A library of ancient climate
The Penny Ice Cap is a 6,000-square-kilometer sheet of ice on Baffin Island, located almost entirely within Auyuittuq National Park. It is a remnant of the massive Laurentide Ice Sheet, which covered most of North America during the last ice age. At its dome, nearly 2,000 meters above sea level, annual layers of snow compress into ice, capturing bubbles of the atmosphere. This process creates a layered archive of past climate conditions.
In 1995, a team of researchers drilled through the ice cap to the bedrock below, extracting a 333.78-meter-long ice core. Analysis of this and other cores provides a high-resolution climate record going back more than 100,000 years. As snow falls and turns to ice, it traps air but also dust, pollen, volcanic ash and other airborne particles. Each layer records the world at the time it was formed. The composition of the water molecules themselves contains chemical data. Scientists measure the ratio of heavy oxygen-18 to light oxygen-16 isotopes (δ¹⁸O) to reconstruct past atmospheric temperatures. Colder periods show less of the heavier isotope.
A record of human impact
The upper, more recent layers of the Penny Ice Cap cores tell a different story. They contain clear evidence of industrial activity from thousands of kilometers away. The ice records a sharp increase in lead particles that correlates with the rise of industrial economies in Europe and North America. Sulfate concentrations spike in layers corresponding to the widespread burning of fossil fuels.
Researchers can even trace specific events. Tritium, a radioactive isotope of hydrogen, appears in distinct peaks in cores from the 1950s and 1960s, a direct result of atmospheric nuclear weapons testing. These markers, along with identifiable volcanic ash layers like the 1783 Laki eruption in Iceland, help scientists precisely date the ice layers. The presence of soot, or black carbon, from modern wildfires and industrial combustion is also evident in the snowpack. This pollution darkens the ice, reducing its reflectivity and potentially accelerating melt rates.