Kiwi scientists have reconstructed more than 34 million years of our climate - including a period when New Zealand was up to 8C warmer - using 2000 samples of fossilised tree pollen.

Their study, published this month in the journal Earth and Planetary Science Letters, sheds new light on an intriguing land before time, when New Zealand's islands lay further south and were home to tree species no longer found here.

The work was based on fossilised tree pollen that scientists have extracted from sedimentary rocks from around the country over the past 60 years.

By comparing the pollen found in the rocks to the modern distribution of closely-related trees, the scientists were able to reconstruct changes in air temperature and rainfall from Oligocene time, 34 million years ago, to the present day.

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This was the first time such a long record of New Zealand terrestrial climate had been compiled in this way.

In their database, the scientists have used pollen from about 75 plant genera - mostly trees - and the distribution of different species tells a story about temperature and rainfall in different epochs in New Zealand's history.

Prominent in this study were pollen from tawhai and other southern beeches, some of which are today restricted to New Caledonia and New Guinea, rimu, rewarewa, and other members of the family Proteaceae that are today only found in Australia.

The pollen grains are microscopic and generally need to be magnified several hundred times for meaningful study by scientists.

Palynologist Joe Prebble, of GNS Science, with the new pollen-based New Zealand climate record (left), and a false-colour scanning electron microscope image of a pollen of the daisy family. Photo: Margaret Low / GNS Science.
Palynologist Joe Prebble, of GNS Science, with the new pollen-based New Zealand climate record (left), and a false-colour scanning electron microscope image of a pollen of the daisy family. Photo: Margaret Low / GNS Science.

"Other than computer climate models, these types of fossil climate records are the only way we have to understand how climate responded to elevated carbon dioxide concentrations in the past, and might respond in the future," lead author and GNS Science palynologist Joe Prebble explained.

The new results effectively filled a gap in scientists' understanding of the New Zealand climate during the elevated carbon dioxide conditions of the middle Miocene period - 15 million years ago - and how the climate subsequently cooled.

Their record shows a close coupling of New Zealand temperature with changes in Antarctic climate, particularly after the Antarctic ice sheet enlarged substantially about 14 million years ago.

Before 14 million years ago, a time interval known as the "Miocene Climatic Optimum", mean annual temperatures in New Zealand ranged between 18C in the south and 21C in the north.

For comparison, modern mean annual temperatures in New Zealand ranged between 10C and 16C.

The elevated temperatures in the Miocene occurred when the New Zealand landmass was further south than today.

During the Miocene, the islands of New Zealand were further south; between the latitudes of Christchurch and Campbell Island.

The pollen samples used for the work were taken from a publically accessible national database of New Zealand fossils and represent observations made by many scientists.

"This database of fossil observations is a really essential resource to help us answer a range of questions about New Zealand's natural history," Prebble said.

"As well as helping us to understand past climate, it is an essential tool for assessing the age of rocks during exploration for minerals and petroleum, and for understanding the evolution of our unique plants and animals."