The National Institute of Water & Atmospheric Research (NIWA) has produced new climate change projections for New Zealand.

The new projections represent the most up-to-date scientific knowledge about likely effects of climate change in New Zealand, including regional impacts. They form the basis of the second edition of the Ministry for the Environment guidance manual for local government, Climate Change Effects & Impacts Assessment Manual: A Guide for Local Government, which was released on 27 May 2008.

The new projections are based on the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), and the global climate modelling results produced for that report. The results are broadly consistent with NIWA’s projections for the first edition of the manual in 2004, which drew on the IPCC’s 3rd Assessment Report (2001). But the new projections use more, and better, information. “The evidence of climate change continues to mount, climate models are becoming more sophisticated, and scientific knowledge of the climate is improving all the time,” says Dr David Wratt, NIWA’s General Manager Climate Change.

What’s happening already

NIWA says evidence that New Zealand is already experiencing climate change includes:

• Increasing temperatures: about 0.9ºC over the past 100 years.
• Reduced frost frequency over most of the country: Canterbury and Marlborough experience about 20 fewer frosts per year now than in the early 1970s.
• Retreat of major South Island glaciers: volume of ice in the Southern Alps reduced by almost 11 per cent in the past 30 years. Twelve of the largest glaciers are unlikely to return to their earlier lengths without extraordinary cooling of the climate.

The projections

Future climate changes will be affected, amongst other things, by how much extra greenhouse gas goes into the atmosphere. For this reason, NIWA looked at likely changes in New Zealand across six greenhouse gas emission scenarios, but focusing on the mid-range.

Temperature:

Temperature is projected to increase by about 2ºC by 2090 for a mid-range scenario. This is about the difference in the annual median temperature between Wellington and Auckland.

An unusually warm year now could be the norm in 30–50 years, and could be a very cold year by the end of the century.

Extreme temperature:

NIWA is projecting fewer frosts and more hot days.

Initial simulations from the regional climate model for New Zealand suggest large decreases in the number of frost days in the central North Island and in the South Island. For example, in the central plateau of the North Island, away from the actual alpine areas such as Ruapehu, the number of air frosts is projected to decrease from around 30–40 frost days per year currently to 5–15 days or so per year under a low-medium emission scenario, and to a few less days with a medium-high emission scenario, by the end of the 21st century.

The initial regional climate model runs also suggest substantial increases in the number of days above 25°C, particularly at already warm northern sites. For example, depending on the scenario, the number of hot days in Auckland could more than treble. Currently, the city experiences about 21 days per year above 25°C. By the end of the century, Aucklanders could face an extra 40 days or more of high temperatures under a low-medium scenario, and more than 60 extra hot days under a medium-high scenario.

Rainfall:

NIWA says the overall picture continues to be for a drier climate in most of the east coast and north of the North Island, coastal Canterbury & coastal Marlborough, and for a wetter climate in the west and south of South Island.

In addition, NIWA has produced rainfall projections for each season:

• winter & spring: all the models point to increased westerly winds, especially over the South Island. This means drier in the east & north, but more rain in the west of both islands.
• summer & autumn: different models point in different directions, but the average pattern is that westerlies decrease slightly over almost all of New Zealand. NIWA scientists have less confidence in this result than in the annual, winter, and spring projections. If borne out, less frequent westerlies would mean drier summers and autumns in the west of North Island, and possibly wetter in Gisborne & Hawkes Bay for those seasons.

Extreme rainfall:

NIWA says heavy rainfall is likely to get heavier and/or more frequent. For a mid-range scenario, a 1-in-100 year event now could become a 1-in-50 year event by the end of the century.

It advises planners to assume that extreme rainfall could be about 8 per cent heavier for every 1°C of warming. But it says there will be considerable geographical variation, related to such factors as where and how often storms cross the country and the intensity of ex-tropical cyclones in future. Further research is required.

Snow:

Based on air temperature and precipitation projections, it is likely that average snowlines would rise as the temperature increases, but snowfall is influenced by complicated atmospheric processes. NIWA intends to use a specialised snow model to quantify likely changes at the small scale required for management purposes (eg, effects on skifields or the amount of water in a catchment).

Strong winds:

A simulation using the regional climate model suggests an increase in the strongest winter winds over much of the country by 2100. The changes are fairly small for the most part (averaging out at a 2.3 per cent increase over all land points in the model), but reach about 10 per cent in some eastern locations.

To see the full projections for individual regions, click here.