The Greenhouse Effect is a Natural Phenomenon That Is Critical for Life as We Know It

GHGs—which include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others—are present in relatively low concentrations in the atmosphere, but, because of their ability to absorb and re-radiate infrared energy, they trap heat near the Earth's surface, keeping it much warmer than it would otherwise be (Figure 2.1). The atmospheric concentrations of GHGs have increased over the past two centuries as a result of human activities, especially the burning of the fossil

2 http://www.globalchange.gov/publications/reports/scientific-assessments/us-impacts

3 http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm

4 http://national-academies.org/climatechange/

5 For additional discussion and references, see Chapter 6 in Part II of the report.

NATURAL WARMING

(1) Sunlight brings energy into the climate system; most of it is absorbed by the oceans and land.

(2) Heat (infrared energy) radiates outward from the warmed surface of the Earth.

(3) Some of the infrared energy is absorbed by greenhouse gases in the atmosphere, which re-emit the energy in all directions.

(4) Some of the infrared energy further warms the Earth,

(5) Some of the infrared energy is emitted into space.

FIGURE 2.1 The greenhouse effect. SOURCE: Marian Koshland Science Museum of the National Academy of Sciences.

AMPLIFIED WARMING

(6) Higher concentrations of CCL and other "greenhouse" gases trap more infrared energy in the atmosphere than occurs naturally. The additional heat further warms the atmosphere and Earth's surface.

fuels—coal, oil, and natural gas—for energy. The increasing concentrations of GHGs are amplifying the natural greenhouse effect, causing Earth's surface temperature to rise. Human activities have also increased the number of aerosols (small liquid droplets or particles suspended in the atmosphere). Aerosols have a wide range of environmental effects, but on average they increase the amount of sunlight that is reflected back to space, a cooling effect that offsets some, but not all, of the warming induced by increasing GHG concentrations.

Earth Is Warming

There are many indications—both direct and indirect—that the climate system is warming. The most fundamental of these are thermometer measurements, enough of which have been collected over both land and sea to estimate changes in global average surface temperature since the mid- to late 19th century. A number of inde-

FIGURE 2.2 Global surface temperature change from 1880 to 2009 in degrees Celsius.The black curve shows annual average temperatures, the red curve shows a 5-year running average, and the green bars indicate the estimated uncertainty in the data during different periods of the record. For further details see Figure 6.13. SOURCE: NASA GISS (2010; based on Hansen et al., 2006, updated through 2009 at http://data. giss.nasa.gov/gistemp/graphs/).

FIGURE 2.2 Global surface temperature change from 1880 to 2009 in degrees Celsius.The black curve shows annual average temperatures, the red curve shows a 5-year running average, and the green bars indicate the estimated uncertainty in the data during different periods of the record. For further details see Figure 6.13. SOURCE: NASA GISS (2010; based on Hansen et al., 2006, updated through 2009 at http://data. giss.nasa.gov/gistemp/graphs/).

pendent research teams collect, analyze, and correct for errors and biases in these data (for example, accounting for the "urban heat island" effect and changes in the instruments and methods used to measure ocean surface temperatures). Each group uses slightly different analysis techniques and data sources, yet the temperature estimates published by these groups are highly consistent with one another.

Surface thermometer measurements show the first decade of the 21st century was 1.4°F (0.8°C) warmer than the first decade of the 20th century (Figure 2.2). This warming has not been uniform, but rather it is superimposed on natural year-to-year and even decade-to-decade variations. Because of this natural variability, it is important to focus on trends over several decades or longer when assessing changes in the heat balance of the Earth. Physical factors also give rise to substantial spatial variations in the pattern of observed warming, with much stronger warming over the Arctic than over tropical latitudes and over land areas than over the ocean.

Other measurements of global temperature changes come from satellites, weather balloons, and ships, buoys, and floats in the ocean. Like surface thermometer measurements, these data have been analyzed by a number of different research teams around the world, corrected to remove errors and biases, and calibrated using independent observations. Ocean heat content measurements, which are taken from the top sev eral hundred meters of the world's oceans, show a warming trend over the past several decades that is similar to the atmospheric warming trend in Figure 2.2.

Up until a few years ago, scientists were puzzled by the fact that the satellite-based record of atmospheric temperature trends seemed to disagree slightly with the data obtained from weather balloon-based measurements, and both seemed to be slightly inconsistent with surface temperature observations. Recently, researchers identified several small errors in both the satellite and weather balloon-based data sets, including errors caused by instrument replacements, changes in satellite orbits, and the effect of sunlight on the instruments carried by weather balloons. After correcting these errors, temperature records based on satellite, weather balloon, and ground-based measurements now agree within the estimated range of uncertainty associated with each type of observation.

The long-term trends in many other types of observations also provide evidence that Earth is warming. For example:

• Hot days and nights have become warmer and more frequent;

• Cold snaps have become milder and less frequent;

• Northern Hemisphere snow cover is decreasing;

• Northern Hemisphere sea ice is declining in both extent and average thickness;

• Rivers and lakes are freezing later and thawing earlier;

• Glaciers and ice caps are melting in many parts of the world (as described in more detail below); and

• Precipitation, ecosystems, and other environmental systems are changing in ways that are consistent with global warming (many of these changes are also described below).

Based on this diverse, carefully examined, and well-understood body of evidence, scientists are virtually certain that the climate system is warming. In addition, scientists have collected a wide array of "proxy" evidence that indicates how temperatures and other climate properties varied before direct measurements were available. These proxy data come from ice cores, tree rings, corals, lake sediments, boreholes, and even historical documents and paintings. A recent assessment of these data and the techniques used to analyze them concluded that the past few decades have been warmer than any other comparable period for at least the last 400 years, and possibly for the last 1,000 years or longer (NRC, 2006b).

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