Climate Change: Evidence, Impacts, and Choices PDF Booklet (2012) / Chapter Skim
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Part I
Part I
Pages 2-19
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climate change, scientists have understood for more than a century that emissions from the burning of fossil fuels could lead to increases in the Earth's average surface temperature. Decades of research have confirmed and extended this understanding.
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Consider, for example, sure the data aren't skewed by such things as that 1.4°F is greater than the average annual E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e FIGURE 1 FIGURE 2 NASA's Global Surface Temperature Record Esti- (bottom left) Climate monitoring stations on land and sea, mates of global surface temperature change, relative such as the moored buoys of NOAA's Tropical Atmosphere to the average global surface temperature for the Ocean (TAO)
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Weather balloons are shorter and milder, snow and ice cover are used to probe the temperature, humidity, and winds decreasing in the Northern Hemisphere, glaciers in the atmosphere. A key breakthrough in the ability and ice caps around the world are melting, and to track global environmental changes began in the many plant and animal species are moving to cooler 1970s with the dawn of the era of satellite remote latitudes or higher altitudes because it is too warm sensing.
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Human activities -- especially burning fossil fuels -- are increasing the concentrations of many of these gases, amplifying the natural greenhouse effect. Image courtesy of the Marian Koshland Science Museum of the National Academy of Sciences Box 1 Early Understanding of Greenhouse Gases In 1824, French physicist Joseph Fourier (top)
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Source: National Research Council How do we know that humans are causing greenhouse gas concentrations to increase? D iscerning the human influence on greenhouse gas concentrations is challenging because many greenhouse gases occur naturally in Earth's atmo tional CO2 began to be released into the atmosphere much more rapidly than in the natural carbon cycle.
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Atmospheric concentration units changes in plant activity (plants take up CO2 during indicate the number of molecules of the greenhouse spring and summer in the Northern Hemisphere, gas per million molecules of air for carbon dioxide E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e where most of the planet's land mass and land and nitrous oxide, and per billion molecules of air for ecosystems reside, and release it in fall and winter)
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Furthermore, a forensic-style concentrations in the atmosphere, scientists have analysis of the CO2 in the atmosphere reveals the amassed detailed records of how much coal, oil, chemical "fingerprint" of carbon from fossil fuels and natural gas is burned each year. They also (see Box 2)
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some aerosols -- which are tiny liquid or solid When all human and natural forcing agents are particles suspended in the atmosphere, such as those considered together, scientists have calculated that that make up most of the visible air pollution -- have the net climate forcing between 1750 and 2005 is Box 3 Warming and Cooling Effects of Aerosols Aerosols are tiny liquid or solid particles suspended in the atmosphere that come from a number of human activities, such as fossil fuel combustion, as well as natural processes, such as dust storms, volcanic eruptions, and sea spray emissions from the ocean. Most of our visible air pollution is made up of aerosols.
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Even if we could hold all human-produced forcing agents at present-day values, Earth would continue to warm well beyond the 1.4°F already ob served because of human emissions to date. Climate Feedback Loops The amount of warming ARCTIC SEA that occurs because of increased greenhouse gas AS REFLECTIVE ICE MELTS emissions depends in part on feedback loops.
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temperature, scientists have examined records of Prior to the satellite era, solar energy output had solar activity to determine if changes in solar output to be estimated by more indirect methods, such as might be responsible for the observed global warm- records of the number of sunspots observed each ing trend. The most direct measurements of solar year, which is an indicator of solar activity.
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. This is a result of solar changes can be found in the exactly the vertical pattern of temperature changes temperature trends in the different layers of the expected from increased greenhouse gases, atmosphere.
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Based on a rigorous assessment of available temperature records, climate forcing estimates, and sources of natural climate variability, scientists have concluded that there is a more than 90% chance that most of the observed global warming trend over the past 50 to 60 years can be attributed to emissions from the burning of fossil fuels and other human activities. Such statements that attribute climate change to human activities also rely on information from Short-term Temperature Effects of Natural Climate Variations Natural factors, such as volcanic eruptions and El Niño and La Niña events, can cause average global temperatures to vary from one year E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e to the next, but cannot explain the long-term warming trend over the past 60 years.
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When greenhouse gas emissions tures would have evolved if only natural factors and other activities are included in the models, how (volcanoes, the Sun, and internal climate variability) ever, the resulting surface temperature changes more were influencing the climate system.
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Many of the 20th century saw intense winter warming of the world's glaciers and ice sheets are melting in across parts of Canada, Alaska, and northern Europe response to the warming trend, and long-term avand Asia, while summer warming was particularly erage winter snowfall and snowpack have declined strong across the Mediterranean and Middle East in many regions as well, such as the Sierra Nevada and some other places, including parts of the U.S. mountain range in the western United States.
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and much drier in the Southwest. Another example of a climate change observed Warmer air holds more water vapor, which has led during the past several decades has been changes in to a measurable increase in the intensity of precipita 16
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The timing of different The next section describes how observed climate seasonal activities is also changing. Several plant trends and impacts are predicted to continue if species are blooming earlier in Spring, and some emissions of human-produced greenhouse gases are birds, mammals, fish, and insects are migrating maintained during the next century and beyond.
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. Through a convergence of theory, observations, and modeling, scientists have deduced that the ice ages are caused by slight recurring variations in Earth's orbit that alter the amount and seasonal distribution of solar energy reaching the Northern Hemisphere.
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The ice age cycles nicely illustrate how climate forcing and feedback The U.S. Geological Survey National Ice Core Lab stores ice cores samples taken from polar ice caps effects can alter Earth's temperature, but and mountain glaciers.
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