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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization





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How Do We Know that the Atmospheric Build-up of Greenhouse Gases Is Due to Human Activity?

Four lines of evidence prove conclusively that the recent buildup of carbon dioxide arises largely from human activities.

The nuclei of carbon atoms in carbon dioxide emitted by burning coal, oil, and natural gas (fossil fuels) differ in their characteristics from the nuclei of carbon atoms in carbon dioxide emitted under natural conditions. Coal, oil, and natural gas were formed deep underground tens of millions of years ago, and the fraction of their nuclei that were once radioactive has long ago changed to non- radioactive carbon. But the carbon dioxide emitted from natural sources on the Earth's surface retains a measurable radioactive portion. As carbon dioxide has been emitted through fossil fuel combustion, the radioactive fraction of carbon in the atmosphere has decreased. Forty years ago scientists provided the first direct evidence that combustion of fossil fuels was causing a buildup of carbon dioxide and thereby diluting radioactive carbon in the atmosphere by measuring the decreasing fraction of radioactive carbon-14 captured in tree rings, each year between 1800 and 1950.

Secondly, scientists began making precise measurements of the total amount of carbon dioxide in the atmosphere at Mauna Loa, Hawaii, and at the South Pole in the late 1950s. They have since expanded their observations to many other locations. Their data show convincingly that the levels of carbon dioxide have increased each year worldwide. Furthermore, these increases are consistent with other estimates of the rise of carbon dioxide emissions due to human activity over this period.

A third line of evidence has been added since 1980. Ice buried below the surface of the Greenland and Antarctic ice caps contains bubbles of air trapped when the ice originally formed. These samples of fossil air, some of them over 200,000 years old, have been retrieved by drilling deep into the ice. Measurements from the youngest and most shallow segments of the ice cores, which contain air from only a few decades ago, produce carbon dioxide concentrations nearly identical to those that were measured directly in the atmosphere at the time the ice formed. But the older parts of the cores show that carbon dioxide amounts were about 25% lower than today for the ten thousand years previous to the onset of industrialization, and over that period changed little (Figure 5.1).

Figure 5.1

Measured amounts of carbon dioxide in the atmosphere.

The final line of evidence comes from the geographic pattern of carbon dioxide measured in air. Observations show that there is slightly more carbon dioxide in the northern hemisphere than in the southern hemisphere. The difference arises because most of the human activities that produce carbon dioxide are in the north and it takes about a year for northern hemispheric emissions to circulate through the atmosphere and reach southern latitudes.

Carbon dioxide is released to the atmosphere by a variety of sources, and over 95% percent of these emissions would occur even if human beings were not present on Earth. For example, the natural decay of organic material in forests and grasslands, such as dead trees, results in the release of about 220 billion tons of carbon dioxide every year. But these natural sources are nearly balanced by physical and biological processes, called natural sinks, which remove carbon dioxide from the atmosphere. For example, some carbon dioxide dissolves in sea water, and some is removed by plants as they grow.

As a result of this natural balance, carbon dioxide levels in the atmosphere would have changed little if human activities had not added an amount every year. This addition, presently about 3% of annual natural emissions, is sufficient to exceed the balancing effect of sinks. As a result, carbon dioxide has gradually accumulated in the atmosphere, until at present, its concentration is 30% above pre- industrial levels.

Direct atmospheric measurements of other human-produced greenhouse gases have not been made in as many places or for as long a period as they have for carbon dioxide. However, existing data for these other gases do show increasing concentrations of methane, nitrous oxide, and chlorofluorocarbons over recent decades. In addition, ice core data are available for methane and for nitrous oxide that demonstrate that the atmospheric concentrations of these gases began to increase in the past few centuries, after having been relatively constant for thousands of years. Chlorofluorocarbons are absent from deep ice cores because they have no natural sources and were not manufactured before 1930.

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