This page provides an overview of how the climate in the West Coast region is likely to change into the future and what implications this may bring for the region.
Projections of climate change depend on future greenhouse gas emissions which are uncertain. Also, global climate models used to predict future climate vary in their sensitivity to these emissions. The combination of these factors means that projections of future climate are usually expressed as a range of likely values. This information is mostly from 'middle-of-the-range' climate change projections.
Temperatures are likely to be around 0.9˚C warmer by 2040 and 2.0˚C warmer by 2090, compared to 1990. By the end of the century, the West Coast is projected to have up to 10 extra days per year where maximum temperatures exceed 25˚C. The number of frosts could decrease by around 20–70 days per year, with the most marked changes in frosts expected for the southern half of the region.
The West Coast is expected to become wetter, particularly in winter and spring. In Hokitika, average annual rainfall is likely to increase by 5 per cent by 2040 and 8 per cent by 2090. Seasonal projections for Hokitika show little change in summer and autumn rainfall, with winter rainfall likely to increase by 21 per cent by 2090.
Very heavy rainfall events are likely to become more frequent throughout the region. An expected increase in average rainfall in the main divide could increase river flows throughout the region. In a 2005 study, three storm events for the West Coast Buller catchment were modelled, for both the current climate as well as for three different scenarios of temperature increase. Rainfall increased on average for the three storms by 3 per cent, 5 per cent and 33 per cent for temperature changes of 0.5°C, 1.0°C and 2.7°C, respectively. Flooding was estimated to increase from 4 per cent of Westport being inundated under the current climate, to 13 per cent, 30 per cent and 80 per cent for each of the respective temperature scenarios.
New Zealand tide records show an average rise in relative mean sea level of 1.7 mm per year over the 20th century. Sea levels are expected to continue to rise into the future. The Ministry for the Environment recommends planning for future sea-level rise of at least 0.5 m, along with consideration of the consequences of a mean sea-level rise of at least 0.8 m (relative to the 1980–1999 average) by the 2090s.
The number of storms crossing the Tasman Sea is expected to increase in summer and decrease in winter, by the end of the century. The intensity of these storms is likely to decrease in both summer and winter. Preliminary results from recent research suggests that the frequency and intensity of thunderstorms are likely to increase, particularly in winter. For example, thunderstorms in winter may occur up to four times as often by the end of the century. Similarly, the amount of energy available to storms could double by 2100, making the storms more intense.
The frequency of extreme winds over this century is likely to increase by between 2 and 5 per cent in almost all regions of New Zealand in winter, and decrease by a similar amount in summer. Changes in wind direction may lead to an increase in the frequency of westerly winds over the South Island.
Overall glacier ice mass has decreased by 25 per cent over the last 60 years in New Zealand, and is expected to continue to do so into the future. Some of our most iconic glaciers (such as Franz Josef) have advanced in recent times. This is a result of more precipitation falling at their glacier heads. Whether these glaciers continue to advance into the future will depend on the balance between increased melting due to warmer temperatures and increased precipitation in the mountains. For example, one climate modelling study suggests the Franz Josef glacier may retreat approximately 5 km and lose around 38 per cent of its mass by 2100.
By 2090, seasonally the region could expect*:
1.7°C temperature rise
8 per cent more rainfall in Hokitika
2.2°C temperature rise
Little change in rainfall in Hokitika
2.1°C temperature rise
3 per cent more rainfall in Hokitika
2.1°C temperature rise
21 per cent more rainfall in Hokitika
*Projected changes are relative to 1990 levels. The numbers provided are mid-range estimates of what the change is projected to be, and should not be taken as definitive values. To view the ranges within which each change could lie, see Table 2 and Table 3 in the Preparing for Climate Change report.
What does this mean for the West Coast?
Coastal hazards – Coastal roads and infrastructure will face increased risk from coastal erosion and inundation, increased storminess and sea-level rise.
Flooding and landslides – More heavy rainfall will increase the risk of flooding, erosion and landslides, which is already high in many parts of the region. Many West Coast communities are located along narrow coastal and river strips beneath mountain ranges, leaving them exposed to increased risks of storms, flooding and landslides.
Biosecurity – Warmer, wetter conditions could increase the spread of pests and weeds.
Agriculture – Warmer temperatures, a longer growing season and significantly fewer frosts could provide opportunities to grow new crops and farmers might benefit from faster growth of pasture and potentially better growing conditions. However, these benefits may be limited by negative effects of climate change such as increased flood risk or greater frequency and intensity of storms. The Ministry for Primary Industries website has more information on the regional impacts of climate change on agriculture.