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Variability
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Introduction | The Bounds of Natural Variability | Recent Global Warming
IntroductionThe Earth's climate has undergone many natural changes in the past, and it will continue to change naturally in the future. Today, however, there is another factor to consider: During the past century, people have burned millions of tons of fossil fuels to produce energy, releasing large quantities of greenhouse gases and other substances that affect the climate. How much of today's global warming trend is due to natural factors and how much is due to humans? Researchers cannot be certain, but according to the Intergovernmental Panel on Climate Change, "there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."
The climate system, consisting of the atmosphere, the oceans, sea ice, ecosystems, clouds, and other components, has its own natural random variability. This variability can lead to short-term changes in climate, such as El Niņo events, or longer-term changes such as those brought on by shifts in ocean circulation.
Natural forces outside the climate system, such as volcanoes, changes in solar activity, and changes in the Earth's orbit, also can affect climate. For example, scientists recently discovered an 1,800-year cycle in global warming and cooling that is driven by periodic changes in the strength of ocean tides. The changes in the tides are caused by gradual shifts in the relative astronomical positions of the Sun, Moon, and Earth. When tides are strong, cool water from the ocean depths is brought to the surface where it cools the climate. When tides are weaker, the climate warms. Presently, tides are weakening.
The Bounds of Natural VariabilityClimate change from human activities is superimposed on the climate's natural variability. Thus, if researchers could determine the range and character of natural variability, they might be able to tell the extent to which natural causes could explain current trends.
Direct measurements of temperature and precipitation are available from locations around the world for only the past 150 years. But scientists have been able to piece together a picture of the Earth's climate for the past 5 million years by analyzing a number of surrogate or "proxy" measures of climate such as ice cores, tree rings, pollen remains, and ocean sediments.
The evidence shows that global and regional climates have changed dramatically at times. During the past two million years, for example, ice ages alternated with periods of relative warmth on a roughly 100,000-year cycle. Researchers believe these events were triggered by slow changes in the tilt of the Earth's axis, along with changes in the Earth's distance from the Sun.
During the past 1,000 years, however, the climate appears to have been fairly stable. Two exceptions to this stability, known as the Medieval Warm Period and the Little Ice Age, occurred between the 9th and 14th centuries and the late 16th to early 19th centuries respectively. Those two periods, along with short-term fluctuations in climate on the scale of decades or years, provide boundaries for what could be considered the natural range of climate variability during the past millennium.
Recent Global WarmingThe global warming of the 20th century, particularly the warming that has occurred since 1950, falls well outside those boundaries. Based on the available record, the Intergovernmental Panel on Climate Change (IPCC) concluded in 2001 that the warming of the northern hemisphere in the 20th century is probably greater than any warming than has occurred during the past 1,000 years. The IPCC also found it likely that globally, the 1990s were the warmest decade and 1998 the warmest year of the past millennium.
The IPCC concluded that most of the warming during the past 50 years is attributable to anthropogenic (human) emissions of greenhouse gases. Greenhouse gases leave a distinctive "fingerprint" on climate, affecting temperature and precipitation in patterns that differ from those caused by fluctuations in solar output or natural variability. While not all scientists agree on the extent to which humans contribute to global warming, the great majority believe that warming is underway.
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