Dear EarthTalk: With all the talk of rising seas, what could happen to the rivers that flow into the oceans? Will they reverse flow? Will rising seas back up into fresh water lakes? And what happens to our groundwater should saltwater flow backwards into it?
—Sandy Smith, concerned Michigander
The intrusion of saltwater from the sea into rivers and groundwater is a serious issue, but the threat is not from a reversal of flow, and our far inland lakes and rivers are not expected to be directly affected by the salty water of our oceans. However, the sensitive areas around the edges of our continents, where fresh water meets salt water, are at risk, and greater efforts must be taken to protect them. Some 40 percent of world population lives less than 40 miles (60 kilometers) from the shoreline.
According to the Intergovernmental Panel on Climate Change (IPCC), global average sea levels should rise eight to 34 inches by the year 2100, a much faster pace than the four to 10 inch increase of the past century. Seas rise because of higher global temperatures, melting mountain glaciers and polar ice caps, and other factors. Higher temperatures also cause thermal expansion of ocean water, intensifying the problem.
Rising sea levels cause major problems as they erode and flood coastlines and, yes, as they mix salt water with fresh. A November 2007 article in ScienceDaily posited that coastal communities could face significant losses in fresh water supplies as saltwater intrudes inland. And whereas it had been previously assumed that salty water could only intrude underground as far as it did above ground, new studies show that in some cases salt water can go 50 percent further inland underground than it does above ground.