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Observing Climate Variability and Change

The 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:

  • On the global average, the surface air temperature of the Earth has increased by about 1.0 ± 0.4°F (0.6 ± 0.2°C) since the late 19th century.
  • The decade of the 1990s was very likely the warmest decade in the instrumental record, which dates back to 1861.
  • On average, between 1950 and 1993, nighttime daily minimum air temperatures over land increased by about 0.2°C per decade. This has lengthened the freeze-free season in many mid- and high latitude regions.
  • It is very likely that precipitation has increased by 0.5 to 1.0% per decade in the 20th century over most mid- and high latitudes of the Northern Hemisphere continents, and it is likely that rainfall has increased by 0.2 to 0.3% per decade over the tropical land areas. It is also likely that rainfall has decreased over much of the Northern Hemisphere subtropical (10°N to 30°N) lands areas during the 20th century by about 0.3% per decade. No comparable changes have been observed over the Southern Hemisphere land areas.
  • Global sea level has risen by between 4 and 8 inches (0.1 and 0.2 meters) over the past 100 years, and much of the increase is thought to be related to the rising global average temperature.

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):

There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

The 2001 NRC report noted that:

The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.

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.


Graph of selected observations shows an El Nino warm phase.

An index of six observed variables over the tropical Pacific is used to monitor the coupled ocean-atmosphere phenomenon known as the El Niño-Southern Oscillation (ENSO). (larger image)


Data graphs show variations in temperatures measured from thermometers over the past 140 years; and thermometers, tree rings, corals, ice cores and historical data over the last millennium. Both graphs show a rise in temperatures over the past 140 years.

Long records of past changes in greenhouse gases show the effects of large and increasing growth in anthropogenic emissions during the Industrial Era (~1750-present). (larger image)


Data graphs show s show a rise in emissions of carbon dioxide, methane and nitrous oxide during the Industrial Era.

Variations of the Earth's surface temperature over the last 140 years and the last millenium. (larger image)


A huge dust stom moves across the prairie.

The Dust Bowl of the 1930s is an example of climate variability that is not easily explainable.

NOAA Research programs that study Climate Variability and Change

checkmarkClimate Program Office
Sponsorship of scientific research aimed at understanding climate variability and change and its predictability over a continuum of timescales from seasonal to decadal and beyond.

checkmarkEarth System Research Laboratory (ESRL)
Within ESRL, areas of research and climate variability and change include:

checkmarkAir Resources Laboratory
Studies of diurnal to multi-decadal variations in the global climate system, with a focus on analysis of observational data; measurement of the surface radiation budget over the United States.

checkmarkAtlantic Oceanographic and Meteorological Laboratory
Predict and Assess Decadal to Centennial Change

checkmarkGeophysical Fluid Dynamics Laboratory
Development of earth system models for the production of timely and reliable knowledge and assessments on natural climate variability and anthropogenic changes.

checkmarkPacific Marine Environmental Laboratory
Ocean observations in support of long-term monitoring and prediction of the ocean environment on time scales from hours to decades, to define the forcing functions and the processes driving ocean circulation and the global climate system.

checkmarkJoint Research Institutes
Research relevant to understanding the Earth's oceans, the Great Lakes, inland waters, Arctic regions, solar terrestrial environment, intermountain west and the atmosphere.



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