Up in flames

What do atmospheric rivers have to do with forest fires? (8 minute read)

In Wellington, it feels as if I’m wading my way through a season when nothing ever dries out. In the park across the road where I walk my dog in the mornings, I can’t remember it ever being quite so sodden. These days, I’m not taking a walk in the park, I’m squelching through a swamp.

The data back me up too. June was particularly wet, with close to double the normal rainfall at Paraparaumu, just to the north of Wellington. There are still a few days before the summary is released for July, but data from the MetService suggest Wellington has already had around 240 millimetres. The July average is less than 150 millimetres, so we have already had an unusually large amount of rain, and the month isn’t finished with us yet. There’s still another heavy rainfall due before July is over.

Exactly why it has been so wet isn’t immediately clear, but the pattern of rainfall over last month gives a clue. It didn’t rain every day, just most days. And some of those days saw a lot of rain. These heavy rainfalls were predicted by New Zealand’s National Institute of Water and Atmospheric Research (NIWA), which does a seasonal forecast every month. The seasonal forecasts for June and July spoke of “plumes of tropical moisture” causing heavy rainfall and potential flooding, and, sure enough, that’s exactly what we’ve seen.

It’s an evocative image – plumes of tropical moisture. It calls to mind a graceful fountain rather than something which brings destructive floods and slips. In fact, it doesn’t really sound like a scientific term at all. So what is a plume of tropical moisture, and what does it really mean?

Another stunning Wellington morning: image credit Melanie Newfield

We are probably all familiar with the water cycle, where liquid water from the land and sea evaporates to form water vapour, condenses into clouds and then falls as rain (there’s a good explanation including a basic video here). But most explanations of the water cycle depict it as rather static, with water falling as rain not far from where it evaporates. But things aren’t that simple.

The expression “plume of tropical moisture” refers to a pattern in the atmosphere also known as an atmospheric river. These are air currents which originate in the tropics and, like rivers on land, carry moisture. Unlike rivers on land, though, they don’t follow the same path all the time, but move around. They can be thousands of kilometres long and hundreds of kilometres wide, and the volumes of water they carry can be huge – sometimes greater than the Amazon River.

Atmospheric rivers are an important part of rainfall cycles in temperate areas. Around 40 atmospheric rivers reach New Zealand every year, but they vary in size and only four or five a year are classed as “strong”. Although they cover only a small area of the atmosphere at any one time, they can account for up to 90% of the moisture travelling from the tropics to the poles. In some parts of New Zealand, such as the west coast of the South Island, they are responsible for much of the heavy rain. They are also important in the US state of California, where they can account for 50% of the annual rain and snow fall. There’s a good video about atmospheric rivers in California here.

When I stumbled across the information about atmospheric rivers, I wasn’t actually looking at the topic of rainfall at all. I was reading about fires, because I wanted to understand more about the serious fires which have been affecting the Mediterranean and California lately. So why did I end up writing about Wellington’s wet winter and atmospheric rivers?

I’ll be honest, it’s partly because I had one of those days where nothing I wrote seemed to make sense and I kept getting stuck. What I had written about fire wasn’t working, and atmospheric rivers seemed interesting, so I decided to follow that path instead. Imagine my surprise, then, when I found the story of atmospheric rivers taking me back to the risk of forest fires in California.

California, it turns out, is dependent on atmospheric rivers for much of its rainfall, and a reduction in atmospheric rivers results in drought. That is what has been happening in California lately. Instead of making landfall over California, during the last year most of the atmospheric rivers travelled further north, dumping their rain over Oregon and Washington.

Without atmospheric rivers to dampen things down, California has been experiencing a drought. Because of the drought, the vegetation has dried out and is more likely to burn. But it’s not just the last year that has seen a drought in California. The current drought has been going on for three years. Before that there was a drought from 2012-2016, another from 2007-2009 and another from 2001-2002. There was a long drought from 1987-1992. There were earlier droughts too – California is a drought-prone area.

But there is evidence that Californian droughts are getting worse because of climate change. This isn’t entirely due to changes in rainfall. The absolute amount of water entering a system is only one factor contributing to drought. Another factor is temperature – if conditions are warmer, then more water evaporates. There is evidence that some of California’s recent droughts were made worse because dry years and warm years coincided. There’s another temperature-related factor too. Some of the water which supplies California falls not as rain but as snow in the Sierra Nevada mountains. With warmer temperatures, there is less snow. How does this contribute to droughts? The snow acts as a form of water storage – water from snow which falls in winter is released during the spring and summer. The streams and rivers are kept flowing by melting snow, even if there is little rainfall. And what about the atmospheric rivers? There’s some evidence that they will decrease in number, but get wider and longer, as climate change gets worse.

So, if California’s droughts are getting worse due to climate change, does this mean that California’s fires are becoming worse too?

The answer turns out to be “yes, but...”

California has always had fires. The indigenous inhabitants of the region worked this out and began burning the forests in a controlled way. This prevented the understorey of the forest from getting too dense, and, with less plant material to burn, the fires were less intense. But Europeans changed all that. The last century saw a deliberate policy of stopping all forest fires. That policy inevitably resulted in much denser vegetation. When that vegetation dried off, the result was more fuel for the forest fires, so they became more intense.

Atmospheric rivers can also add to the problem in an unexpected way. The heavy rainfall they bring results in a flush of extra plant growth, which then dies off as everything dries out. In the years immediately following intense atmospheric rivers, there is evidence for an increased risk of forest fires, because of that increase in dry plant material.

Worsening droughts under climate change certainly won’t help the forest fire situation, but now the problem with fire prevention is recognised, the management of California’s forests is changing. Prescribed burning – that is, burning the forest with small, controlled fires – is increasingly being used as a tool to prevent larger, more damaging fires. This raises some hope that the residents of California won’t face as many severe fires in the future.

But what I’ve learned about California’s fires has made me wonder about fire in New Zealand. Fire is not a natural part of our landscape as it is in California and much of Australia. But, in the years since the arrival of people, especially Europeans, we have seen a lot more fire. Now we’re facing a changing climate. And we have planted huge areas of New Zealand in a fire-adapted, Californian native tree, the radiata pine. Does this make us more vulnerable?

The bad news is that New Zealand will experience more drought conditions, simply because higher temperatures result in increased evaporation. But does that make planting lots of pine trees a good idea or a bad idea[1]? Although it is a fire-adapted species, that is, it grows well in areas subjected to frequent fires, radiata pine isn’t actually a highly flammable species. In fact, there are many New Zealand native species which are more flammable. So, just because we plant a lot of pines, it doesn’t mean New Zealand is about to go the way of California.

But I did come across one other little morsel of information about fire risk in New Zealand. The real culprit when it comes to fire risk is that familiar foe of the New Zealand farmer – gorse. Scientists tested the flammability of a range of New Zealand native and introduced species, and found that gorse was the most flammable species. They even tested a number of fire-adapted and highly flammable Australian plants, but none of them could match gorse. So, as we think about how we are going to manage an increased drought risk in the future, we would do well to keep an eye on the growth of gorse.

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[1] I’m going to leave the issue of whether it is a good idea to plant lots of trees to offset emissions for another time, as it’s worth a closer look on its own.