Farmers who thought they saw bad droughts in the past 30 years may have had only a taste of the horrors yet to come from climate change, though Otago farmers will suffer less than those in other parts of the country, a new report shows.
A consortium of researchers from Niwa, AgResearch, Motu, Landcare, Infometrics and GNS showed that while "average" years will cause little change in farm productivity over the coming century, some regions will be badly hit by droughts.
"The projected driest years in the 2030s and 2080s are worse for national average production than the worst climatic year between 1971 and 2002," the scientists warned in the EcoClimate report.
In the worst year of the past three decades, estimated farm production was 64 percent of the long-term average for dairy, and 67% of the long-term average for sheep and beef.
"Under the climate change scenarios, the projected worst years reach only 52% (for dairy) and 50% (for sheep/beef) of the long-term average production," they said today.
"Extremes, such as drought, may become worse and there are differences in how individual regions are affected".
By the 2080s, some regions will be gutted in the worst droughts: export revenues for dairy will fall to 16% of average in Gisborne, 19% in Hawke's Bay, 21% in Bay of Plenty, 22% around the Wellington region, and 29% in the Auckland area.
Even traditional dairying regions will face tough times in these droughts, with production from Northland down to 39%, Waikato down to 49% and Taranaki down to 57% of average export revenues.
Farmers in some parts of the South Island will fare comparatively well in this extreme scenario: Southland will still produce 90% of its usual export milkflows, the West Coast will barely be affected, at 98%. Otago will only fall back to 72%.
Even the worst affected of the South Island regions, Tasman, at 58%, will continue producing more of its export milk than the best of the North Island regions.
"Improvements in production are projected for both dairy and sheep/beef in the West Coast and Southland," the EcoClimate reports said. "These regions are likely to remain wetter with a warming climate".
The EcoClimate report, Costs and Benefits of Climate Change and Adaptation to Climate Change in New Zealand makes projections on future temperatures, rainfall and droughts.
Changes in El Niño conditions may influence the impacts of climate change.
For an average year later in the century, falls in production are forecast in some dry east coast locations and Northland, with increases in Southland and the South Island West Coast.
The projections show no strong increase or decrease during the coming century in "average year" production when averaged over the whole country.
Projected national dairy production ranges from 96% to 101%, and projected sheep/beef production from 91% to 96% of the 1972-2002 average.
Flows in the rivers fed from the Southern Alps in Canterbury and Otago are expected to increase (on average) under most climate change scenarios.
But though the water supply reliability from irrigation systems fed from this source may increase, "it is yet unclear whether the increased water supply will compensate for an increase in demand".
Outside Canterbury and Otago, agricultural areas in eastern New Zealand - Northland, Hawke's Bay and parts of the Tasman and Marlborough regions - are likely to face greater shortages of water in future.
Rising average temperatures mean that compared to 1990, the number of growing degree days in the 2080s may increase by as much as 500-800 growing degree days for most of the North Island - where the average growing degree days now exceed 3000 - slashing the numbers of frosts.
As the century progresses, the drying of pasture in spring is likely to begin earlier and an earlier start to pasture growth in the late winter or spring may enable farmers to bring forward their lamb slaughter.
"The primary production sector is the engine room of our economy and it will be affected by climate change driven by global warming," said MAF's director of natural resources policy Mike Jebson.
"For an average year in the future, the predicted changes are small when averaged across the country, but different parts of the country are affected differently, with the west becoming wetter, the east drier and all of the country becoming warmer," he said.
Mr Jebson said the projected national decline in production for the driest years in the future calls for more emphasis on the need to drought-proof farms.