With the impact of global warming, temperatures across the country appear to be rising. The warmest year since the start of the NIWA ‘seven-station’ record in 1909 is the year 2016, with an annual mean temperature of 13.44°C, or +0.83°C above the 1981–2010 average (see Figure 3).
The warmest years on the basis of monthly mean temperatures are: 2016 (+0.83°C), 1998 (+0.80°C), 1999 (+0.74°C), 2013 (+0.72°C), with 2005 and 1971 tied at +0.50°C.
The black trendline indicates an increase in temperature from 1909–2016 of 0.95⁰C over a 100-year period (+/- 0.25⁰C).
Rising temperatures will have a significant impact for the rural economy and land uses across the country.
Climate change in the Bay of Plenty
In 2011, Bay of Plenty Regional Council asked the National Institute of Water and Atmospheric Research (NIWA) to update a 2003 climate change report for the region with new information from more accurate climate models and projections. A summary of the information is as follows:
- Bay of Plenty temperatures will rise over the rest of this century. By 2040, the region’s annual mean temperature is expected to be around 1.2°C warmer than it was in 1990. By 2090, it is expected to warm by between 2.7°C under a mid-emissions scenario and 3.6°C under a high-emissions scenario. (See Emissions scenarios below)
- Hot days, i.e. 25°C or more, are tipped to become the summer norm by the end of the century. Whakatāne, for instance, currently gets about 22 hot days a year; by 2090, it may experience between 80 and 100. Rotorua currently gets about 12 hot days a year; under a high emissions scenario, it could expect to get between 50 and 60.
- Tauranga currently gets around 22 hot days annually. By 2040, under a mid-range emissions scenario, it could see twice that number, and more than 70 a year by the century’s end.
- Warming will be fairly uniform across the region; Tauranga, for instance, will warm at much the same rate as Whakatāne.
- Not all seasons will warm at the same rate. Autumn and winter are projected to warm slightly more than summer and spring.
- The warmer air gets, the more moisture it can hold – about seven or eight percent more for each degree of warming – so rain is likely to fall more heavily in future.
- The region will get roughly the same average annual rainfall in 2090 as it does now, but rain may fall at different times. For instance, winters are expected to become drier as the century unfolds; by 2090, coastal and south-eastern areas may receive 10 percent less rain than they do now. On the other hand, summer rainfall is projected to increase – particularly inland – and to become more variable. We may see a sharp year-to-year contrast of either very dry summers, or very wet ones.
- The Bay of Plenty will get more easterly winds during summer, and more westerlies during winter.
- By analysing historical weather maps that have produced extreme winds in the past, and comparing them against the sort of maps we might expect in a warmer climate, scientists calculate that extreme winds may be less frequent during future summers, but more common during winters.
- By 2090, frosts will be a rare thing in the Bay of Plenty. At present, Ōpōtiki gets around five frosts a year, while Rotorua may get 20. By the end of the century, Rotorua is projected to experience frost just once or twice a year – none at all in some years. Other locations may get perhaps one frost every three years. There will be fewer cold nights.
Therefore, the future climate for Rotorua may mean warmer temperatures and more hot days in summer along with an increased risk of droughts, heavier summer rainfall and fewer frosts.
Warmer temperatures, a longer growing season and fewer frosts would provide opportunities for new crops and land uses in the catchment. Farmers might also benefit from faster growth of pasture and better crop-growing conditions. However, these benefits may be limited by negative effects of climate change such as prolonged drought and greater frequency and intensity of storms.
Also, even decades into the future, the nitrogen-leaching capacity of new crops in the catchment would need to be analysed to ensure they still fell within low nitrogen leaching guidelines.
The mid-range scenario describes a future world of very rapid economic growth. It assumes that the global population will peak mid-century, then decline, and that we will rapidly adopt new, more efficient technologies. It also assumes that our future energy will come more or less equally from fossil and non-fossil sources. In this scenario, carbon dioxide concentration doubles from pre-industrial levels by the 2060s, and is about 140 percent higher than pre-industrial (17th century) levels by 2100.
The high-range emissions scenario describes a more piecemeal response to climate change. Regions and nations adopt their own strategies and technologies, and their economies develop at differing rates. This slows the uptake of new technologies, while the global population goes on increasing. In this scenario, carbon dioxide concentration doubles from pre-industrial (17th century) levels by the 2060s and is almost 200 percent higher than pre-industrial by 2100.