What's fuelling our weather extremes

I am a climate scientist who has been around watching the climate crisis grow from one of little concern to one where enough extremes of weather have grabbed the attention of the public. It is now well established that human activities are the cause of global warming and climate change mainly through increases in carbon dioxide and other greenhouse gases in the atmosphere primarily from emissions from burning fossil fuels.

The floods in Auckland, and their devastating impact are unprecedented for the region. Weather is always unpredictable to a certain extent, but these extreme weather events are likely to become more and more common. The floods in Auckland have brought home that extreme weather, and our lack of preparation for it, is not the preserve of those living in remote regions or countries. It has affected people living in south and west Auckland, as well as well-heeled suburbs of Mt Eden, Grey Lynn and Remuera, and on the North Shore.

Houses have been lost, so have lives. You might say the thousands of fans disappointed by the cancellation of Elton John’s concerts, which happened to include my wife and daughter, is a first world problem and in some ways, it is, as ‘first worlds’ are largely responsible for climate change, while its effects have, so far, largely affected developing countries.

Nearby our house is a coastal trail on which I love to walk our dog and it has been undermined by erosion and is no longer passable. I have been appalled, but not altogether surprised at the damage in parts of Auckland from the flooding and swirling rushing of waters that evidently had not adequately been designed for in drainage systems, and the erosion is very sad to see and often hard to fix.

This extreme weather is unpredictable, but we know what is behind it. Recently, as part of a team of international scientists that just published a paper on ocean changes, I’ve been commenting on the fact that the oceans are at their warmest state ever and that it has consequences. Over 90 percent of the warming ends up in the oceans. The record warm ocean provides not only higher sea surface temperatures (SSTs) but also high upper ocean heat content to reinforce those sea temperatures as storms ride on top of the ocean and as winds mix the upper ocean up and extract heat and moisture in the process.

A warmer ocean means a lot of extra fuel for storms and the atmosphere can hold increasing levels of moisture at a rate of seven percent per degree Celsius warming. With sea temperatures running over 3C above normal around parts of New Zealand, and over 1C above normal over broad regions to the north there has likely been 10 to 25 percent more moisture lurking around for storms to gather up and rain on nearby land.

The most outstanding examples of consequences of the warm ocean in the past year have been the Pakistan floods in July-August 2022, where over 1,700 lost their lives and huge regions of Pakistan were flooded especially in the Indus River flood plain. This was on the heels of record high temperatures from March to May that also ruined crops and led to many deaths.

More recently it was the California floods that arose from a pattern of weather that brought atmospheric rivers and storms into the West Coast of the US fuelled by record-high SSTs over the North Pacific. The very high SSTs in the North Pacific mirror to some extent those in the South Pacific and both hot spots were created in part by the La Niña phenomenon of cold SSTs along the equator east of the dateline and the atmospheric teleconnection wave patterns that result. So, the regional hot spots become marine heat waves and are part global climate change and part La Niña.

We have also seen a series of floods in Australia in 2022, especially on the east coast; they too were part of the La Niña pattern of where the rainy areas occur.

As well as earlier wet spells in 2021 that led to flooding in Westport, Canterbury and Northland, in 2022-23 we have had more than our share of tropical and subtropical low-pressure systems bombarding New Zealand, especially over the past four months when we would expect spring and summer to begin to bring us more settled, sunny weather.

The South Island has certainly benefitted, but in the north and east of the North Island everyone is wondering what happened to summer and are the heavens picking on us? The reason is the latest storm brought air from the tropics rich in moisture (nearly double the normal atmospheric amount), which has led to prolonged heavy rains and, as we’ve seen, catastrophic flooding.

This has enormous costs for New Zealand and its citizens, but we are not doing enough to curtail emissions that cause the problem.

In our recently published paper on the ocean temperatures in 2022, we also analysed salinity and found that the fresh areas are getting fresher, while the salty areas are getting saltier. Over the oceans there are very strong and distinctive patterns of salinity related to patterns of rainfall associated with monsoons and the Intertropical Convergence Zone and South Pacific Convergence Zone. This has meant that typically wet areas are getting wetter, and dry areas are becoming drier, which translates to heavier rains and risk of flooding often alongside drought. Or drought and wet conditions taking turns.

In New Zealand this means more rains in a weather pattern that favours more storms, while during a weather pattern that favours fewer storms, like last summer perhaps, drought is more likely. New Zealand is unlikely to suffer dry conditions very long as we are surrounded by ocean, but it does mean we need to pay attention to water management: saving water when we get too much for the times we do not have enough. This is universally true as climate change effects rear up all over the world.

So, while the recent flood disasters are not directly caused by climate change, the warming climate contributes substantially to making all extremes more so, and the damage increases exponentially; a 10 percent increase in rain might lead to 100 percent increase in damage. I call this “the straw that breaks the camel’s back syndrome”.

This has enormous costs for New Zealand and its citizens, but we are not doing enough to curtail emissions that cause the problem.