Our seas are heating up. Here's why

Tuesday, 10 April 2018

Sea surface temperatures around New Zealand were up to six degrees Celsius warmer than usual in areas.

New Zealand's record-breaking summer was partly driven by a marine heatwave, an occurrence which is increasing in number, frequency and intensity.

From 1925 to 2016, the worldwide frequency of marine heatwaves has increased on average by 34 per cent and the length of each heatwave has increased by 17 per cent, according to new research.

Together, that led to a 54 per cent increase in the number of marine heatwave days every year, the study found.

Lead author of the study published in Nature Communications, Dr Eric Oliver from Dalhousie University, Canada, said: 'Our research also found that from 1982 there was a noticeable acceleration of the trend in marine heatwaves.

* New Zealand's warmest start to a year on record

* Why record sea surface temperatures are going 'off the chart'

Total number of marine heatwave days globally. Globally averagedtime series of total marine heatwave (MHW) days over 1982–2016.

* It was officially New Zealand's hottest summer on record**

'While some of us may enjoy the warmer waters when we go swimming, these heatwaves have significant impacts on ecosystems, biodiversity, fisheries, tourism and aquaculture. There are often profound economic consequences that go hand in hand with these events.'

Over summer, sea surface temperatures were as much as six degrees Celsius above average in the Tasman Ocean, with the marine heatwave beginning about the start of November.

The country has just had the warmest start to a calendar year on record.

Niwa has said marine heatwaves were also present in other years with warm starts - 1998, 1999 and 2016.

Victoria University lecturer and Raumati resident Professor James Renwick.

WHAT THE EXPERTS SAY

Dr Jim Salinger, Honorary Research Associate at the University of Otago, told the Science Media Centre the study was 'particularly relevant' given our recent, unprecedented, marine heatwave.

'With further global warming, the likelihood of marine heatwaves is very likely to increase with anthropogenic climate change. This summer's marine heatwave, as well as having impacts on ocean life, had widespread impacts on terrestrial and managed ecosystems such as New Zealand's glaciers and farming, as examples. Should the frequency of these increase so will the impacts on New Zealand's land as well as marine ecosystems.'

Professor James Renwick from Victoria University said the increasing trend in the occurrence, duration, and intensity of marine heatwaves lined up closely with observed increases in temperatures and temperature extremes over land - and with more rapid melting of glacier ice worldwide this century, and accelerated loss of ice from the major ice sheets in Greenland and Antarctica.

'Climate change is obvious everywhere we look. As the paper points out, further global warming will go hand in hand with even more marine heatwaves, as well as increases in many other extremes of weather and climate.

'Many marine species are very sensitive to temperature and such rapid changes in the oceans are bound to be affecting marine food webs, especially when combined with the continued acidification of the oceans. The only way to arrest these changes is to drastically reduce global emissions of greenhouse gases.'

WHAT DROVE THE HEATWAVE

In November, NIWA meteorologist and forecaster Ben Noll said the marine heatwave was driven by a heatwave in the south of the country.

La Niña conditions and higher than usual surface air pressure during summer also contributed.

'Ocean temperatures can bring about warmer than average air temperatures and likewise warmer than average air temperatures can increase ocean temperatures,' he said.

'It's a bit of a positive feedback, if you will.'

Higher atmospheric pressure created tranquil weather which calmed conditions over land and sea, leading to less wind.

'Winds are what churn up the sea and bring colder seas to the top, and with high pressure you don't get that mixing so the sea at the top is stagnant and is heated more effectively by the sun with no storms mixing it up,' Noll said.

Noll has added that a persistently positive Southern Annular Mode and climate change, acting as a long-term tailwind to temperatures, also contributed.

'Our marine heatwave was a striking feature on both a regional and global climate scale, as at one point it represented some of the most unusually warm seas in the world.'

HOW THE RESEARCH WAS CONDUCTED

The researchers contributing to the new study used a variety of observational datasets to reveal the trend of increasing marine heatwaves, combining satellite data with a range century long datasets taken from ships and various land based measuring stations.

They then removed the influences of natural variability caused by the El Nino Southern Oscillation, the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation to find the underlying trend.

Co-author Professor Neil Holbrook from IMAS at the University of Tasmania said: 'There was a clear relationship between the rise in global average sea-surface temperatures and the increase in marine heatwaves, much the same as we see increases in extreme heat events related to the increase in global average temperatures.

'With more than 90 per cent of the heat from human caused global warming going into our oceans, it is likely marine heatwaves will continue to increase. The next key stage for our research is to quantify exactly how much they may change.

'The results of these projections are likely to have significant implications for how our environment and economies adapt to this changing world.'

You can find out more about marine heatwaves here.