Climate change impacts increasing severity of extreme weather
A specialist in physical geography says there is a link between climate change impacts and the extreme weather events like Cyclone Pam.
Transcript
A specialist in physical geography says there is a link between climate change impacts and the extreme weather events like Cyclone Pam.
James Renwick from Victoria University's School of Geography, Environment and Earth Sciences says factors such as increased moisture in the atmosphere and warmer seas are consistent with increased severity of weather.
He spoke to Johnny Blades who began by asking him if the frequency of extreme weather in the Pacific has increased.
JAMES RENWICK: In some ways, yes. And in other ways, no. So one thing that people focus on a lot is the number of tropical cyclones that are occurring and perhaps even the number of very strong ones such as Cyclone Pam. And the evidence is that there's no actual sign of an increase in the numbers of tropical cyclones in the Pacific. But the flipside is that when we have one of these storms it's likely to be a bit stronger and guaranteed to hold more moisture just because more warmer atmosphere by definition has more moisture in it. So when the rains from a tropical cyclone come they will be bound to be heavier and probably more prolonged. Whether or not we get stronger winds with a severe tropical cyclone, it's just too difficult to tell really, because that sort of thing is really sensitive to a whole lot of conditions. The temperature of the sea surface where these storms get most of their energy from, that's one thing. But the detailed structure of the atmosphere of the vertical in the bottom ten or twelve kilometres, that has quite an effect too. So it's a bit hard to tell, I guess what I'm saying here is, the relative number of strong storms is likely to increase a little bit but the total number of storms is likely to decrease.
JOHNNY BLADES: So with the increased likelihood of stronger or more severe cyclones, if not the instance of them, that's clearly linked to the warmer seas, things like that?
JR: Yes, that's linked to warmer seas, warmer atmospheres and so on. And I think one of the most important things around risks associated with tropical cyclones is sea level rise. Because that's been observed to be going on for over a century already and very likely to continue and accelerate through the rest of this century so even a twenty or thirty centimetre rise in sea level increases the risk of coastal inundation hugely. And it just makes it easier for the sea to broach whatever coastal barriers there are. So you put a bit of sea level rise in place, you then get a storm surge and the raised sea level that goes with... the low pressure in the middle of a tropical cyclone actually pulls the level of the sea up: this is called the inverse barometer effect. Basically the storm sucks the sea level upwards a little bit as it passes over, so all of those things combine to sea level rise, which may not sound much, to twenty or thirty centimetres, half a metre. It adds on to those other effects and it can be often enough just to push the seas over the top of whatever barriers there might have been or push the water inland significantly further than it would have gone before, just because there's a bit more of it, and the water's a bit deeper basically.
JB: And I suppose you've got king tides as well which seem higher than ever, devastating to places like Tuvalu and Kiribati.
JR: Well, that's exactly right, yeah. It's a symptom of the rise in sea level so the average sea level has gone up significantly in the last fifty to a hundred years. And again, that's the average sea level. You have a king tide and you're starting from a higher base, so the high tide goes up that much higher and because beaches are relatively flat in a lot of places, a small vertical rise can translate to a long distance horizontally, if you get what I mean. Waves can travel a lot further inland with just a quarter to a half a metre of sea level rise. We're expecting on the order of a metre or so of sea level rise for the rest of this century.
JB: In proximity to all this, where would you put things like torrential rain and droughts which seem to be hitting the Pacific Island countries quite frequently in recent months and years.
JR: Well, those events at least are consistent with the picture of how we're seeing the climate changing. Again, you can't quite ascribe a particular drought or deluge to climate change. You can talk about how much more or less likely one of those things is because the climate has already changed, and that approach is being used quite a lot these days. But yeah, what we're seeing and what the models are saying for the future is that both ends of the rainfall spectrum, the chances of extremes in both directions are increasing, so the increasing chance of droughts as well as the increasing chances of deluges. The idea there is that a warmer atmosphere does hold more moisture so when it does rain, you're likely to get more rain than you would have got before, so the chances of a flood or landslips or so on go up. But with this somewhat decreasing frequency or number of storms, you're basically having to wait longer between the storm events, so you can get longer dry spells as well. And for the tropical Southwest Pacific, one big factor in the rainfall climate at least is the South Pacific convergence zone, which lies from roughly Papua New Guinea down to Fiji and out into the central Pacific. And that convergence zone looks set to vary around more. We already know with an El Nino or a La Nina, the convergence zone moves around a bit and the amount it moves, only a few degrees latitude, is enough to bring the drought in one place and floods in another. It looks from some of the model results we've seen that the variations and the location of the South Pacific convergence zone might become greater. So that becomes more variable and the year to year rainfall becomes more uncertain in a lot of places in the Pacific.
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