The mercury keeps rising

The heat is on. Millions of people across Europe, western parts of Canada and the United States, South Korea and Japan have been sweating their way through a blisteringly hot summer. Little long-term relief appears likely in some places for a month or more yet.

Weeks of temperatures in the high 30s and 40s have sucked every gram of moisture out of soils and vegetation. The inevitable, deadly, fires have turned furnace into inferno, most notably in California, British Columbia and Greece.

New Zealand firefighters have answered the call and are flying in to provide overseas colleagues with much-needed support and to give them time to rest. The Californian fires are now the largest in the United States' history.

The heat has been on in our part of the world too, albeit with winter's moderating influence.

Parts of Australia, notably New South Wales and swathes of inland Queensland, have had an incredibly dry autumn and winter, bad enough for politicians, farmers and others in what is naturally a desiccated continent to wonder if this is the most severe drought in its history.

Reports this week suggest 99% of New South Wales, which produces more than a quarter of the country's agricultural income, is now in drought. Farmers are said to be praying for rain after planting crops, and stock are being sent to the freezing works because they cannot be fed.

The weather has been less harsh on this side of the Tasman Sea, as is often the case, but here also the seasons appear confused. What we might expect in the South in winter has largely failed to eventuate.

Mild nor'westers continue rolling in, keeping polar southerlies and their snowy, frosty accompaniment at bay. Spring, in fact, looks to have sprung, though there will no doubt be nasty surprises to come. And now forecasters are already making tentative predictions of the summer to come.

Both Niwa and MetService are flagging a chance of an El Nino weather cycle during spring and summer. Sea-surface temperatures around the equator across the Pacific Ocean are already slightly warmer than average, MetService says, and the prospects of El Nino-influenced weather have risen to about 65% during spring and 70% in summer.

Some media commentators have pounced on these early prognostications and have predicted another much warmer-than-average summer. But it is not as clear-cut as that.

El Nino generally brings drier-than-normal conditions along the east coasts of both islands and, in the late summer and autumn, this can combine with hot northwesterlies to bring droughts. But in the South, especially during spring and early summer, the phenomenon often generates lots of cold, squally, southwesterlies with frequent rain.

It would be unlikely an El Nino summer would bring heat to top last year's record-setting warmest summer since readings began. The chief influence on that was a rare ''marine heatwave'', with sea-surface temperatures around the country up to 8degC hotter than average.

The heat afflicting many northern parts of the globe cannot be sheeted home directly to climate change, in the same way no single heavy rainfall event or snowstorm can be. But it is the frequency of such occurrences that may reflect changing climate.

In the case of the torrid northern hemisphere summer, meteorologists blame it on changes in the tracks of the jetstreams - ribbons of very strong winds high in the atmosphere that steer weather systems. This has set up a blocking pattern of light winds at the surface, allowing heat to persist.

Whether the jetstream anomaly is down to climate change is difficult to know, although it may be.

Such broad-scale changes, however, give more urgency to efforts to reduce emissions before it is too late.

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