Wildfires and floods will increase as New Zealand's climate changes, but we'll have cheaper heating bills and farmers will have better spring pasture growth.
A draft copy of the Australasian chapter of the latest United Nations Intergovernmental Panel on Climate Change report was leaked to the ABC in Australia ahead of its formal release on March 31.
It predicts a rise in average temperatures of 2 degrees Celsius across Australia and New Zealand.
The risk of wildfires will increase due to the drier and hotter conditions but rain, when it falls, will be heavier and in larger quantities.
Climate scientist Jim Salinger, who worked on the previous IPCC report, said it would lead to an "accentuation" of what New Zealand was experiencing already.
It might seem contradictory to predict drier conditions and more flooding, but that would be the result of warmer atmospheres holding more water vapour and producing heavier rainfall.
"It is an accentuation of the current New Zealand climate . . . it will be wetter in the west, drier in the east."
High pressure would be more likely to settle for long periods over the North Island, while dry northwesterly winds would be hotter.
"The forest fire risk would increase. That doesn't mean there will be more of them [but the] forest fire danger [season] extends a month at either end."
The report points out that the climate changes will bring benefits as well as risks. Winters will be warmer, meaning lower heating bills, and spring pasture growth will increase, leading to more agricultural production.
The national power bill could fall 1-2 per cent for each degree of warming, it says.
But fresh water in the northeast of the South Island and east and north of the North Island will probably decline as years become hotter and drier.
Meanwhile, rainfall over the Southern Alps will increase the flows of the Clutha, Waimakariri and Rangitata river systems, among other alpine-sourced rivers, by up to 10 per cent by 2040.
The seas will warm, affecting fish stocks. Tropical species that might occasionally arrive off the North Island coast at present may establish permanent populations.
More details will come with the release of the full report at the end of this month.
The report, which is the IPCC's fifth assessment of the global climate, was compiled by 310 lead authors from 73 countries.
The fourth IPCC report, released in 2007, was criticised for some of its methodology and predictions, including the claim that Himalayan glaciers would melt completely by 2035. This claim was rescinded in 2010.
- The Dominion Post