Climate change in New Zealand

Until a few year ago it was widely believed that New Zealand would be spared the worst consequences of climate change. Temperature rises more over land than over sea, and the New Zealand climate is dominated by the oceans. Although the weather is changeable and often stormy, the climate is temperate, startlingly so to people used to continents: in Wellington typical maximum temperatures are 20°C in summer and 12°C in winter. In addition, we are not close to the north pole, where rapid changes have led to large regional climate changes in the northern hemisphere. And tropical cyclones are rare.

While all this is true, there are signs that climate change is affecting us.

NIWA’s main long-running temperature record is their ‘seven city’ series. It shows a warming trend of 1.1° since 1909, close to the global average.

January 2018 was the warmest month since reliable records began in 1867 – 3°C above the 1981-2010 average.

A 2001 study in Nature, “Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change“, found that the ozone hole has led to an increase in a atmospheric pattern called the Southern Annular Mode, with significant changes to the summer climate in New Zealand. For example, the record-breaking summer of 2017-2018 has been linked to the Southern Annular Mode. 

The East Australia Current, an energetic warm current linking the Pacific and Indian oceans that eventually turns east towards New Zealand, has become warmer (2.28°C/century), saltier, and extended southwards by 350km in the past 60 years. These changes have been linked both to ozone depletion, changes in the Southern Annual Mode, and to increasing atmospheric CO2. A study by Ridgeway and Hill concluded that “There is strong consensus in climate model simulations that trends observed over the past 50 years will continue and accelerate over the next 100 years.”

There is some evidence that New Zealand may be beginning to suffer from changes in rainfall patterns, similar to the “weather bombs” that have affected parts of the northern hemisphere in recent years. For example, the central Bay of Plenty experienced a “1 in 100 year” rainfall event in July 2004; “Phenomenal, unprecented high rainfall”, a “1 in 500 year event” in May 2005; and in April 2017, the remnants of Cyclone Debbie led to record flows on the Rangitaiki river, which breached the stopbanks and flooded Edgecumbe. This last event was just a month after widespread extreme rain events affected many parts of the North Island, in a pattern linked to climate change. The rainfall in the Hunua ranges (275mm in 1 day, 454mm in 5 days) affected Auckland’s water supply.

Glaciers

New Zealand is proud of its glaciers. There are more than 3000, although only the Tasman, Fox, and Franz Josef glaciers are well known, the latter two being famous for their combination of low altitude and low latitude, and the drama of a glacier in a rainforest. They also became famous for growing for some decades. A recent study in Nature, “Regional cooling caused recent New Zealand glacier advances in a period of global warming“, examines this in depth. (In 2005, more than half of all known advancing glaciers were in New Zealand!)

Overall, however, New Zealand’s glaciers lost 25% of their volume in the past 20 years. Since 2012, the front face of Franz Josef glacier

has been too dangerous to visit, while east of the main divide, in 1990 Lake Tasman formed at the terminus of the Tasman Glacier and is now 7km long:

Some of the events of the past three years, such as the record loss of snow cover in the Southern Alps

and the Port Hills fires

are likely related to El Nino and the record Tasman sea temperatures of 2016-2017. If so, they may be a harbinger of what we can expect in most years after another decade of global warming.

Robert McLachlan