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Observing Climate Variability and ChangeThe Earth's climate is dynamic and naturally varies on seasonal, decadal, centennial, and longer timescales. Each "up and down" fluctuation can lead to conditions which are warmer or colder, wetter or drier, more stormy or quiescent. Analyses of decadal and longer climate records and studies based on climate models suggest that many changes in recent decades can be attributed to human actions; these decadal trends are referred to as climate change. The effects of climate variability and change ripple throughout the environment and society - indeed touching nearly all aspects of the human endeavor and the environment. These factors underlie NOAA's mission to observe, understand, and predict climate variability and change. What are the variations and changes occurring in Earth's climate?Variability. Perhaps the most well understood occurrence of climate variability is the naturally occurring phenomenon known as the El Niño-Southern Oscillation (ENSO), an interaction between the ocean and the atmosphere over the tropical Pacific Ocean that has important consequences for weather around the globe. The ENSO cycle is characterized by coherent and strong variations in sea-surface temperatures, rainfall, air pressure, and atmospheric circulation across the equatorial Pacific. El Niño refers to the warm phase of the cycle, in which above-average sea-surface temperatures develop across the east-central tropical Pacific. La Niña is the cold phase of the ENSO cycle. The swings of the ENSO cycle typically occur on a time scale of a few years. These changes in tropical rainfall affect weather patterns throughout the world. Because of the importance of ENSO, NOAA has established a special ocean - atmosphere observing system in the tropical Pacific; this enables forecasts of El Niño to be made several seasons in advance. Climate variability is manifested in other ways as well. Decadal and seasonal shifts in wind patterns and sea surface temperatures in the Atlantic cause changes in hurricane frequency, for example. Sometimes climate varies in ways that are random or not fully explainable. The Dust Bowl of the 1930s in the United States is one such example. Because of the social and economic importance of understanding such climate fluctuations, NOAA routinely monitors past and current climate. Many of the longer period fluctuations are linked to the ocean. NOAA with its international partners is implementing a new global ocean observing system. This will lead to new understanding which will improve our predictions of climate variability and change. Change. The amount of energy entering and escaping from Earth is the determining factor in climate. Any changes to that balance-either the input or the output-will cause a directional change in climate. Observations have conclusively demonstrated that the atmospheric abundance of greenhouse gases has risen dramatically since the onset of the Industrial Age. Human activities such as fossil fuel burning and deforestation have caused this increase. The atmospheric concentration of carbon dioxide (CO2 ), has risen by 31% since 1750 and is now at the highest concentration seen in the last 420,000 years (and likely higher than any concentration seen for the last 20 million years). Other greenhouse gases that have increased since 1750 are methane (up by 151%), nitrous oxide (up by 17%), halocarbons (rising rapidly since 1950 but slowing or decreasing in recent years because of international agreements to protect the ozone layer), and tropospheric ozone (up by 36%). These gases absorb heat that would otherwise escape to space. The natural greenhouse effect warms the surface of the planet to temperatures that are hospitable for life. Indeed, without it we would have a frozen planet with surface temperatures of 0°F (-18°C), some 59°F colder than the current value. OAR has taken the lead internationally to build and maintain a system to monitor greenhouse gases and atmospheric fine particles (such as soot and sulfate aerosols) that influence climate, as well as other important trace gases (see www.esrl.noaa.gov/goals/climate.html and www.arl.noaa.gov). The energy input to Earth from the Sun can vary, for example because of changes in Earth's orbital path, changes in the tilt angle of the Earth, or variations in the energy coming from the Sun. These changes occur over long time scales and lead to the cycling of Ice Ages and interglacial periods that have occurred over the last 500,000 years. An additional factor which can cause climatic fluctuations on yearly to decadal timescales are volcanoes. Volcanic eruptions lead to enhancements of stratospheric and tropospheric aerosols which for the most part reflect solar radiation, hence leading to global cooling on a global average. However, regionally these can lead to warmer conditions by changing atmospheric circulation patterns, especially in the wintertime. Global observations of climate-related quantities have made it possible to document and analyze the behavior of Earth's climate. An international scientific body known as the Intergovernmental Panel on Climate Change (IPCC) periodically updates the state of scientific understanding with regard to climate science. Hundreds of scientists worldwide participate in the preparation and review of these reports. The latest such IPCC report is Climate Change 2001: The Scientific Basis. Also in 2001, the U.S. National Research Council issued a report on Climate Change Science: An Analysis of Some Key Questions. Some of the findings of the IPCC report and the NRC report with regard to the occurrence of climate change are:
Much climate research has focused on determining whether the human-caused increase in greenhouse gases is the cause of the observed changes in the last century's climate. As noted in the most recent climate change report of the IPCC (2001): The 2001 NRC report noted that: These statements reflect the progress made in the recent decade in reducing the uncertainties associated with quantifying the factors that influence climate, along with the difficulties in distinguishing between human-induced climate change and natural variability. Underlying recent progress are: a longer and more closely scrutinized temperature record; improved model estimates of climate variability; reconstructions of climate data for the past 1,000 years; new estimates of the climate response to natural and human-caused forcing; improved understanding of the driving forces of the global climate, in both the tropics and the polar regions; and new techniques for the detection of change and the attribution of change to causal factors.
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