of people, the absorption spectrum of the atmosphere can be used to identify the various gases and particles in it. The ozone layer near the tangent point (the point in space at which the instrument’s line of sight appears to touch the edge of the curve of the Earth) absorbs radiation at several wavelengths of light. SOLSE-2 infers the presence of ozone by using a spectrometer to separate the light it detects into individual wavelengths, and then measuring the relative absence of radiation at wavelengths that are strongly absorbed relative to wavelengths that are weakly absorbed. | The atmosphere is a blue haze that thins as it rises above the curve of the Earth’s surface. Called the “limb,” it provides a view of the structure of the atmosphere. Orbiting scientific instruments look at the limb to measure how the concentrations of trace gases vary with altitude. One of these instruments, the Shuttle Ozone Limb Sounding Experiment-2, flew in the cargo bay of the Space Shuttle Columbia on its final flight. (Image STS073-E-5113 courtesy NASA-JSC Gateway to Astronaut Photography of Earth.) | ||
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| The graphs above show preliminary results from the SOLSE-2 experiment. The top panel illustrates the structure of the light—all the different wavelengths—detected by SOLSE at an altitude of 25 kilometers (15.5 miles) above the Earth. The dips in the line at specific wavelengths (listed along the horizontal axis) indicate that less light at those wavelengths made it back to SOLSE-2’s detectors. The broad dip in the line near 600 nanometers (600 billionths of a meter) shows where ozone absorbed light most strongly, while the other wavelengths have more subtle dips. The narrow dip near 760 nanometers is absorption by oxygen. The bottom panel is an ozone index that shows the amount of light in a triplet of wavelengths (one strongly absorbed relative to two weakly absorbed) detected by SOLSE-2 between 0 and 50 kilometers above the Earth. Where ozone is high, index values will be low because ozone absorbs the light, leaving less to be detected by the instrument. Index values (green diamonds) were lowest (farthest to left on the graph) around 20 kilometers above the Earth, which is just where years of research and model calculations indicate the “ozone layer” should be. Indeed, in both cases, SOLSE-2’s measurements nearly duplicate what model calculations predict they should be in an atmosphere containing typical amounts of ozone. In the bottom panel, the red line shows the observations that atmospheric ozone models predict we should see; the blue line shows the observations that would be expected if the atmosphere contained no ozone. The measurements made by the SOLSE-2 mission on the Space Shuttle Columbia demonstrated that the limb-sounding technique will work very well for monitoring ozone in next-generation satellites. Although the primary data storage for SOLSE-2 was on board Columbia and was therefore destroyed, data were sent down to Earth for about 15 minutes during each orbit, yielding about 40 percent of the data collected during the mission. That amount was more than sufficient to demonstrate that the experiment was successful. The efforts of the Columbia crew (Rick D. Husband, William C. McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel Blair Salton Clark, and Ilan Ramon) resulted in the collection of valuable scientific data that will help design future Earth-observation satellites. Tragically, the Columbia and her crew were lost during re-entry into Earth’s atmosphere on February 1, 2003. The data used in this study are available in one or more of NASA's Earth Science Data Centers. |
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