Context: Researchers have unearthed evidence of rainforests near the South Pole 90 million years ago, a finding which suggests that the climate at this time was exceptionally warm with a higher level of carbon dioxide in the atmosphere than previously thought.
Background 

• The scientists discovered forest soil pertaining to a time between 145 and 66 million years ago within 900 kilometres of the South Pole.

Key Findings

• In the study, they analysed preserved roots, pollen, and spores from this soil, and showed that the world at that time, the Cretaceous period, was a lot warmer than previously thought. Even during months of darkness, swampy temperate rainforests were able to grow close to the South Pole, revealing an even warmer climate than expected.
• According to the study, the carbon dioxide levels in the atmosphere were higher than expected during the mid-Cretaceous period, 115-80 million years ago, challenging current climate models of the period. 
• The mid-Cretaceous was the heyday of the dinosaurs but was also the warmest period in the past 140 million years, with temperatures in the tropics as high as 35 degrees Celsius, and sea level 170 metres higher than today.
• However, little was known about the environment south of the Antarctic Circle at this time. Now, researchers have discovered evidence of a temperate rainforest in the region, such as would be found in New Zealand today.
• This was despite a four-month polar night, meaning for a third of every year there was no life-giving sunlight at all.
• According to the researchers, the presence of the forest suggests average temperatures in this region were around 12 degrees Celsius, with little likelihood for the presence of an ice cap at the South Pole at the time. 
• The study noted that the evidence for the Antarctic forest is based on a core of sediment drilled into the seabed near the Pine Island and Thwaites glaciers in West Antarctica. 
• On scanning this section with an X-ray CT scan, the scientists discovered a dense network of fossil roots, which was so well preserved that they could make out individual cell structures.
• To reconstruct this ecology, the team assessed the climatic conditions under which the plants’ modern descendants live, as well as analysing temperature and rainfall indicators within the sample. 
• They believe the average summer temperatures may have been around 19 degrees Celsius and water temperatures in the rivers and swamps to be 20 degrees. 
• According to the study, the amount and intensity of rainfall in West Antarctica at this time may have been similar to those in today’s Wales.
• To get these conditions, the researchers conclude that 90 million years ago the Antarctic continent was covered with dense vegetation, there were no land-ice masses on the scale of an ice sheet in the South Pole region, and the carbon dioxide concentration in the atmosphere was far higher than previously assumed for the Cretaceous.
• Earlier it was general assumption that the global carbon dioxide concentration in the Cretaceous was roughly 1000 ppm. But in latest model-based experiments, it took concentration levels of 1120 to 1680 ppm to reach the average temperatures back then in the Antarctic.