Dansgaard was born in the Danish capital of Copenhagen and educated at the university there, obtaining his PhD in 1961. He has studied the applications of environmental isotopes to meteorological, hydrological, and glaciological problems, and in particular to the climate of the last 100,000 years. Oxygen is present in two stable isotopes – the normal oxygen–16 and a much smaller proportion of oxygen–18 with two extra neutrons in its nucleus. In 1947 Harold Urey demonstrated that the variation of the two isotopes in sea water depended on temperature, i.e. the colder the temperature the smaller the oxygen–18 content of the seas. He had further established that a slight change of temperature would produce a measurable alteration in oxygen–18 levels.

In the early 1960s the US army drilled down into the Greenland icecap, producing an ice core 4600 feet (1400 m) long and with a 100,000-year history. Dansgaard realized that by making careful measurement of the core's varying oxygen–18 level he should be able to reconstruct the climatic history of the last 100,000 years. The most recent ice age, ending 10,000 years ago, was clearly marked, as was evidence of a weather cycle during the last 1000 years.