Feature

Our Shifting Climate

Evidence is mounting that there has been a shift in New Zealand's climate over the past 20 years.

Dr Jim Salinger and Dr Brett Mullan

East Coast and Northland dairy farmers have had enough of the 1 in 50-year drought happening more frequently, and are shifting their herds to Southland. On the West Coast of the South Island, and in the far South, devastating floods are becoming a familiar, rather than a rare event. In the Bay of Plenty, where winters have been warmer than usual, kiwifruit flowering has been poor.

Recent scientific research shows these unusual droughts, floods and crop failures are not just a short-term anomaly. A marked change in the Pacific climate has had widespread effects, reaching as far as New Zealand.

From 1977, the frequency of El Niño events in the Pacific increased, and at the same time the anticyclonic belt, which brings fair weather to northern New Zealand, strengthened. Westerly winds of the roaring forties became stronger over central and southern New Zealand, and brought more rain to the west and south of the South Island.

Average annual rainfall increased by 10%, annual sunshine dropped by five percent, and there was a big increase in the number of severe floods, from one in the two decades before 1977, to 11 damaging floods since that date. The Franz Josef glacier, which had receded almost two kilometres since 1950, started to advance again, fed by the moisture-laden westerly winds. By this year, it had moved forward a kilometre.

Areas sheltered from prevailing westerly winds experienced opposite effects. It became sunnier in the east, with a five percent increase in annual sunshine hours in Gisborne and Hawke's Bay. In the north and east of the North Island, annual rainfall decreased by 10%. Those regions suffered four devastating droughts since 1977 -- twice as many as the previous 21 year period.

Pacific Decadal Oscillation

These changes around 1977 in Pacific weather did not just affect New Zealand. A much bigger picture has emerged in the last decade from intensive international research into "decadal climate variability".

It appears that the 1977 climate shift is but the most recent flip in what a research group at the University of Washington, Seattle, is calling the "Pacific Decadal Oscillation". Previous reversals around 1925 and 1947 have been identified in North Pacific climate data, where the effects of the climate shift seem to be most dramatic.

The picture of periodic reversals in the Pacific Decadal Oscillation adds a new perspective to our interpretation of climate records and global warming.

The Intergovernmental Panel on Climate Change, an international scientific body established by the United Nations to assess information on climate change, has concluded that the globe has warmed substantially this century. We can expect to see increased warming next century as a consequence of rising greenhouse gas concentrations in the atmosphere. Regionally, however, these climate trends could be masked, or enhanced, by decadal circulation fluctuations.

It might have been expected that the 1977 shift that brought more El Nio events and a strengthening of westerly winds over New Zealand, would have also lowered New Zealand temperatures. This has not happened. Instead, global warming has seen temperatures, particularly those at night, continue to increase.

Since 1977, average night minimum temperatures have been 0.2^oC warmer than they were between 1951 and 1976. This has reduced the number of frosts. Many frost-prone areas now get at least ten frosts fewer each year than they used to. The more frequent westerly winds have also increased the number of hot days in eastern areas.

Fewer frosts have also lowered the quality of pastures in the central and lower North Island. Between 1976 and 1986 pastures with 40% or more of the lower-quality paspalum grass spread south from the north of the North Island between Hamilton and Gisborne, to the vicinity of Palmerston North. In addition, warmer winter temperatures brought more potential problems with pests and diseases.

Higher temperatures have been bad news for Bay of Plenty kiwifruit orchardists. Kiwifruit vines need chilly nights when their flower buds are developing, and in warm winters like this last one, Bay of Plenty kiwifruit orchardists are having to put up with relatively poor flowering unless they use dormancy-breaking chemicals.

Wine, however, is another story. The drier, hotter summers in eastern areas of the North Island have been perfect for grape growing, leading to bumper wine vintages.

Past Changes

New Zealand's climate has changed through the ages, sometimes quite abruptly. Last century, for example, when the first European colonists arrived, the country was much colder than it is today. This century there have been two significant shifts in New Zealand's climate. In the 1950s, the climate warmed abruptly. Records show that New Zealand temperatures increased by an average of 0.5^oC.

From 1951 to 1976 the westerly and south westerly winds which prevail over New Zealand weakened, and there were more frequent winds from the north east. It is tempting to link this 1950 change in New Zealand climate with the reversal around 1947 in the Pacific Decadal Oscillation.

The second significant shift, as our research has just recently shown, came from 1977, with changes in the frequency of El Nio events. The big question remaining unanswered is "When will the next Pacific Decadal Oscillation reversal occur?".

It would be sensible for New Zealanders who depend on the climate for their livelihood to start planning ahead.

Predicting the climate is never a certain science, but there's a better than even chance the recent changes in climate may be with us for at least the next several years.

Massive Ocean Current May Provide Clues to Global Warming

In the depths of the remote southwest Pacific Ocean there flows the world's largest deep ocean current, one that is 100 times the size of the mighty Amazon River. Racing at depths between 2,000 and 5,500 meters this Deep Western Boundary Current (DWBC) forms part of a global system of ocean circulation which distributes heat around the planet, and may play a key role in controlling climate change. This current channels 40% of the world's newly formed, cold deep water to the oceans.

The DWBC's role in controlling these climatic changes will be studied by a team of 26 scientists representing nine countries during the next research expedition of the Ocean Drilling Program (ODP). Scientists from New Zealand's National Institute for Water and Atmospheric Research have provided most of the site survey data for this expedition, enabling scientists to pin-point the spots to be drilled with no chance of hitting pockets of oil or gas.

The Earth has undergone global warming many times before. During the last 2.5 million years, Earth has been subjected to 50 cycles of glaciation followed by deglaciation, the last glacial period ending only 15,000 years ago. The ODP scientists will seek to resolve questions about climate change by analyzing samples of deep-sea mud, which are shaped by the currents to form great mounds on the sea floor. Core samples will be collected using the world's largest scientific drill ship, JOIDES Resolution.

As the DWBC passes from the Southern Ocean into the Pacific, it runs adjacent to the landmass of New Zealand. Mountains associated with the active faults and volcanoes of New Zealand provide an abundant source of eroded rock detritus. This sand and mud is fed into the path of the DWBC along several large deep-sea channels. Under the influence of the current, the fine-grained muds are then molded into huge deep-sea sediment drifts. Some of these drifts are several hundred kilometers long, and their sedimentary layers preserve a unique archive of the changes which have occurred in climate and current strength.

"Previous ODP studies of deep-sea sediment drifts in the North Atlantic have contributed enormously to our understanding of climate change in the northern hemisphere," explains team leader Nick McCave of Cambridge University. "I anticipate that southern hemisphere drilling will result in exciting advances in knowledge, and enable a truly global picture to be ascertained. The area east of New Zealand is essentially unexplored territory for the drilling project. There are only three deep sea drill holes in this area, which is equivalent to the size of the North American Basin of the North Atlantic Ocean, where nearly 100 holes have been drilled. This is pioneering work."

To investigate the history of the DWBC and its sediment drifts, the scientific team will take core samples as deep as 500 metres below the seafloor, using advanced drilling technology aboard the JOIDES Resolution. A hydraulic piston corer pushes directly into the upper layers of sediment, enabling scientists to recover the delicately layered deep-sea muds in an almost undisturbed state.

Subsequent studies of the core materials, both aboard the ship and in land-based laboratories, will allow the scientists to reconstruct the climate changes which have occurred in the southern Pacific ocean, and changes in the strength of the DWBC and associated oceanic fronts such as the subtropical convergence. Scientists will continue to study whether global warming changes the strength of the current. They also want to know if climatic feedback resulting from changes in the strength of the current would cause further warming still (super-greenhouse), or might it rather trigger a cooling and the onset of the next glaciation.

"Computer models are an important aid to understanding global climate change," says Bob Carter of Australia's James Cook University. "To be useful they must be based on an understanding of the experiments in climate change that have been undertaken already by planet Earth. The results of our expedition will undoubtedly contribute greatly to our understanding of such past climatic experiments by providing hard data to constrain computer-based speculation."

See the ODP Web Site for reports on the work undertaken on Leg 181:
http://www-odp.tamu.edu/

Brett Mullan is a senior scientist with NIWA in Auckland.