Dan Kulpinski

At an October 29 seminar on Capitol Hill, scientists explained how and why the world is getting wetter as the globe gets warmer.

It has to do with water vapor. Warmer air holds more water vapor, so as the climate warms, scientists expect more water to evaporate and more to fall down to Earth as precipitation. This has already started to happen.

Water vapor is also the most abundant – and most important — greenhouse gas and accounts for 60 percent of the greenhouse effect. A positive feedback loop results: As the atmosphere warms up, water vapor in the atmosphere increases, which causes the atmosphere to warm further.

These were some of the findings presented at the seminar, which was titled “Climatically-Induced Increases in Water Vapor and Precipitation: Causation and Implications” and sponsored by the American Meteorological Society.

Dr. Brian J. Soden, an associate professor at the University of Miami, led off with a presentation about how water vapor amplifies warming. He noted that there is a fundamental relationship between water vapor and temperature, with water vapor increasing 7 percent per degree Celsius. He noted that the trend for the past 20 years has been for water vapor to increase with El Nino (which has a warming effect) and decrease with La Nina (which has a cooling effect).

He also showed how temperatures and water vapor levels dropped in parallel after the Mount Pinatubo volcanic eruption of 1991.

The model for the year 2100, he said, has wet regions becoming wetter and dry regions getting drier.

Frank Wentz, head of Remote Sensing Systems (a company that processes and analyzes microwave data collected by satellites), then discussed satellite measurements of water vapor, precipitation and evaporation. He noted that satellite data only goes back 20 years, but those
observations show water vapor has been increasing. Precipitation and evaporation have increased 1.3 percent over the ocean during that time. One area of the western Pacific Ocean did get wetter, but satellites have not shown any dry areas getting drier.

He also pointed out that in 100 years, the total amount of water vapor in the atmosphere will rise 13 percent, based on the predicted temperature increase of at least 2 degrees Celsius. Climate models show only a 4 percent increase for water vapor, however, and that is an important discrepancy that needs to be resolved.

Dr. Francis Zwiers, director of climate research for Environment Canada – their federal department of the environment – discussed rainfall changes on land. He said that discerning a human influence on the amount of rain has been difficult. He has looked at latitudinal bands across the globe and found that wet areas have gotten wetter and dry areas have become more dry since 1950. More precipitation has fallen in the tropics and at high latitudes.

Climate models that include human influences (greenhouse gas emissions) match observations better than models that exclude human influences. So it seems that we are affecting the distribution of rainfall already.

Dr. Benjamin Santer of Lawrence Livermore National Laboratory in Livermore, Calif., discussed his work with “climate fingerprinting,” wherein scientists search the climate records for patterns of climate change predicted by computer models.

Using data from satellites and 22 climate models, Santer discovered that water vapor over the oceans increases over time in both models and observations. He noted that changes from the sun and volcanoes along cannot explain this increase, but adding human influences can explain the increase.

He also discovered that the main contributor to the rise in water vapor is the increase in well-mixed greenhouse gases. He concluded that the water vapor feedback is operating and that we can identify a model fingerprint in the response of water vapor to human factors with high statistical confidence.

In Summary:
Real-world observations show that water vapor levels in the air are increasing; this has led to a rise in precipitation and evaporation. Water vapor is increasing as the atmosphere warms up – and it amplifies the warming. Climate models predict this warming will continue, so we can expect to see average annual worldwide precipitation levels to rise as well. Areas that are wet now will get wetter (such as equatorial regions and areas at high latitudes); areas that are dry will get drier (subtropical regions).

The next step is to try to measure such impacts regionally, which is more challenging. Dr. Santer said we’d see more emphasis soon on identifying regional effects.

Three of the presenters have contributed to reports issued by the Intergovernmental Panel on Climate Change (IPCC); two of them, Soden and Zwiers, contributed to the 2007 reports.

The IPCC’s report “Climate Change 2007: The Physical Science Basis,” released in February, states that the likely range for temperature increase by 2100 is 1.1-6.4 degrees Celsius (1.98-11.5 degrees Fahrenheit). Since water vapor increases some 6.5-7.0 percent per degree Celsius, we can expect a 7.1-41.6 percent rise in water vapor. If that occurs, we could see significantly more rain and snowfall.

So if you live in an area that already gets a lot of rain, prepare to get wetter.

(Photo from rambla via Flickr)