Hill Heat

The Role of Water Vapor in Climate: The Outlook from Models, Observations and Theory

Water vapor is the dominant greenhouse gas, the most important gaseous source of infrared opacity in the atmosphere. As the concentrations of other greenhouse gases, particularly carbon dioxide, increase because of human activity, it is centrally important to predict how the water vapor distribution will be affected. To the extent that water vapor concentrations increase in a warmer world, the climatic effects of the other greenhouse gases will be amplified. Models of the Earth’s climate indicate that this is an important positive feedback that increases the sensitivity of surface temperatures to carbon dioxide by more than a factor of two.

Prevailing evidence strongly suggests that the increased water vapor resulting from the warming effect due to CO2 and other greenhouse gases does serve to significantly amplify climate warming as models predicted. In addition, observationally-based estimates of the strength of this climate amplification are in agreement with model predictions. In other words, the warming due to the increase in greenhouse gases is driving the increase in water vapor which, in turn, is significantly amplifying the climate warming.

Water Vapor, Precipitation and Evaporation – The View from Satellites

Water vapor is a natural greenhouse gas that is very important to the climate. It can be measured by satellites more accurately than most other climate variables. Satellite observations show that water vapor has increased by 2.4% during the last 20 years. Satellites can also measure the temperature of the atmosphere, but not as accurately as water vapor. The satellites indicate the troposphere has warmed by 0.4 C during the last 20 years, which is in general agreement with surface thermometers. Thus the water vapor has increased at a rate of 6% per degree of global warming. Climate models predict a similar rise with temperature. There is no serious discrepancy between the satellite observations and the climate models with regards to the increases in water vapor and temperature.

Satellites also measure global precipitation and evaporation, although for the evaporation estimates additional surface observations are required. When averaged over the globe, evaporation must equal precipitation. This equality provides us with a useful consistency check. We find that the precipitation and evaporation trends do agree; they both indicate an increase of 6% per degree of warming, the same as water vapor. This observational result disagrees with climate models, which indicate a smaller increase of 1-3%. This is a significant discrepancy that needs to be resolved.

Have Humans Affected 20th Century Precipitation Trends?

Models suggest that anthropogenic forcing should have caused a small increase in global mean precipitation over the 20th century. However, human influence on global mean precipitation has been difficult to detect, partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal. Models further suggest that anthropogenic forcing should have caused a latitudinal redistribution of precipitation, increasing precipitation at high latitudes, decreasing precipitation at sub-tropical latitudes, and possibly changing the distribution of precipitation within the tropics by shifting the position of the Intertropical Convergence Zone. The 20th century instrumental precipitation record, which represents changes over land, indicates a latitudinal redistribution of precipitation with increasing precipitation at high latitudes, decreasing precipitation at northern sub-tropical latitudes, and increasing precipitation in southern subtropical latitudes.

Analysis of multiple climate models indicates that the observed changes in latitudinally averaged land precipitation are best explained by anthropogenic forcing (i.e, humans), and that they cannot be explained by internal climate variability or by the combined effect of natural solar and volcanic forcing of the climate system. We therefore estimate that anthropogenic forcing contributed significantly to the observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation.

Searching for Human “Fingerprints” in Atmospheric Water Vapor Changes

“Fingerprinting” involves searching for a computer model-predicted pattern of climate change (the “fingerprint”) in observed climate records. Fingerprint techniques allow researchers to make rigorous statistical tests of different possible explanations for an observed climate change. Most fingerprint work has focused on temperature changes at the Earth’s surface, in the free atmosphere, or in the oceans. Recently, a number of new studies have applied fingerprint methods to changes in the cycling of moisture between atmosphere, land, and ocean.

One recent fingerprint study looked at the possible causes of the increase in the total amount of atmospheric moisture over oceans. As noted above, satellite measurements indicate that the total amount of water vapor has increased by roughly 2.4% since 1988. Results from 22 different computer models show that this increase is consistent with the simulated climate response to human influences. This model-data consistency holds for both the overall size and the complex fingerprint pattern of water vapor changes. Climate models suggest that the main driver of the observed water vapor increase is the human-caused increase in well-mixed greenhouse gases. The observed atmospheric moistening cannot be explained by current model estimates of natural climate variability, and is highly unlikely to be due to the effects of solar variability or recovery from the 1991 eruption of Mt. Pinatubo.

Biographies

Dr. Brian J. Soden is an Associate Professor of Meteorology and Physical Oceanography at the University of Miami’s Rosenstiel School for Marine and Atmospheric Science. Dr. Soden specializes in the use of satellite observations to test and improve computer model simulations of climate change. During the past 15 years he has published over 60 peer-reviewed papers on a variety of topics, but most often related to the response of the atmospheric hydrological cycle to global warming. He recieved his B.S. degree from the University of Miami, and M.S. and Ph.D. degrees from the University of Chicago. Before returning to the University of Miami, Dr.Soden was a Visiting Scientist at Princeton University, and a Physical Scientist with NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, NJ.

Dr. Soden also served as a Lead Author of the chapter on atmospheric observations for the 2007 IPCC Report. His awards include the AMS Henry G. Houghton Award, the National Space Club’s David S. Johnson Award, and several outstanding research paper awards from NOAA and NASA.

Frank J. Wentz is the Director of Remote Sensing Systems (RSS), a research company specializing in microwave remote sensing. Over the last 25 years, he has been one of NASA’s leading principal investigators. He obtained a B.S. and M.S. in physics from Massachusetts Institute of Technology. His early research focused on radiative transfer models that relate satellite observations to geophysical parameters, with the objective of providing research-quality geophysical data sets to the Earth science community. As a member of NASA’s SeaSat Experiment Team (1978-1982), he pioneered the development of physically based retrieval methods for microwave scatterometers and radiometers. In 1987, he took the lead on providing the community with high-quality ocean products derived from a new generation of satellite microwave imagers: the SSM/I. Since then, observations from many more satellite sensors have been added to the RSS climate archive, which now includes data from over 20 satellites spanning the period for 1979 to present. Mr. Wentz has served on many NASA review panels and several NRC committees. He has a long list of about 100 publications in the peer-reviewed literature on remote sensing and its application to oceanography, hydrology, and climate.

Dr. Francis W. Zwiers is the Director of the Climate Research Division of Environment Canada. Dr. Zwiers is recognized as a world leader in developing and implementing statistical tools for the study and prediction of climate change. His work is being applied to determine and understand changes in the climate that may be resulting from the build-up of greenhouse gases in the earth’s atmosphere. Author of more than 50 research papers in the past decade, he has also co-authored the chapter on the “Detection of climate change and attribution of causes” in the Intergovernmental Panel on Climate Change 2001 Assessment Report. He recently served as a convening lead author of the chapter “Understanding and Attributing Climate Change” in the IPCC 4th Assessment Report, which has just been published, and he currently serves as a lead author of the CCSP Synthesis Product 3.3 report entitled “Weather and Climate Extremes in a Changing Climate”. In addition, he has co-authored the textbook Statistical Analysis in Climate Research, considered to be the standard reference for the application of statistical methods in climate science.

Dr. Zwiers received his PhD in Statistics from Dalhousie University in 1980. He was appointment as a research scientist with Environment Canada in 1984, and has subsequently fulfilled increasingly responsible roles, progressing to the level of senior scientist in 2002. He served as Chief of the Canadian Centre for Climate Modelling and Analysis in Victoria, B.C., from 1997 to 2006, before being appointed to his current position. Dr. Zwiers is a Fellow of the American Meteorological Society and a Fellow of the Royal Society of Canada.

Dr. Benjamin D. Santer is an atmospheric scientist at Lawrence Livermore National Laboratory (LLNL). His research focuses on such topics as climate model evaluation, the use of statistical methods in climate science, and identification of natural and anthropogenic “fingerprints” in observed climate records. Dr. Santer holds a doctorate in climatology from the University of East Anglia in England, where he studied under Prof. Tom Wigley. After completion of his Ph.D. in 1987, he spent five years at the Max-Planck Institute for Meteorology in Germany, where he worked with Prof. Klaus Hasselmann on the development and application of climate fingerprinting methods. In 1992, Dr. Santer joined the Program for Climate Model Diagnosis and Intercomparison at LLNL.

Dr. Santer served as convening Lead Author of the climate change detection and attribution chapter of the 1995 IPCC report, an experience best described as “character building”. His awards include a MacArthur Fellowship (1998), the Norbert Gerbier-MUMM international award from the World Meteorological Organization (1998), the U.S. Dept. of Energy’s E.O. Lawrence Award (2002), and a U.S. Dept. of Energy Distinguished Scientist Fellowship (2005). Dr. Santer has over seventy publications in the peer-reviewed scientific literature, and has contributed to ten books.