Jamie Morton is the NZ Herald's science reporter.

Grim message on climate change

New Zealand authors spell out extreme outlook for our country

Scientists are now almost certain that mankind's carbon emissions are warming up the planet.

As the world's most important climate report was released internationally last night, its New Zealand authors spelt out the outlook for our country and our closest neighbours.

The UN Intergovernmental Panel on Climate Change's draft fifth assessment report (AR5) warned that if the world could not rein in carbon emissions to a cap of one trillion tonnes of carbon - a budget already half spent - it would not be able to hold global warming back within 2°C, causing widespread extreme weather, drought and rising sea levels.

The report says that by the end of this century, the world's climate would have warmed by at least this level.

The report says it is a virtual certainty that natural variables alone could not have fuelled changes that since 1950 have warmed the troposphere and warmed the stratosphere.

Around the globe over this period, it was considered "very likely" the number of cold days and nights had dropped while the rate of warm days and nights had increased.

It pinpointed the level of warming across land and ocean temperatures since the turn of the 20th century to about 0.9°C, while global mean surface temperature change between 2016 and 2035 would likely be in the range 0.3°C to 0.7°C.

Stretching to more than 2,000 pages drawing on thousands of peer-reviewed studies, and put together by more than 250 lead authors, the report updates its 2007 predecessor, which stirred controversy after critics found some errors.

In New Zealand, extreme rainfall events will become more frequent and intense by the end of the century, while drought risk would increase substantially, especially in the east and north of the country.

Elsewhere in the country, more high temperature extremes and fewer cold extremes were virtually certain to become the norm.

"Longer observational records, improved models and better understanding tell us that climate change will be on-going this century and beyond and will bring significant changes to New Zealand and to the Pacific," said Victoria University climate scientist Dr James Renwick, a contributor to the report.

"The South Pacific Convergence Zone, a major feature of rainfall variability in the tropical Southwest Pacific, may become more variable in its movement and rainfall intensity, which would be associated with increased risk of both floods and droughts for many of our Pacific neighbours."

The number of tropical cyclones was not likely to increase, but they would become more powerful.

Professor Tim Naish, the director of Victoria University's Antarctic Research Centre, said the report also showed carbon dioxide was now at levels unprecedented in at least the last 800,000 years.

"It is very likely that the Arctic sea ice cover will continue to shrink and thin as the region is expected to warm more rapidly than other areas of the world," he said last night.

"In addition, the volume of the polar ice sheets and glaciers globally will continue to decrease, contributing to a global mean sea-level rise between 26-82cm by the end of the century, depending on the greenhouse gas concentration pathway we end up following."

"The report ... suggests many aspects of climate change, including polar ice sheet melt and sea-level rise will continue for centuries, even if carbon dioxide emissions were stopped."

The UN report at a glance

• It is "extremely likely" human activities caused more than half of the observed increase in global mean surface temperature since 1950 - and almost certain that natural variability alone has not been responsible.

• For New Zealand, extreme heavy rainfalls will have become more frequent and more intense by the end of the century, while drought risk will increase substantially - notably in the east and north of the country. More high temperature extremes and fewer cold extremes are virtually certain almost everywhere.

• It is very likely Arctic sea ice will continue to shrink and thin. The volume of the polar ice sheets and glaciers globally will continue to decrease, contributing to a global mean sea-level rise between 26-82cm by the end of the century.

• The frequency of tropical cyclones is unlikely to increase, but they will become stronger. Rainfall intensity in the South Pacific Convergence Zone may become more variable, increasing the risk of floods and droughts to many of our Pacific neighbours.

- NZ Herald

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