The world has successfully halted human-made changes to the southern jet stream that left New Zealand increasingly prone to drought, a new study concludes.
But if we don’t cut greenhouse gas emissions, the researchers say, any improvement may be temporary.
Although most people know that climate change from greenhouse gas emissions raises the risk of droughts, it’s less well-known that the ozone hole was also leaving our country hotter and drier.
A new study, which was published in the scientific journal Nature, examined the position of the southern hemisphere’s jet stream, a fast-moving air current that can play havoc with New Zealand’s weather. This air stream varies from year to year, but had been moving south courtesy of the ozone hole.
The ozone layer acts as a filter for ultraviolet radiation from the sun. The ozone hole – which worsens the country’s sunburn risk – is caused by ozone-depleting chemicals, such as chlorofluorocarbons and halons, that were once widely used in appliances from air conditioners to fridges.
International cooperation successfully led to significant drops in potent ozone-depleting chemicals – in stark contrast to the lack of progress the world has made in cutting emissions of greenhouse gases such as carbon dioxide and methane that are driving global warming and climate change.
Ozone and climate scientists have kept an eye on the southern jet stream. As there was less ozone to react with ultraviolet rays from the sun, the upper atmosphere above Antarctica cooled, triggering the jet stream’s migration towards the icy continent. The new study found, from the turn of the millennium, this drift came to a halt.
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Between 1980 and 2000, the jet stream shifted approximately 220 kilometres towards the south pole during the summer. The air current influences the number of low-pressure weather systems, known for bringing storms and chilly weather, that pass over New Zealand.
As these lows became less likely to arrive, especially in northern regions, our temperatures rose and our rainfall fell, leaving the country increasingly prone to drought. Northern regions experienced a record-breaking warm and dry summer this year.
Study co-author and Cooperative Institute for Research in Environmental Sciences researcher Dr Antara Banerjee said the data indicated that, from the year 2000, the jet stream’s drift “has not only paused, but might even be reversing … moving back towards the equator” as the ozone hole repairs itself.
“In the coming decades, we might see this reversal more clearly,” she said.
However, this climate repair may only be temporary. Rising carbon dioxide levels in the atmosphere also influenced the jet stream’s position, though the effect was not as strong as the pull of depleted ozone, Banerjee said.
“The Antarctic ozone hole is projected to close around the 2060s. When it is near-to-fully healed, we might then expect the effect of carbon dioxide to start winning out over the effect of ozone recovery, causing the jet stream to continue its poleward migration,” she said.
Banerjee said the research proved the world had the power to avert large-scale climate impacts.
“The success of the Montreal Protocol should send a powerful message to governments that they have the ability to reverse the damage that we have done to our planet, only if they cooperate effectively and honour their commitments with strong action,” she said.
Commenting on the research, Niwa ozone scientist Dr Olaf Morgenstern said this summer’s extreme dry weather was “made more likely” but not solely caused by the ozone hole.
The migrating jet stream was one contributing factor to New Zealand’s changing climate, Morgenstern said. “Under global warming, there are other reasons why drought might become more prevalent,” he said.
Though the ozone hole worsened the effects of climate change, greenhouse gas emissions will have the biggest impact on our country’s risk of drought and other extreme weather in future.
Ozone-depleting chemicals were phased out after the international Montreal Protocol agreement was signed in 1987.
Although banned chemicals may still be in use, Morgenstern said the decline was “a little slower than we had hoped for” but steady. “It’s going down, which is better than going up.”