New Zealand's climate could revert to a hot, tropical state if the world's emissions aren't reined in, scientists have warned.
While one paper published this month outlined a risk of global average temperatures rising up to 5C higher than pre-industrial temperatures, another study by UK scientists has suggested New Zealand and Western Europe might experience yet more such dramatic "hothouse" conditions.
Scientists were interested in the early Paleogene period, which lasted from 56 million to 48 million years ago, because current CO2 levels were similar to those predicted for the end of this century.
"We know that the early Paleogene was characterised by a greenhouse climate with elevated carbon dioxide levels," said Dr David Naafs, of the University of Bristol's School of Earth Sciences.
"Most of the existing estimates of temperatures from this period are from the ocean, not the land - what this study attempts to answer is exactly how warm it got on land during this period."
In a study published in the journal Nature Geoscience, Naafs and his colleagues used molecular fossils of micro-organisms in ancient peat to provide estimates of land temperature 50 million years ago.
These revealed how annual land temperatures in Western Europe, as well as New Zealand, were actually higher than previously thought - between 23 and 29C - which was currently 10C to 15C higher than current average temperatures in these areas.
The results suggested that temperatures similar to those of the current heat wave that is influencing western Europe and other regions would become the new norm by the end of this century if CO2 levels in the atmosphere continue to increase.
"Our work adds to the evidence for a very hot climate under potential end-of-century carbon dioxide levels," added co-author Professor Rich Pancost, of the University of Bristol Cabot Institute.
"Importantly, we also study how the Earth system responded to that warmth. For example, this and other hot time periods were associated with evidence for arid conditions and extreme rainfall events."
The research team now planned to turn their attentions to geographical areas in lower-latitudes to see how hot land temperatures were there.
"Did the tropics, for example, become ecological dead zones because temperatures in excess of 40C were too high for most forms of life to survive?" Naafs said.
"Some climate models suggest this, but we currently lack critical data.
"Our results hint at the possibility that the tropics, like the mid-latitudes, were hotter than present, but more work is needed to quantify temperatures from these regions."
Victoria University climate scientist Professor James Renwick said that while the late Palaeogene climate was an example of a hothouse climate, it was important to note that continental forms and locations were vastly different at that point in Earth's history.
"New Zealand was pretty different-looking then, and was located [at a latitude] between 50S and 60S."
Australia was also connected to Antarctica, so there was no Antarctic Circumpolar Current to help regulate climate.
"The climate operated rather differently to today, though it was pretty warm because of the level of CO2 in the atmosphere."
Renwick said such a scenario would see an absence of ice, sea levels that were up to 60m higher, and a climate that wouldn't support the level of food production necessary to feed seven billion people.
While it would take centuries to millennia to reach such a point, he said, the threshold that committed the planet to an extreme climate future could be much sooner on the horizon.
New Zealand and climate change
• Under present projections, the sea level around New Zealand is expected to rise between 30cm and 100cm this century. Temperatures could also increase by several degrees by 2100.
• Climate change would bring more floods; worsen freshwater problems and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world.
• The latest greenhouse gas emissions inventory, which gives a picture of how much human-generated greenhouse gas is being emitted into and removed from our atmosphere, shows emissions as at 2016 have increased from 1990 levels by 19.6 per cent.
• New Zealand has pledged to slash emissions by 30 per cent below 2005 levels, and 11 per cent below 1990 levels, by 2030. The Government has also proposed new Climate Commission and a Zero Carbon Act, with goals for a carbon-neutral economy by 2050 and 100 per cent renewable energy by 2035.