Observation is a keystone of science. For millennia, people have observed nature to discover patterns in the weather. We’ve used these patterns as guides for where we’ve built our towns and cities, when and where to plant crops and much more. As we continue to observe weather patterns in Aotearoa, it’s becoming increasingly obvious that patterns are changing.
Climate change has well and truly arrived in New Zealand and is affecting the climate where we live.Our atmosphere and climate 2020, Ministry for the Environment
Establishing benchmarks for temperature and rainfall
The Ministry for the Environment and Stats NZ used weather information from 1961–1990 to create a standard reference. When examining climate change, this 30-year period is recommended by the World Meteorological Organization as a benchmark for calculating anomalies – the data points above or below a standard reference. The National Institute of Water and Atmospheric Research (NIWA) gathers data from more than 100 electronic weather stations across the country. The Ministry for the Environment has chosen 30 representative sites to report on temperature, rainfall, heatwaves, first and last frosts, drought and fire danger.
Nature of science
Scientists depend on empirical evidence to produce scientific knowledge. The Ministry for the Environment and Stats NZ analyse numerous datasets (which are rigorously assessed to ensure relevance and accuracy) when reporting about environmental topics such as climate change.
The evidence for temperature rise
The average land-surface temperature in Aotearoa has risen by 1.1℃ since records began in 1909. This may not sound like much of an increase, but small changes have large consequences.
Many of the largest temperature anomalies occur in summer. The hot summer in 2018 was unprecedented in New Zealand’s climate record. Really hot days often make the news headlines, but changes to minimum temperatures can have impacts too. Changes to 24-hour temperature cycles like higher night-time temperatures impact people and other living things as they do not get a break from the heat.
Winters are becoming warmer. The maximum winter temperatures increased at all 30 sites while the minimum temperatures increased at all but three sites. There are benefits to warmer winters, such as improved air quality if we are not using our fireplaces as much and diversified land use such as planting kiwifruit in areas that have been too seasonally cold before. However, changing seasonality affects the flowering of some native trees, species migrations and the timing of bird egg-laying. The economy is also affected. Cold temperatures are crucial for some fruit trees to blossom, and winter tourism relies on predictable snowfall.
Rain and snow also determine climate. Experts think that changes to rainfall patterns will likely align with current rainfall patterns. Wet areas are expected to get wetter, and dry areas are expected to get drier. In the last 60 years, sites with decreasing rainfall tended to be in the northern half of Te Ika-a-Māui/North Island while southern sites in Te Wai Pounamu/South Island – already amongst the wettest places in New Zealand – had gains in annual rainfall. Spring appears to be the season with the most number of sites with a change.
Extreme weather events are where we often feel the most immediate impacts of climate change. The number of warm days – where the maximum temperature is above 25℃ – has increased at 19 of the 30 sites. This is up from the 14 sites that were observed between 1972 and 2016.
Changes in rainfall intensity are mixed. The proportion of total rainfall arriving in extreme events (downpours versus light rain) increased at 13 sites and decreased at 11. Most sites with increasing annual rainfall are also experiencing more extreme rainfall events, increasing the risk of flooding. For example, extreme rainfall hit the South Island’s West Coast in March 2019. Stretches of roads were damaged over a distance of hundreds of kilometres, a bridge was destroyed and floodwaters washed away about 135,000 kg of rubbish from an old landfill. Extreme rainfall hit the North Island in early 2023. The Auckland Anniversary Weekend floods and Cyclone Gabrielle both caused loss of life and forced thousands of people to evacuate their homes.
On the other end of the scale is drought – much less moisture compared to what is expected. Auckland experienced its longest streak of dry days in a row in early 2020. Observational data shows that Hamilton experienced short but extremely dry conditions six times within a recent 10-year span. Rural communities that depend on rainfall for crops, livestock and drinking water also find droughts stressful. Native plants tend to have low levels of drought resistance, and when dry conditions persist, there are also greater risks of wildfires.
Aotearoa’s glaciers are strongly influenced by temperature and precipitation. Aerial surveys show that our glaciers are retreating and losing mass. The oceans around the country are both warming and becoming more acidic.
NIWA has a selection of drought indicator maps.
Cyclone Gabrielle from February 2023 got a lot of us thinking more about the impacts of climate change which this Stuff news article explores. Professor James Renwick wrote this article for The Conversation Floods, cyclones, thunderstorms: is climate change to blame for New Zealand’s summer of extreme weather?
This resource has been produced with the support of the Ministry for the Environment and Stats NZ. (c) Crown Copyright.