New Zealand faces a future of flood and fire
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New Zealand has grappled with two consecutive extreme weather events – massive flooding followed by a cyclone – that have claimed at least 12 lives and left hundreds of thousands without power. The strong winds and waters of Cyclone Gabrielle washed away coastal roads in the North Island and splintered and destroyed bridges. Landslides have blanketed the tarmac in slippery mud, and homes and roads on the other side have been submerged just weeks after heavy rains also caused widespread flooding. For only the third time in its history, the country has declared a national emergency.
New Zealand’s climate change minister, James Shaw, wasted no time in pointing the finger at the root cause of the weather disasters, telling New Zealand’s parliament: “It’s climate change.”
He may be right, but evidence from attribution studies is still lacking, says James Renwick, a climate scientist and professor at Victoria University of Wellington. The cyclone itself is not uncommon for New Zealand, as they regularly whirl out of the tropics and come close enough to sound the alarm, he says. “We line up for these things fairly regularly. Some of them aren’t that remarkable and some are absolutely disastrous,” says Renwick.
But our warming planet may have increased this cyclone’s ferocity due to warmer ocean water, says Olaf Morgenstern, an atmospheric scientist at New Zealand’s National Institute of Water and Atmospheric Research. Hotter oceans mean that when a cyclone hits, “it gets stronger, holds more moisture and more energy, and maintains its energy longer,” he says.
New Zealand has also experienced marine heat waves associated with La Niña, a cyclical Pacific weather system that has dominated the region for the past three years. These could have boosted the tropical cyclone. “Because it was unusually warm, it didn’t lose that much intensity — it was still pretty strong when it got here,” Morgenstern says.
Record-breaking rain and flooding preceded the tropical cyclone and wreaked havoc on the North Island in late January — again appearing linked to climate change. January broke a century-old record for Auckland’s wettest month with 539 millimeters of rain, half of which fell in a single day. That was truly unprecedented, Renwick says, but the likely impacts of climate change on New Zealand will be more complex than just more rain.
The winds that blow across the country from west to east have the greatest influence on the regional climate. These ensure enormous amounts of rain, especially on the west coast of the South Island. Milford Sound, the famous fjord popular with tourists, is one of the wettest places on earth with a mean annual rainfall of 6.8 metres. The island’s mountains then displace moisture from the air as it flows over them, casting a rain shadow that leaves the east coast relatively dry.
But even small changes in wind direction or wind speed can result in large changes in local climates, Renwick says. Climate models suggest these westerly winds are likely to get stronger. “Whether or not they are that much over New Zealand is difficult to answer because there are a few touching parts to this story, but the bigger picture gets a little stronger over time,” he says. An increase in strength is expected to bring more rain to the west coast and less to the east coast, resulting in hotter temperatures.
