Tropical Antarctica

Antarctica was once covered in highly diverse, near-tropical forests, according to 2012 published research from an international team, including several New Zealanders. The researchers, part of Expedition 318 to Wilkes Land, analysed pollen and spores buried in offshore Antarctic sediments to determine the ancient climate of the continent.

Warm climate in Eocene epoch

The pollen and spores collected tell a remarkable tale – about 48–55 million years ago during the Eocene epoch, the climate was very warm, even at the poles, due to high levels of atmospheric CO₂ that are estimated to be twice that of what they are today. The climate supported the growth of a diverse range of palms and ferns similar to those found in modern-day New Zealand.

Expedition 318 set out from Wellington in 2010 as part of the Integrated Ocean Drilling Program on board the specialised drilling ship JOIDES Resolution. The researchers drilled down over a kilometre into the ocean floor off the coast of Wilkes Land and extracted long cores of sediment for analysis.

Climate response to high atmospheric carbon dioxide levels

In their published research paper, the authors write that “the warmest global climates of the past 65 million years occurred during the early Eocene epoch, when the Equator-to-pole temperature gradients were much smaller than today and atmospheric carbon dioxide levels were in excess of one thousand parts per million by volume. Recently the early Eocene has received considerable interest because it may provide insight into the response of Earth’s climate and biosphere to the high atmospheric carbon dioxide levels that are expected in the near future as a consequence of unabated anthropogenic carbon emissions.”

By examining the sediment cores, the researchers found that the climate in lowland settings along the Wilkes Land coast supported the growth of a diverse range of plants and trees including:

• ferns and tree ferns (Lygodium Cyatheaceae)
• palms (Arecaceae)
• Bombacoideae (Malvaceae)
• Strasburgeria (Strasburgeriaceae)
• Beauprea (Proteaceae)
• Anacolosa (Olacaceae)
• Spathiphyllum (Araceae).

This vegetation tells the researchers what the climate was like – for example, palm and Bombacoideae pollen imply that winter temperatures remained substantially above freezing. Extant palms occur naturally only in regions with a coldest-month mean temperature of 5°C. Even warmer conditions are suggested by the record of Bombacoideae, which today occur where the coldest-month mean temperature are 10°C or more.

The researchers conclude that winters were extremely mild, warmer than 10°C, and frost-free despite polar darkness. The authors say this “provides a critical new constraint for the validation of climate models and for understanding the response of high-latitude terrestrial ecosystems to increased carbon dioxide forcing”. Summer temperatures likely ranged between 20°C and 27°C.

Role of warm ocean currents

The research underlines the extreme contrast between modern and past climate conditions in Antarctica and the extent of global warming during periods of elevated carbon dioxide levels in the atmosphere. However, the CO₂ content of the atmosphere 50 million years ago was not enough on its own to explain the almost tropical conditions in the Antarctic. Warm ocean currents reaching Antarctica were also an important factor in transferring heat to the continent. When the warm ocean current collapsed and the Antarctic coast came under the influence of cooler ocean currents, the tropical rainforests and lush coastal palm trees also disappeared.

The international research team, led by researchers from Goethe University and the Biodiversity and Climate Research Centre in Frankfurt, Germany, included micro-paleontologist Dr Ian Raine of GNS Science in Lower Hutt.

In a press release from GNS Science, Dr Raine said studies of warm periods in the geological past increased the knowledge of climate system mechanisms.

“Computer models indicate that future climate warming will be particularly pronounced in regions near the poles. Until now, however, it has been unclear how Antarctic terrestrial ecosystems responded in the geological past to a greenhouse climate with high atmospheric CO₂ concentrations.”

The study was published on 2 August 2012 in the leading science journal Nature.