Jason Briner, PhD, University at Buffalo associate professor of geology, who led the study, said that on land, the atmosphere was warmest between 9,000 and 5,000 years ago, maybe as late as 4,000 years ago while the oceans were warmest between 5-3,000 years ago.
Growing ice sheets are like bulldozers, pushing rocks, boulders and other detritus into heaps of rubble called moraines.
Researchers looked at 250 ancient clams from moraines in three western regions, and discovered that most of the fossils were between 3-5,000 years old.
Briner and his colleagues looked at the structure of clam's amino acids - the building blocks of proteins - in the fossils of ancient clams. Amino acids come in two orientations that are mirror images of each other, known as D and L, and living organisms generally keep their amino acids in an L configuration.
When organisms die, however, the amino acids begin to flip. In dead clams, for example, D forms of aspartic acid start turning to L's.
Because this shift takes place slowly over time, the ratio of D's to L's in a fossil is a giveaway of its age.
Briner's research team matched D and L ratios in 20 Arctic clamshells to their radiocarbon-dated ages to generate a scale showing which ratios corresponded with which ages.
The researchers then looked at the D and L ratios of aspartic acid in the 250 Greenland clamshells to come up with the fossils' ages.
The study has been published in the journal Geology.