Every 100,000 years or so over the past million years, relatively small changes in the amount of sunlight that reaches the Arctic region in summer have spurred dramatic shifts in the proportion of Earth’s surface that is covered in ice. During the most recent of these massive transformations, up to 30% of the planet’s surface was covered in glaciers, including an ice sheet over Canada and the northern United States that at places reached more than a kilometer and a half thick.
Now, researchers have produced a new, more detailed model of this latest glacial cycle, spanning from 150,000 years ago to the present. To reconstruct the volume of ice covering the globe, Lisiecki and Stern analyzed the shells of single-celled ocean floor creatures called foraminifera, which capture two different isotopes of oxygen in their shells. In the environment, the lighter isotope, 16O, evaporates easily from the ocean and falls on land as rain or snow. When the climate cools, it becomes trapped in ice sheets as snow freezes to the ground. Meanwhile, the heavier isotope, 18O, stays in the ocean. As they get buried in layers of ocean sediment over time, foraminifera shells provide a snapshot of the isotope ratio left in the ocean, which can be used to calculate the volume of global ice sheets and changing deep-sea temperatures.
The authors’ prior work averaged this ratio (known as δ18O) from 57 sediment cores from oceans across the globe to reconstruct the evolution of glacial ice sheets over the past 5 million years. Because of the small number of samples and certain assumptions built into this model, however, the margin of error for the timing of events such as sea level rise could be as much as 4000 years.
By including 5 times as many records as in the previous version, the authors have reduced that error rate to approximately 1000 years over the past glacial cycle. First, the team took δ18O data from 263 cores from eight regions in the North and South Atlantic, Pacific, and Indian Oceans. Then they correlated ice-rafted debris and temperature data from North Atlantic cores with independent data on temperature from an ice core in Greenland and stalagmites from China and the Alps.
The new, more accurate model suggests that sea levels rose more rapidly than previously thought in response to increases in solar energy. This finding could improve future predictions of how quickly glaciers will respond to future solar radiation and other factors that drive climate change, such as the buildup of greenhouse gases in the atmosphere. (Paleoceanography, doi:10.1002/2016PA003002, 2016)
—Emily Underwood, Freelance Writer