With sixteen percent of New Zealanders living in the coastal zone, and two thirds in areas prone to flooding, most of us are likely to feel the watery effects of climate change. A new Royal Society of New Zealand report, Climate Change Implications for New Zealand, says your local council should be thinking about this. And they are, according to panel chair Professor James Renwick. He says that his is the easy job – telling councils what the science is. Councils have the hard job of figuring out what to do about it.
The report seems to have mainly considered water impacts from climate change – sea level rise, flooding. Why is that?
We spent quite a bit of time talking about what we were going to cover in the report – things that are important for New Zealanders, and water is important for us in so many ways. Some areas have been quite well covered elsewhere, e.g. there has been quite a lot written about agriculture and climate change already. So when we came up with our list of risk factors for New Zealand, part of the aim was to shine a light on areas that haven’t had so much public coverage. Sea level rise is pretty well known, but what’s going on in the ocean and with ecosystems hasn’t had so much press.
Issues around water will be the major stressor for societies everywhere. Just about everything we do is reliant on water – drinking supplies, irrigation for agriculture, marine fisheries, etc. Water pervades everything. A few commentators have said that wars in the 21st century will be fought over water. I’m not sure we’ve seen that yet but there’s pretty good evidence that the recent drought in Syria helped precipitate the war there.
Ministry for the Environment guidance tells Councils to consider the consequences of 80 centimetres of sea level rise by 2100, though it’s obvious from your report it actually depends on how emissions evolve. Is it really realistic to expect Regional Councils to look that far into the future?
Definitely. Long term plans go out only 10 years, but infrastructure lasts for many decades, perhaps a century or more. District and Regional Councils are thinking pretty hard about these issues. I’ve recently been speaking about climate change and sea level rise to the Waimakariri District Council, as part of their hazard planning process. I know that Kāpiti Coast District and Greater Wellington are also thinking about these things. The level of planning does vary around the country though. There really isn’t coordination from central government at the moment, which I think is not helpful.
One of the questions I was asked by the people in the Waimakariri District was that they’ve been told to expect one metre of sea level rise and how realistic is that? I told them that one metre is absolutely realistic – it may not be there on 1 January 2100, but if it isn’t, just give it a few years. Sea levels will continue to rise for a long time even if we scale back emissions, it’s just a question of the rate of rise.
If we don’t scale back the emissions sea level rise could be pretty open ended, if the big ice sheets really start to melt. But mainstream thinking is still for up to one metre or so this century, plus more after 2100. Even if we do commit to losing the West Antarctic Ice Sheet, it would still take several hundred years to melt. If that happens, then dealing with the idea of 5-10 m or more of sea level rise – something that hasn’t been seen in the whole of human history – is going to be a huge issue for the global community, obviously not something a regional council can face by itself.
What is going to cause more trouble first – sea level rise or flooding? Or will it be a combination of both, for particular areas?
This is an area of active research. For instance, in the MBIE-funded Climate Change Impacts and Implications programme, there has been quite a lot of work on some of these questions, for instance with a focus on the Hutt River. Some detailed modelling has been done for a few locations at least. There are also plans to develop a set of new regional sea level rise projections for New Zealand, using improved digital elevation models. That proposal builds in part on the recent report from the Parliamentary Commissioner for the Environment on sea level rise.
Whether river flooding or sea level rise is more important varies around the country, but one or both will affect a large majority of the New Zealand population into the future.
Will warmer temperatures change patterns like the Southern Annular Mode? And how would that affect New Zealand?
The Southern Annular Mode (SAM) has been trending towards its positive phase, where storms and the strongest westerly winds contract towards the pole. For New Zealand that means more settled and drier weather like we had this past summer, with storms tending to stay south of the country.
The SAM is driven by the difference in temperature between the equator and the pole, and in the Southern Hemisphere that’s been increasing. That’s partly because while the tropics to the north of us have been warming, the oceans to the south, and most of Antarctica, haven’t been warming much. The southern oceans are very turbulent and good at absorbing heat and transporting it to depth, so the deeper ocean around Antarctica has been heating up but the surface hasn’t. The net result is an increase in the north-south temperature difference felt by the atmosphere. But the main driver behind the trend towards the positive SAM is actually the ozone hole. Taking ozone away from the stratosphere over the pole cools the atmosphere there, so that increases the north-south temperature difference too. The ozone effect has been stronger than the greenhouse gas effect over the last 40 years or so.
The complicating factor is that the ozone hole is now recovering – it is expected to recover completely in about 50 years or so. As that happens, the polar air in the stratosphere warms, which would decrease the temperature difference between the tropics and the South Pole. Nobody is sure just how that’s going to play out, compared to greenhouse warming. This was discussed in the latest IPCC report; the conclusion is that the effect of the recovering ozone hole will probably cancel out the effect on the SAM of greenhouse gas increases for the next 50 years or so. If greenhouse gas concentrations are still increasing in another 50 years (and I really hope they aren’t) we’d start to see the upward trend in the SAM resuming. If the SAM continues its positive trend, the effect on New Zealand would be towards less rainfall and calmer conditions – but the SAM is just one component of our weather and there will always be lots of variability from day to day and week to week.
Thinking about the SAM, ozone depletion and greenhouse gas increase and how it all plays out over New Zealand illustrates why we can’t be so certain about regional climate change. The global-scale temperature increases and broad patterns of rainfall change are fairly well understood. At smaller regional scales many local factors come into play and the range of possible future outcomes is a lot broader. For example, although we can say that a warmer atmosphere has more moisture in it, generally making rain events heavier, we can’t say exactly how this will affect regional rainfall variations around New Zealand, as the patterns of wind changes across the country play a huge role.
You also work on Antarctic sea ice?
Yes, I’ve been drawn to thinking about it as there’s been a curious upward trend in sea ice around Antarctica in the last few decades. It’s odd to have more ice in a warming world! Being able to model the trend properly has been a big challenge, and my recent Marsden grant is trying to address that. As the oceans and atmosphere warm you’d expect that the ice would have more trouble forming. But melting ice sheets also means more fresh water, which freezes more easily than salt water. So that’s definitely part of the reason there is more sea ice. But how much of a part of it, nobody knows right now. Changes in wind patterns over the past few decades have obviously also played a role. James Hansen led a paper recently saying fresh water input from Antarctica is the main reason for the sea ice increase. He and his co-authors argue that the rate of melting is going to increase rapidly so we’ll see even more sea ice around the Antarctic. I’m not entirely persuaded that melt rates will increase so quickly, or that the input of fresh water is such a dominant reason for the sea ice trend.
Around the Antarctic Coast there are significant impacts from El Niño and Southern Annular Mode changes. The question I’d like to answer is when we’ll see a turnaround in sea ice extent in Antarctica – when will it stop increasing? Obviously it can’t just go on expanding forever. In fact in 2015 the sea ice didn’t expand so much in the winter, and that appears to be the result of the big El Niño event we’ve had the past year.
The question of what effect Antarctic sea ice has on us is another interesting one – it seems to help define the strength of the westerlies. I have a Master’s student looking at the extent of Antarctic sea ice and the location and strength of storms and how that affects rainfall over New Zealand. A colleague at the Greater Wellington Regional Council told me of a farmer he knows who keeps track of Antarctic sea ice extent because it affects how much rain he gets. We were excited to hear about this and must arrange to meet up!
These interviews are supported by the Royal Society of New Zealand, which promotes, invests in and celebrates excellence in people and ideas, for the benefit of all New Zealanders.