Water Vapor: More information

The maps and bar graphs show how the amount of water vapor in the Arctic atmosphere for different years and months compares to averages from 1979 to 2015. On the map, areas with greater than average water vapor for the selected month and year are indicated in purples (positive anomalies), and areas with less than average water vapor are shown in greens (negative anomalies). The map of anomalies helps to show where water vapor changes are strongest. The values show the mass of water vapor in a column of the atmosphere that stretches from the surface to the top of the atmosphere.

The bar graph indicates the water vapor anomaly for the selected month and year, for the entire Arctic (the area north of 70°N). The graph helps illustrate Arctic-wide changes for each month of the year.

water vapor sample imageThis sample image shows water vapor anomalies for October 2007.

Water vapor, the atmosphere’s most abundant greenhouse gas, plays a key role in regulating Earth's climate. As the atmosphere warms, the amount of water vapor that it holds is expected to increase. However, since water vapor is a greenhouse gas, this will lead to further warming. Research continues to assess the contribution of increased water vapor to observed rises in air temperature across the globe.

Increases in water vapor in the Arctic region begin to appear after about the year 2000.  However, as with air temperature, the magnitudes of these increases depend on time of year as well as location. The largest increases tend to occur during summer and early autumn. Smaller anomalies are the rule for other months. This is in large part because of low temperatures and because much of the land and ocean surfaces are covered by sea ice and snow cover, limiting the potential for evaporation of water. A notable exception is the North Atlantic.  Here, cold Arctic air overlying fairly warm and ice-free water can result in high rates of evaporation, supplying water vapor to the atmosphere.

Patterns of change in water vapor are more complex than for air temperature, but some level of organization can be seen. In recent years, in early summer, regions with large water vapor anomalies are apparent over land areas. This is likely a result of snow cover that is less extensive than in the past, and thawing of seasonally frozen soils, both of which allow evaporation of water. The tundra of Siberia and North America is especially boggy at this time of year. Patterns of change in water vapor are also influenced by changes in the circulation of the atmosphere.

In recent years, there are also large positive anomalies in water vapor over parts of the Arctic Ocean, primarily in late summer and early autumn. In many cases these correspond with the regions of Arctic Ocean that have lost their summer ice cover. Positive anomalies centered over the Beaufort and Chukchi seas in 2007 correspond with the large loss of sea ice in this area in this year.

The data shown here are from the NASA MERRA reanalysis project.  Reanalyses are forms of retrospective numerical weather prediction. Observations of surface air pressure, air temperature, humidity and wind speed are blended with short term forecasts from weather forecast models to provide the best estimate of atmospheric and surface conditions given available data. Observations of atmospheric water vapor are made with instruments carried by weather balloons (radiosondes) that are released twice daily from weather stations, and by satellite-borne instruments. Coverage by radiosondes is very sparse over the Arctic Ocean. Estimates of water vapor from reanalyses are not as reliable as, for example, surface pressure. All reanalysis products tend to have biases as compared to direct observations. This is especially the case in winter.  At this time of year, reanalyses tend not to capture low-level temperature inversions, and overestimate both temperatures and water vapor near the surface. Nevertheless, both reanalyses and observations tend to show positive trends in atmospheric water vapor albeit with varying strengths and levels of statistical significance.

Reference

Serreze, M. C., A. P. Barrett, and J. Stroeve. 2012. Recent changes in tropospheric water vapor over the Arctic as assessed from radiosondes and atmospheric reanalyses, J. Geophys. Res. 117, D10104, doi:10.1029/2011JD017421.

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