KEY POINTS:
The impact of climate change on agriculture is not all bad, but the bad effects - especially more extreme and more frequent droughts - are worse than those experienced in the past.
That is the conclusion of a report by EcoClimate, a consortium of scientific and economic research organisations, for the Ministry of Agriculture and Forestry.
It is based on updated climate modelling by the National Institute of Water and Atmospheric Research (Niwa) aimed at providing more regional and seasonal detail of the expected effects of global climate change on New Zealand.
"For an average year in the future the predicted changes are small when averaged across the country," said MAF's director of national resources policy, Mike Jebsen.
"But different parts of the country are affected differently, with the west becoming wetter, the east drier and all of the country becoming warmer."
The growing season will get longer, especially in Southland and the West Coast. But although production is expected to increase there, it will decrease in Northland and some east coast areas.
In an average year in the 2030s or 2080s the expected impact on national export revenue from dairying, compared with now, is expected to be minimal. For sheep and beef farming it would be between 4 and 9 per cent lower.
But in extreme years droughts are expected to be worse than those experienced in recent decades, like the 1977/78 and 1997/98 episodes, and national production from pastoral farming would be almost halved.
Econometric modelling by the Treasury a few years ago found drought to be the second most potent influence on the New Zealand economic cycle, after export prices.
Droughts would become more frequent, said Niwa scientist David Wratt, and the work suggested the likelihood of the especially damaging case of a two-year drought might be higher.
"What is now a one-in-20-year drought might become a one-in-five-year one later in the century."
In the winter and spring all the models point to increased westerly winds, meaning more rain in the west of both islands while the east and north are drier.
For summer and autumn, the Niwa scientists are less certain. Different models point in different directions, but the average is for less frequent westerlies, which would mean drier summers and autumns in the west of the North island and wetter ones in Gisborne and Hawkes Bay.
The work takes no account of any increased risk of pests and diseases which might arise from higher average temperatures (about 2C over the rest of the century, twice the increase of the past 100 years), with fewer frosts and more hot days.
Nor does it attempt to model how farmers would react to these climatic changes.
"It takes no account of adaptation, for example, water harvesting and storage or going to drought-resistant plants," Jebsen said.
More precipitation on the Southern Alps might mean more water coming down the South Island rivers, increasing the potential for irrigation.
