Kenneth R. Minschwaner of the New Mexico Institute of Mining and Technology and Andrew Dessler of the University of Maryland have found that warming caused by carbon dioxide build-up will cause less evaporation of water and therefore less additional warming than climate modellers have been assuming. (same article here and both have graphic illustrations)
A NASA-funded study found some climate models might be overestimating the amount of water vapor entering the atmosphere as the Earth warms. Since water vapor is the most important heat-trapping greenhouse gas in our atmosphere, some climate forecasts may be overestimating future temperature increases.
In response to human emissions of greenhouse gases, like carbon dioxide, the Earth warms, more water evaporates from the ocean, and the amount of water vapor in the atmosphere increases. Since water vapor is also a greenhouse gas, this leads to a further increase in the surface temperature. This effect is known as "positive water vapor feedback." Its existence and size have been contentiously argued for several years.
Ken Minschwaner, a physicist at the New Mexico Institute of Mining and Technology, Socorro, N.M., and Andrew Dessler, a researcher with the University of Maryland, College Park, and NASA's Goddard Space Flight Center, Greenbelt, Md, did the study. It is in the March 15 issue of the American Meteorological Society's Journal of Climate. The researchers used data on water vapor in the upper troposphere (10-14 km or 6-9 miles altitude) from NASA's Upper Atmosphere Research Satellite (UARS).
Their work verified water vapor is increasing in the atmosphere as the surface warms. They found the increases in water vapor were not as high as many climate-forecasting computer models have assumed. "Our study confirms the existence of a positive water vapor feedback in the atmosphere, but it may be weaker than we expected," Minschwaner said.
Water evaporation will contribute to warming but not as much as previously predicted.
In most computer models relative humidity tends to remain fixed at current levels. Models that include water vapor feedback with constant relative humidity predict the Earth's surface will warm nearly twice as much over the next 100 years as models that contain no water vapor feedback.
Using the UARS data to actually quantify both specific humidity and relative humidity, the researchers found, while water vapor does increase with temperature in the upper troposphere, the feedback effect is not as strong as models have predicted. "The increases in water vapor with warmer temperatures are not large enough to maintain a constant relative humidity," Minschwaner said. These new findings will be useful for testing and improving global climate models.
While gradual warming is the scenario most widely discussed possible future climate scenario there is a real possibility that the Earth's climate could undergo a rapid cooling initiated by natural processes or human-caused changes.
While policymakers have worried long and hard about global warming, which might raise Earth's temperature 1.4 to 5.8 degrees C by century's end, a growing body of evidence suggests natural forces could just as easily plunge Earth's average temperatures downward. In the past, the planet's climate has changed 10 degrees in as little as 10 years.
For example: Regional and global climates have undergone quick and dramatic changes even after what would appear to be only gentle prodding by natural influences, Dr. Eakin says. In many cases, that prodding has been far less severe than the changes humans have wrought via industrial emissions of carbon dioxide.
We have not seen sudden drastic climate changes in the lifetimes of anyone now living. But the Little Ice Age temperature dips have happened as recently as the US Revolutionary War period and into the 19th century.
The general trends reflected in the tree-ring record include cooler conditions in the early 1700s, followed by warming that started mid-century. An abrupt cooling occurred in the late 1700s and continued for much of the 1800s. The coldest period was between the 1830s and 1870s, after which a steadily increasing warming trend began.
We shouldn't be completely suprised if the Earth's climate suddenly begins to undergo some sudden change in the coming years and decades. It has happened often enough in the past that it is really a question of when and not if this will occur.
|Share |||Randall Parker, 2004 March 18 07:58 PM Climate Trends|