Source: Daily Climate
OPINION: Last week’s devastating floods over parts of the North Island are the latest in a series of extreme weather events that New Zealand has experienced in the past few years. Other explosive rainfall events have been felt in Nelson, Northland, Dunedin, Gisborne and Christchurch, to name just a few.
Scientists in New Zealand are trying to understand if and how New Zealand’s extreme events are linked to climate change. The question matters because hazards like floods and droughts bring large personal, social and economic impacts.
The science that lets us examine the frequency of extreme weather events is called “probabilistic event attribution”. It investigates how the odds of specific events appear to have been affected, or not, by the changing climate.
The technique has parallels with epidemiological approaches in which doctors try to estimate the links between specific illnesses and causes, enabling them to make statements about the difference in the chances of a smoker and a non-smoker getting lung cancer.
In the climate change case, we attempt to understand the odds of specific events in a changing climate, versus the odds of those events in a climate without significant human influences.
This requires large numbers of climate model experiments; more than are feasible on a supercomputer. The only way we can get that many model runs is by harnessing the power of the internet through citizen science. The “weather@home” project, headquartered at the University of Oxford, in England, involves members of the public running climate models on their PCs.
Event attribution for New Zealand is conducted by Niwa and is funded by the Deep South National Science Challenge.
Large numbers of model runs are needed to get reasonable statistics regarding extreme events, which are of course rare by definition. The experiment compares simulations of the climate as it is now – with high levels of carbon dioxide and warm sea surface temperatures – with simulations of “what might have been” without the elevated levels of carbon dioxide and warming we have actually observed.
This comparison between the climate as it is today and as it might have been in a world without climate change lets us investigate how aspects of climate have changed, including the frequency of extreme weather events.
The jury is still out on the links between tropical cyclones and climate change, but some clear physical links are emerging.
One result common to mid-latitude climates such as New Zealand’s is that many of the wettest events are getting wetter. Broadly speaking, the cause seems to be that air parcels are getting warmer, and simple physics dictates that they can therefore support more water vapour.
When the time comes for these parcels of air to dump their moisture, they have more to dump.
Since the warmest and most moist parcels of air originate in the tropics, we need to look to the North for the source of these explosive downpours. This is consistent with a general pattern of the emergence of climate change, in which the tropics – which act as the engine of the climate system – show faster emerging changes in a relative sense than the more variable storm track regions.
But, as is usually the case in atmospheric science, the picture is not completely straightforward. Although we generally expect the heaviest events to get heavier, there appears to be significant regional variation within New Zealand. In some parts of the country, circulation changes may compete with the energy-related effect already described.
This means some areas may experience an overall decrease in extreme precipitation events. But the energy-related effects still apply. So when they do come, heavy rainfall events will probably be more damaging than those our parents and grandparents experienced, even in those areas that can on the whole expect fewer heavy rainfall events.
Droughts are trickier, for a variety of fairly subtle reasons. A recent paper by Victoria University graduate and Oxford postdoctoral researcher Luke Harrington suggests that weather patterns such as those seen in the 2013 drought were 20 per cent more likely to occur in the present day than would have been the case without climate change.
The 2013 drought had a huge impact on New Zealand – according to Treasury estimates, it cost the New Zealand economy at least $1.3 billion.
Events like floods and droughts are not just numbers in Treasury spreadsheets. They’re life-altering shocks to families and communities.
When I was 14, my home town of Invercargill was hit by terrible flooding. My dad was a building surveyor and builder. In the wake of the flood I helped him assess the damage.
I remember the rotten stench of the land in the wake of the retreating floodwaters, and the hideous, incongruous sight inside houses that looked perfectly fine above the high water mark, but which were rotten and warped and completely despoiled below that mark. Floods are eerie, devastating things which break hearts and ruin people’s livelihoods.
The reason we conduct research into near-term climate change risks is to work out how much and how fast things are changing. They are changing, and we know why. But now we need to quantify those changes. If we understand emerging climate risks, we can help make people less vulnerable, since better information and better preparedness can and should lead to better management of the risks.
That will be of little solace to those trying to reconstruct their lives in the wake of last week’s deluges, but it should help those seeking to avoid future damages.
Professor Dave Frame is director of the Climate Change Research Institute at Victoria University of Wellington.
– The Dominion Post