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  • NCDC Frequently Asked Questions

    How do we know the Earth's climate is warming?

    [ Return to FAQs ]

    Thousands of land and ocean temperature measurements are recorded each day around the globe. This includes measurements from climate reference stations, weather stations, ships, buoys and autonomous gliders in the oceans. These surface measurements are also supplemented with satellite measurements. These measurements are processed, examined for random and systematic errors, and then finally combined to produce a time series of global average temperature change. A number of agencies around the world have produced datasets of global-scale changes in surface temperature using different techniques to process the data and remove measurement errors that could lead to false interpretations of temperature trends. The warming trend that is apparent in all of the independent methods of calculating global temperature change is also confirmed by other independent observations, such as the melting of mountain glaciers on every continent, reductions in the extent of snow cover, earlier blooming of plants in spring, a shorter ice season on lakes and rivers, ocean heat content, reduced arctic sea ice, and rising sea levels.

    The Global Surface Temperature is Rising

    Global Surface Temperature
    Global annual average temperature measured over land and oceans. Red bars indicate temperatures above and blue bars indicate temperatures below the 1901-2000 average temperature. The black line shows atmospheric carbon dioxide concentration in parts per million.

    Global average temperature is one of the most-cited indicators of global climate change, and shows an increase of approximately 1.4°F since the early 20th Century. The global surface temperature is based on air temperature data over land and sea-surface temperatures observed from ships, buoys and satellites. There is a clear long-term global warming trend, while each individual year does not always show a temperature increase relative to the previous year, and some years show greater changes than others. These year-to-year fluctuations in temperature are due to natural processes, such as the effects of El Ninos, La Ninas, and the eruption of large volcanoes. Notably, the 20 warmest years have all occurred since 1981, and the 10 warmest have all occurred in the past 12 years.

    U.S. Surface Temperature is also Rising

    US Temperature
    Annual surface temperatures for the contiguous U.S. compared to the 20th Century (1901-2000) average. Calculated from the U.S. Historical Climatology Network (USHCN version 2). More information: U.S. Surface Temperature Data, USHCN v2.

    Surface temperatures averaged across the U.S. have also risen. While the U.S. temperature makes up only part of the global temperature, the rise over a large area is not inconsistent with expectations in a warming planet. Because the U.S. is just a fraction of the planet, it is subject to more year-to-year variability than the planet as a whole. This is evident in the U.S. temperature trace.

    Sea Level is Rising

    Sea Level Rise
    Annual averages of global sea level. Red: sea-level since 1870; Blue: tide gauge data; Black: based on satellite observations. The inset shows global mean sea level rise since 1993 - a period over which sea level rise has accelerated. More information: Coastal Sensitivity to Sea Level Rise (USGCRP) and Climate Change 2007: The Physical Science Basis.

    Global mean sea level has been rising at an average rate of approximately 1.7 mm/year over the past 100 years (measured from tide gauge observations), which is significantly larger than the rate averaged over the last several thousand years. Since 1993, global sea level has risen at an accelerating rate of around 3.5 mm/year. Much of the sea level rise to date is a result of increasing heat of the ocean causing it to expand. It is expected that melting land ice (e.g. from Greenland and mountain glaciers) will play a more significant role in contributing to future sea level rise.

    Global Upper Ocean Heat Content is Rising

    Ocean Heat Content
    Time series of seasonal (red dots) and annual average (black line) of global upper ocean heat content for the 0-700m layer since 1955. More information: BAMS State of the Climate in 2008.

    While ocean heat content varies significantly from place to place and from year-to-year (as a result of changing ocean currents and natural variability), there is a strong trend during the period of reliable measurements. Increasing heat content in the ocean is also consistent with sea level rise, which is occurring mostly as a result of thermal expansion of the ocean water as it warms.

    Northern Hemisphere Snow Cover is Retreating

    Northern Hemisphere Snow Cover Extent
    Left: Average of monthly snow cover extent anomalies over Northern Hemisphere lands (including Greenland) since Nov 1966. Right: Seasonal snow cover extent over Northern Hemisphere lands since winter 1966-67. Calculated from NOAA snow maps. From BAMS State of the Climate in 2008 report.

    Northern Hemisphere average annual snow cover has declined in recent decades. This pattern is consistent with warmer global temperatures. Some of the largest declines have been observed in the spring and summer months.

    Glacier Volume is Shrinking

    Glacial Decrease
    Cumulative decline (in cubic miles) in glacier ice worldwide. More information: Global Climate Change Impacts in the U.S.

    Warming temperatures lead to the melting of glaciers and ice sheets. The total volume of glaciers on Earth is declining sharply. Glaciers have been retreating worldwide for at least the last century; the rate of retreat has increased in the past decade. Only a few glaciers are actually advancing (in locations that were well below freezing, and where increased precipitation has outpaced melting). The progressive disappearance of glaciers has implications not only for a rising global sea level, but also for water supplies in certain regions of Asia and South America.

    U.S. Climate Extremes are Increasing

    US Extremes Index
    Annual Climate Extremes Index (CEI) value for the contiguous United States. Larger numbers indicate more acive climate extremes for a year. More information: CEI.

    One way climate changes can be assessed is by measuring the frequency of events considered "extreme" (among the most rare of temperature, precipitation and storm intensity values). The Climate Extremes Index (CEI) value for the contiguous United States is an objective way to determine whether extreme events are on the rise. The figure to the left shows the the number of extreme climate events (those which place among the most unusual of the historical record) has been rising over the last four decades.