Ice sheets at both poles are melting and releasing water into the sea. Scientists want to know, and policymakers need to know, how fast this ice will melt, as the planet warms further. Will it melt at a slow and even rate, causing relatively little disruption to coastal communities? Or will the rate accelerate, making adaptation to sea level rise difficult?
Evidence of tilting of the U.S. coast
Accurate sea level markers
Roe Plains, Australia
The team’s first expedition
Researchers take advantage of a celestial collision
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Many strands of scientific research shed light on this important problem. The Pliomax team hopes to improve understanding of how ice sheets will change in the future by studying the level of the sea in the past. In its 4.5 billion year history, Earth has undergone many changes. The temperature of the planet has fluctuated 10°C or more warmer and many degrees cooler. The composition of the atmosphere has varied. Earth’s early atmosphere was mostly hydrogen and helium. Oxygen levels like today didn’t appear for billions of years.
Scientists sometimes consider a past state of the planet like a test run of a global experiment. Since the experiment has already taken place, researchers cannot alter the conditions. But, in contrast to a laboratory-scale experiment or a mathematical simulation, a past state includes all of Earth’s interacting systems.
The Pliomax team considers Earth during the Pliocene era, 5.3–2.6 million years ago, analogous to the conditions expected later this century. Back then the atmosphere contained the same amount of carbon dioxide, which regulates the planet’s temperature, as the post-industrial atmosphere of today. Global temperature was about 2-3°C hotter than that of today. Researchers expect Earth to rise to that temperature, and possibly beyond, in the coming decades. The current global temperature is not in equilibrium with the amount of carbon dioxide in the atmosphere and is playing “catch-up”.
Earth’s glaciers, including the massive polar ice sheets, will respond sensitively to changes in temperature. And of course, polar ice sheets, with a mass of millions of billions of tons, will also take time to react. Even a tray of ice cubes doesn’t melt right away, when taken from the freezer and left in a warm kitchen. The concern, though, is that the ice sheets will act like a humungous flywheel, slow to respond, but difficult to stop once set in motion.
The Pliocene was the most recent time in Earth’s past that the atmospheric concentration of carbon dioxide has been as elevated as it is today, at about 400 parts per million (ppm) or 4 one-hundreths of 1 percent. Other researchers have tried to determine sea level during the Pliocene, but their results have been contradictory, with estimates ranging from the same as the present to more than 35 meters (115 feet) higher. The Pliomax project is the first systematic effort to search around the world for Pliocene-age beaches and come up with a more accurate estimate of Pliocene sea level.