Ice core study shows plants absorb less carbon in a warming world

The Earth’s land biosphere takes up less carbon in a warming climate according to research published in Nature Geoscience in July.

The research, led by Dr Mauro Rubino of CSIRO and the Seconda Universita di Napoli, used bubbles trapped in Antarctic ice cores during the Little Ice Age (1500 to 1750 AD), to measure the sensitivity of the Earth’s land biosphere to changes in temperature.

The study focused on the Little Ice Age era (a period of severe winters and expanded glaciation, particularly in Europe and North America), as it occurred before the growth of industry and agriculture affected carbon dioxide concentrations.

Previous studies have shown that carbon dioxide levels in the atmosphere went down slightly during the Little Ice Age. But it was not known whether this was due to the response of the terrestrial biosphere (plants and soils) to temperature change (cooling), or to vegetation re-growth due to land-use changes.

Australian Antarctic Division ice core scientist and study co-author, Dr Mark Curran, said the research team used ice cores to look at changes in the carbon-13 isotope ratio — a signature from land plants. This confirmed that the carbon dioxide changes during the Little Ice Age originated from the land biosphere.

“Ice cores preserve atmospheric gases in the bubbles trapped in the ice, and many of the samples used in this study were collected from the Australian Antarctic Program’s Law Dome site,” Dr Curran said.

“The very high detail preserved in the Law Dome ice cores was key to unlocking this information.”

The team then looked at another ice-bound natural gas, carbonyl sulfide. Like carbon dioxide, carbonyl sulfide is taken up by plants during photosynthesis, but unlike carbon dioxide, it is not released back into the atmosphere when the plants respire at night.

Measurements of carbonyl sulfide ruled out vegetation regrowth as a contributor to the decline in carbon dioxide and confirmed that the decline was related to cooling of the land surface.

This result implies that a warmer climate in the future will lead to a reduction in carbon dioxide uptake, providing a positive feedback to greenhouse warming as more emissions remain in the atmosphere.

Study co-author and CSIRO scientist, Dr David Etheridge, said the research showed that for every degree Celsius of global temperature rise, 20 parts per million less carbon dioxide is absorbed by the land biosphere.

“It has long been assumed that as the Earth’s surface warms, the ability of land-based plants to store carbon is reduced. This study puts definitive numbers around this positive feedback,” Dr Etheridge said.

About half the carbon dioxide emitted by human activities since 1850 has been taken out of the atmosphere by the land biosphere and the ocean. Uncertainties in how this uptake might change in the future has been a significant source of uncertainty in climate projections.

These new results will reduce the uncertainties of climate models such as the Australian Community Climate and Earth System Simulator (ACCESS), which is used to project future climates scenarios from human greenhouse gas emissions.

Wendy Pyper and Eliza Grey
Australian Antarctic Division