It’s been a hot month – the hottest in at least 120,000 years, according to climate models – even if it’s the middle of winter in New Zealand. How do we know that, and how do we predict what’s coming next?
The headlines, certainly, are scary. In India, heat is approaching the “limits of survivability”. China has been using record-breaking volumes of electricity as people turn their air conditioning to the max. Pavements in the US are giving people third degree burns. In Turkey, you can fry an egg without using a stove. It’s the middle of winter but Antarctic sea ice is at record low levels. And, not worst of all but nonetheless worrying, olive oil prices are going to rise as a result.
According to the World Meterological Organisation, July 2023 has seen the hottest three-week period ever recorded, the three hottest days on record and the hottest ocean temperatures ever recorded. The average temperature two metres above the land surface was 16.95 degrees, 0.22 degrees above the previous record in July 2019.
The sheer number of broken records can be hard to take in, says Adrian McDonald, a professor at the University of Canterbury who studies climate modelling. “Understanding how the global mean and temperature extremes change helps us understand how the impact of climate change is altering the global distribution of temperatures,” he says. “The extremes in the northern hemisphere do look scary, and this isn’t 1.5 degrees of warming yet.”
Global weather systems are astonishingly complicated, and need enormous computational power with billions of inputs to predict. McDonald says that this makes it difficult to say with any certainty how scorching temperatures in the northern hemisphere summer are impacting weather patterns in New Zealand. “Lots of things we’re seeing in the northern hemisphere [like wildfires and extreme heat] are things that might come to the southern hemisphere.”
Because the southern hemisphere has more ocean than the north, much of the extra heat trapped by greenhouse gases like carbon dioxide is absorbed by water, rather than land. But those warmer ocean temperatures also mean more moisture in the air and heated weather systems; Cyclone Gabrielle, for instance, was shaped by warmer air and ocean temperatures in the Pacific, although it’s difficult to determine the exact effect of climate change.
Weather models are very accurate at predicting temperatures and conditions up to 14 days away, and climate models have, so far, shown to be very accurate for predicting average temperatures and precipitation over time. It’s the medium-term, seasonal prediction that is most difficult to get right, making it impossible to say for sure what high temperatures elsewhere mean for New Zealand’s springtime and summer.
McDonald says it helps to imagine weather as a wiggly line, temperatures and precipitation fluctuating hour by hour and day by day. Weather forecasting zooms in on that line’s movement in the short term. Climate modelling is more about averages: it draws a straight line through the wiggles to predict overall trends. Seasonal modelling, predicting temperatures and rainfalls a few months away, is complex because there are literally millions of variables.
Set weather patterns like El Niño are one element causing high temperatures elsewhere, although an El Niño summer in New Zealand usually means cooler conditions and changed wind directions, as warm water off the coast of South America makes the western Pacific less warm. “It’s hard to know how climate change will alter the form of El Niño,” McDonald says. “The natural cycle could be hiding some of the effects of human-caused warming.”
In the long-term, he can be more confident. “We know that climate change will shift the weather we expect towards the pole, and the tropical belts will expand.” The norms for weather will move south: the steamy humidity of tropical areas will be seen further south; the snow and freezing temperatures that ecosystems (and tourism magnets) are built to rely on will become less and less common in northern parts of New Zealand.
The data that measures and is used to predict temperatures come from hundreds of thousands of weather stations around the world, all feeding into prediction models. Far above the planet’s surface, satellites measure the temperature of the ocean surface, while the Argo network of buoys sink up and down in the water column, recording the temperature and currents of the top 2,000 metres of the sea.
When records like “the hottest month” are declared, scientists use predictions about the past to say how certain they are that a record really is a record, even if we don’t have exact measurements of global temperatures on, say, July 17 in 3440 BC. “Since the instrumental record began, it’s the hottest month,” McDonald explains. “But we have really good confidence this is true for a certain period, like the hottest month in 10,000 years.” It’s even possible that July was the hottest month for 120,000 years.
In New Zealand, July 2023 has been unusually warm too. Niwa’s seasonal climate outlook showed that the temperature was predicted to be above or at average historical norms across the country and the first half of the year broke rainfall records, with more than a year’s worth of rain falling in six months in much of the northern and eastern parts of the North Island. “We are really sure about our record going back – Niwa keeps good records and we use multiple instruments and calibrate them to have a consistent record.
The most unpredictable parts of climate models, McDonald says, are the human elements. With the power of supercomputers and finely tuned algorithms, understanding what different climate futures might look like is relatively easy. “We can model the difference between a climate future with renewable energy where we stick to the Paris Agreement and one where we continue to use coal.” But no amount of knowledge of physics and climate systems can predict the social, economic and political factors that determine when or whether humans are going to stop generating emissions. “That’s the biggest uncertainty in climate science.”