How do ice cores reveal Earth's past climate?

Ice cores are long cylinders drilled from glaciers and ice sheets. Each layer formed from snow that fell in a particular year, so the ice works like an archive, with the youngest ice at the top and the oldest at the bottom. The layers hold three main kinds of evidence: the chemistry of the frozen water, tiny bubbles of ancient air, and particles that settled out of the atmosphere. Although ice sheets form only in cold regions, the cores carry information about conditions across the whole planet. [1] [2]

As snow piles up and compacts, the pores in the old snow (called firn) close off and trap air in bubbles. According to the Australian Antarctic Program, trapping is generally not complete until the bubbles are around 60 metres or more below the surface. Once sealed, the bubbles act as time capsules. Researchers can measure gases such as carbon dioxide and methane directly in them, which gives a direct reading of past greenhouse gas levels. [3] [2] [4]

Temperature is not read from the ice with a thermometer. It is inferred from the ratio of oxygen isotopes in the water molecules. Water containing the heavier oxygen-18 evaporates less readily from the ocean than water containing the usual oxygen-16. In warmer periods more of the heavy water evaporates and falls as snow, so ice from warmer times has a higher share of oxygen-18. Hydrogen isotopes are used in a similar way. Scientists convert these ratios into temperatures using an empirical relationship, for example one derived from modern snowfall measurements. [1] [4]

Winds carry fine particles around the globe, and some settle on the snow. These include soot from fires, ash from volcanic eruptions and dust from dust storms. The size and chemical makeup of dust and ash help show how air circulation changed and where particles came from, and ash from known eruptions helps calibrate the age of a layer. The thickness of a layer indicates how much snow fell that year. Beryllium-10 is an indirect indicator of solar activity. [1]

In the 1960s, glaciologists at Byrd Station in Antarctica drilled a core reaching back to the last glacial period. It showed that cooler temperatures coincided with lower greenhouse gas concentrations. Later, a multinational effort on the East Antarctic Plateau produced an 800,000-year record covering eight glacial cycles. During the Last Glacial Maximum, about 20,000 years ago, Antarctic temperatures were between 4 and 10 degrees Celsius cooler and carbon dioxide was over 100 parts per million lower than in pre-industrial conditions. Greenland cores also show abrupt warmings of about 10 degrees Celsius over a few decades during the last glaciation. The PAGES article notes that these records, together with other long-term records, have contributed to the understanding that glacial cycles are paced by Earth's orbital configuration and amplified by feedbacks within the climate system. [4]