How do humans impact the water supply?

The Earth is getting warmer, and warmer temperatures do cause changes in the balance of water as liquid, vapor and ice. We see it every time the cubes melt in our iced tea.

By Susan Williams

Some are obvious—polluting surface water and groundwater with various agricultural and industrial runoff, for example. Or using more than our fair shares, by letting faucets drip or water sprinklers spray on and on.

But some of the ways are more subtle and potentially more dramatic, even to the point of influencing climate change through global warming.

Since there is always the chance of changing how we do things, the important questions become, how are we impacting the planet and how much? According to Scott Robeson, an associate professor of geography specializing in climatology at IU Bloomington, human interaction with the environment is definitely a primary “how.”

“Human activity, such as heavy use of fossil fuels, deforestation and some types of agriculture, has increased the natural atmospheric concentration of ‘greenhouse’ gases, such as carbon dioxide and methane,” said Robeson. “In turn, these gases change the way that radiation is absorbed within the atmosphere, generally leading to a warmer global atmosphere.”

But Robeson is drawing the line at “how much.”

Some observers think that changes in precipitation evidenced by the past summer’s extended drought in the western United States and torrential rains that flooded Europe represent climate shifts resulting, at least in part, from a warmer globe. Robeson, though, is not sure. He pointed out that we can’t make generalizations based upon a limited geographical area or number of events.

“Some recent weather extremes may be evidence of climatic change, but only when combined with careful analysis of large numbers of similar events in the past,” he said. “We need to look for changes in the frequency of these events for evidence of global warming. And we need to remember that what is happening locally may not be happening globally—our country represents only about 2 percent of the globe’s total area.”

Photo by Paul Martens

Climate change and global warming are two decidedly different things, Robeson emphasized. While changes in human activity might slow down an enhanced greenhouse effect, many other processes over which we have no control impact the climate—solar output and volcanic eruptions are just two of them.

“It’s fairly clear that the global atmosphere near the surface of the earth has warmed by about 1 degree Fahrenheit over the past 100 years,” said Robeson. “While I wouldn’t say that we have proof that human activities are the sole cause for this increase, the change is certainly an important one in the global environment.”

Nearly all global climate model projections show warming over the next 100 years, but the rate varies widely depending on the model used, and what processes are included, according to Robeson. Another big variable is location, location, location.

“During the last few decades, we’ve seen rapid warming in most locations worldwide,” Robeson said. “Much of the warming has occurred at night and during the winter. Some environments, such as Alaska, are warming at a much more rapid rate than other locations, but not all northern locations are warming.”

So what does a warmer globe mean in terms of a wetter world and more water supply? First, remember that there is a finite amount of water on Earth. But warmer temperatures do cause changes in the balance of water as liquid, vapor and ice. We see it every time the cubes melt in our iced tea.

Climatologists are still working to understand all of the ways that an enhanced greenhouse effect can change global and regional environments, said Robeson. In general, though, evaporation of surface and land moisture would be increased in a warmer world, and overall, a warmer world should lead to more precipitation on a global scale. This is known as an “enhanced hydrologic cycle,” said Robeson.

But because precipitation is highly variable from year to year, how available an increased amount due to global warming might be for agricultural use and water supply is uncertain.

“Recognizing ‘signals’ within the ‘noise’ for precipitation is more difficult than it is with air temperature,” said Robeson, who added that the timing of precipitation changes and associated changes in humidity, cloud cover and wind speed all play a role in determining water availability.

The form the precipitation falls in also matters in terms of availability.

A warmer world may be a wetter one, but precipitation in liquid form may not be as beneficial as snow. In the Sierra Nevadas, for example, rain has fallen rather than snow in past years, causing problems in California surface water and groundwater supplies.

“Precipitation that falls as snow is ‘stored’ better and is available later in the year when there is less rainfall,” said Robeson. “Winter is the ‘wet’ season in California, so having additional rainfall at that time of year is less useful than having snow that will melt in late spring and early summer.”

Another possible impact is receiving rain in the form of torrential downpours, which are more likely to run off parched land than soaking into it.

“There’s some evidence for this, although the increase in heavy rainfall events is fairly small,” said Robeson. “In general, though, heavy rain events are associated with warmer conditions, as humidities are higher in warmer air. The atmospheric capacity for water vapor is higher at higher temperatures, so there is a larger ‘atmospheric reservoir’ under warmer conditions.”