Photo by Alex MacLean/Landslides
About 40 percent of the world’s inhabitants work and farm and sleep each night within 100 kilometers of a coastline. As the sea rises in a warming world, many of these people are threatened with increased flooding, storm damage and salt intrusion into groundwater. Billions of dollars of property and millions of lives are at risk.
Researchers know that sea level is rising. It has gone up by about 11 centimeters since 1950. How fast and how high might sea level go in the future? Government planners and residents of shoreline communities are examples of the many people who could plan better if they knew the answer. Scientists have made estimates of future sea level but their task is complicated by the numerous factors that influence the height of the ocean.
Researchers know generally why sea level changes. When water heats up, it expands and creeps up shorelines. Higher air and ocean temperatures also threaten mountain glaciers and the big ice sheets at Earth’s poles. But, since higher temperatures can also produce more snow, warming doesn't necessarily make all glaciers shrink.
There are two primary means of forecasting sea level rise. In one, scientists create a mathematical model of sea level that takes into account how much water will expand and how much glaciers will grow or shrink. This method requires a detailed understanding of factors like how heat penetrates into the ocean’s depths and how changes in air temperature influence precipitation. Researchers have created such models. But they suffer from uncertainty about how Earth’s complex systems work and relate to each other.
Rather than modeling how Earth might behave in the future, the Pliomax team is studying how it did behave in the past. The team has chosen to examine the Pliocene, a period about 3 million years ago, when Earth (on average) was about 2 to 3 degrees centigrade hotter than today. Our planet could easily reach such elevated temperatures later in this century. The Pliomax scientists are trying to determine how much higher sea level was during that previous hot period.
To do this, the Pliomax team is seeking fossil beaches all around the world, beaches and reefs that were left stranded inland when Earth cooled at the end of the Pliocene and sea level fell. So far they’ve examined coastal regions in Australia, India and South Africa. At each site, they measure the height of the ancient beach above present sea level. They determine the age of the beaches by examining fossils with precise laboratory instruments. Because it takes measurements at many sites to eliminate errors caused by confounding factors, such as vertical movements of the edges of continents, they are planning more expeditions in the future.