20 Jul 2016

World's warmest six months on record

8:50 pm on 20 July 2016

The planet has just had its warmest six months on record, with June marking 14 consecutive months of record heat.

The average global temperature so far this year was more than 1°C above the 20th century average, according to latest data from America's National Oceanic and Atmospheric Administration.

Greenpeace New Zealand executive director Russel Norman said humans were to blame.

"That means we're seeing the effects of all those greenhouse gases we've been pumping into the atmosphere are now heating up the planet," he said. "In time, of course, that will mean rising sea levels, increased storm events, increased droughts and all the rest of it."

"It's a very significant indicator of the impact of climate change," he said.

This weekend, tropical winds from north of Vanuatu would stream toward New Zealand, NIWA meteorologist Ben Noll said.

"You have this warm flow coming from the tropics and it's taken to a new level because you have this big high to the north and east and you also have this really big low to the southwest," he said.

"That amplifies the wind, it amplifies the gradient and it pulls the air from the north even further across New Zealand."

Mr Noll said temperatures were forecast to be 5-7°C warmer than average this weekend on what would normally be one of the coldest.

But, it would be very windy, and the West Coast would be wet.

"Saturday's a very wet day on the west of the South Island, some thunderstorms as well are possible, maybe flooding and slips," he said.

"The nasty weather will come up the country later on Saturday."

Wellington would have to look out on Saturday evening, he said.

Winds were expected to reach 120 km/h and were "perhaps the gustiest winds we've seen this Winter season", he said.

But how is the weather on any given day connected to the bigger picture of a warming world?

"There isn't a direct relationship in the sense that you can point at a given storm and say this is due to climate change," said James Renwick, a professor of climate science at Victoria University.

"But there's an overall relationship, and other scientists have made the point that the weather's already different to what it would've been if we hadn't increased greenhouse gas concentrations in the atmosphere," he said.

Dr Renwick said greenhouse gases have helped warm the world, meaning there was more moisture in the air so storms could be bigger and wetter.

In Golden Bay in December 2011, nearly half a metre of rain fell in one day - double the previous levels - and Dr Renwick said that was made more likely because of the greenhouse gas effects.

Similar studies of heatwaves in Europe or the US show the events are two, three or even five times more likely than a couple of hundred years ago.

"The maximum amount of moisture that the air can have in it is purely a function of temperature," he said.

"The warmer the air, the more evaporation [from oceans], and the more moisture the air can hold in it.

"So when it rains - and that moisture condenses and converges together into a cloud - more of it falls out," he said.

So while the relationship between a warm weekend in winter and climate change is unclear, Dr Renwick said, in a warming world, the extremes increased.

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