Graph of the Day: Sea Ice Before Satellites
September 15, 2011
National Snow and Ice Data Center sheds light on an often asked question – what do we know about arctic sea ice before satellites?
Scientists have pieced together historical ice conditions to determine that Arctic sea ice could have been much lower in summer as recently as 5,500 years ago. Before then, scientists think it possible that Arctic sea ice cover melted completely during summers about 125,000 years ago, during a warm period between ice ages.
To look back into the past, researchers combine data and records from indirect sources known as proxy records. Researchers delved into shipping charts going back to the 1950s, which noted sea ice conditions. The data gleaned from those records, called the Hadley data set, show that Arctic sea ice has declined since at least the mid-1950s. Shipping records exist back to the 1700s, but do not provide complete coverage of the Arctic Ocean. However, taken together these records indicate that the current decline is unprecedented in the last several hundred years.
Before the 1950s, the data are patchier. So researchers also use clues from the environment to look into past sea ice conditions. Core samples from the ocean floor allow scientists to study layers of marine sediments laid down hundreds, thousands, even millions of years ago. They have also studied the remains of algae, plants, and animals in the ocean floor or along coastlines. Ice cores pulled from deep within Arctic glaciers contain evidence of past temperatures and periods of cooling and warming. Even the distribution of ancient driftwood can provide clues about where there was open water and where there was ice. While a single piece of evidence does not provide a whole picture of past conditions, many of them together can add up to a more complete picture of historical ice conditions.
The historical data also show how closely Arctic sea ice extent is linked to Earth’s climate. When Arctic sea ice was lower, Earth’s climate was much warmer than it is today, and sea level was higher than it is today. Leonid Polyak, a researcher at the Byrd Polar Research Center, said, “If that’s where we are heading, we should be worried.”