Explain how ice cores and oxygen isotopes are used as proxies for past climate

What this dot point is asking

SCSA wants you to explain how ice cores and oxygen isotopes serve as climate proxies, and what each reveals. These are the most powerful proxies because ice cores uniquely capture both temperature and the actual ancient air, allowing direct study of the link between greenhouse gases and climate.

Why proxies are needed

Instrumental temperature records cover only the last century or so. To study climate over thousands to millions of years, scientists use proxies, natural materials whose properties depend on the climate when they formed. Reading the proxy then reconstructs past conditions.

Ice cores

Ice sheets in Antarctica and Greenland build up year by year, so deep cores drilled through them read back through time.

• Trapped air bubbles preserve the actual composition of past atmospheres, giving a direct record of carbon dioxide and methane concentrations.
• Annual layers of snowfall can be counted, like tree rings, to date the record, and dust and ash layers add markers.
• The deepest Antarctic cores extend back hundreds of thousands of years across many glacial cycles.

Ice-core records show that carbon dioxide and temperature have risen and fallen together through the glacial cycles, and that today's carbon dioxide is far above the natural range of that whole period.

Oxygen isotopes

Oxygen comes in a lighter and a heavier form, and the ratio between them in ice and in the shells of marine organisms depends on temperature and ice volume.

• Lighter oxygen evaporates more easily, so the ratio locked in snow and in sea-floor shells shifts with how cold the climate is and how much water is held in ice sheets.
• Measuring the oxygen isotope ratio in an ice core or in fossil shells in ocean sediment therefore reconstructs past temperature and global ice volume.

Putting the records together

Ice cores combine two proxies in one archive: the trapped air gives past greenhouse gas levels, and the oxygen isotope ratio of the ice gives past temperature. Plotting them together shows carbon dioxide and temperature moving in step through the ice ages, evidence that greenhouse gases and climate are tightly linked. Crucially, the same records show current carbon dioxide is well outside the natural range, supporting the case for human-caused change.

How oxygen isotopes record temperature

It is worth understanding the physics behind the oxygen isotope proxy, because SCSA can ask you to explain rather than just state it. Water contains a lighter oxygen isotope and a heavier one. The lighter isotope evaporates slightly more readily and the heavier one condenses out of clouds slightly sooner, so the isotopic make-up of precipitation depends on temperature: in colder conditions, snow reaching the poles is more depleted in the heavy isotope. The ratio locked into each layer of glacial ice therefore acts as a thermometer for the time the snow fell. The same principle works in reverse in the ocean: during glacial periods, water rich in the light isotope is locked up in expanding ice sheets, so seawater, and the shells of plankton forming in it, becomes enriched in the heavy isotope. Measuring the ratio in fossil shells in sediment thus records both temperature and global ice volume, which is why ice-core and ocean-sediment isotope records can be matched against each other.