Climate explained: why coastal floods are becoming more frequent as seas rise



As sea levels rise, it becomes easier for ocean waves to spill further onto land.
from http://www.shutterstock.com, CC BY-ND

James Renwick, Victoria University of Wellington


CC BY-ND

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz

I saw an article claiming that “king tides” will increase in frequency as sea level rises. I am sceptical. What is the physics behind such a claim and how is it related to climate change? My understanding is that a king tide is a purely tidal effect, related to Moon, Sun and Earth axis tilt, and is quite different from a storm surge.

This is a good question, and you are right about the tides themselves. The twice-daily tides are caused by the gravitational forces of the Moon and the Sun, and the rotation of the Earth, none of which is changing.

A “king” tide occurs around the time when the Moon is at its closest to the Earth and Earth is at its closest to the Sun, and the combined gravitational effects are strongest. They are the highest of the high tides we experience.

But the article you refer to was not really talking about king tides. It was discussing coastal inundation events.




Read more:
King tides and rising seas are predictable, and we’re not doing enough about it


When tides, storms and sea levels combine

During a king tide, houses and roads close to the coast can be flooded. The article referred to the effects of coastal flooding generally, using “king tide” as a shorthand expression. We know that king tides are not increasing in frequency, but we also know that coastal flooding and coastal erosion events are happening more frequently.

As sea levels rise, it becomes easier for ocean waves to penetrate on to the shore. The biggest problem arises when storms combine with a high tide, and ride on top of higher sea levels.

The low air pressure near the centre of a storm pulls up the sea surface below. Then, onshore winds can pile water up against the coast, allowing waves to run further inshore. Add a high or king tide and the waves can come yet further inshore. Add a bit of sea level rise and the waves penetrate even further.

The background sea level rise has been only 20cm around New Zealand’s coasts so far, but even that makes a noticeable difference. An apparently small rise in overall sea level allows waves generated by a storm to come on shore much more easily. Coastal engineers use the rule of thumb that every 10cm of sea level rise increases the frequency of a given coastal flood by a factor of three.

This means that 10cm of sea level rise will turn a one-in-100-year coastal flood into a one-in-33-year event. With another 10cm of sea level rise, it becomes a one-in-11-year event, and so on.

Retreating from the coast

The occurrence rates change so quickly because in most places, beaches are fairly flat. A 10cm rise in sea levels might translate to 30 or 40 metres of inland movement of the high tide line, depending on the slope of the beach. So when the tide is high and the waves are rolling in, the sea can come inland tens of metres further than it used to, unless something like a coastal cliff or a sea wall blocks its way.

The worry is that beaches are likely to remain fairly flat, so anything within 40 metres of the current high tide mark is likely to be eroded away as storms occur and we experience another 10cm of sea level rise. If a road or a house is on an erodible coast (such as a line of sand dunes), it is not the height above sea level that matters but the distance from the high tide mark.

Another 30cm of sea level rise is already “baked in”, guaranteed over the next 40 years, regardless of what happens with greenhouse gas emissions and action on climate change. By the end of the century, at least another 20cm on top of that is virtually certain.




Read more:
Our shameful legacy: just 15 years’ worth of emissions will raise sea level in 2300


The 30cm rise multiplies the chances of coastal flooding by a factor of around 27 (3x3x3) and 50cm by the end of the century increases coastal flooding frequency by a factor of around 250. That would make the one-in-100-year coastal flood likely every few months, and roads, properties and all kinds of built infrastructure within 200 metres of the current coastline would be vulnerable to inundation and damage.

These are round numbers, and local changes depend on coastal shape and composition, but they give the sense of how quickly things can change. Already, key roads in Auckland (such as Tamaki Drive) are inundated when storms combine with high tides. Such events are set to become much more common as sea levels continue to rise, to the point where they will become part of the background state of the coastal zone.

To ensure cities such as Auckland (and others around the world) are resilient to such challenges, we’ll need to retreat from the coast where possible (move dwellings and roads inland) and to build coastal defences where that makes sense. The coast is coming inland, and we need to move with it.The Conversation

James Renwick, Professor, Physical Geography (climate science), Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Election FactCheck: are larger, more frequent storms predicted due to climate change?


Kevin Walsh, University of Melbourne

Certainly larger and more frequent storms are one of the consequences that the climate models and climate scientists predict from global warming. But you cannot attribute any particular storm to global warming, so let’s be quite clear about that. – Prime Minister Malcolm Turnbull, speaking to reporters in Tasmania on June 9, 2016.

In the aftermath of the deadly East Coast Low that swamped eastern Australia, dumping massive amounts of rain in early June, the prime minister toured flood-affected Launceston and announced emergency relief funding.

Turnbull told reporters that larger and more frequent storms were forecast by climate scientists but cautioned that no individual storm could be attributed to global warming.

Is he right?

Checking the source

The Conversation asked the prime minister’s office for sources to support his statement but did not hear back before publication deadline. Nevertheless, we can test his statement against recent published and peer-reviewed research on this question.

The science shows that, just like real estate, climate change is all about location. Different parts of Australia will be affected in different ways by climate change.

And global warming will have different effects on different types of weather systems.

Let’s break Turnbull’s statement into two parts: is it true that we can expect larger and more frequent storms as a consequence of global warming? And is it possible to attribute a specific storm to global warming?

Can we expect larger and more frequent storms as a result of global warming?

Yes – but not for all regions or types of storms.

There are many types of storms that affect different parts of Australia, among them East Coast Lows, mid-latitude cyclones (a category that includes cyclones that happen in the latitudes between Australia and Antarctica), tropical cyclones, and associated extreme rainfall events. Each will be affected in a different way by climate change, and the effect will vary by region and by season.

On East Coast Lows: Acacia Pepler, who is studying extreme rainfall and East Coast Lows in relation to climate change, recently wrote in The Conversation that her research showed that:

… East Coast Lows are expected to become less frequent during the cool months May-October, which is when they currently happen most often. But there is no clear picture of what will happen during the warm season. Some models even suggest East Coast Lows may become more frequent in the warmer months. And increases are most likely for lows right next to the east coast – just the ones that have the biggest impacts where people live.

For all low-pressure systems near the coast, “most of the models we looked at had no significant change projected in the intensity of the most severe East Coast Low each year,” Pepler wrote.

On mid-latitude cyclones: Another study predicted that the overall wind hazard from mid-latitude cyclones in Australia will decrease – except in winter over Tasmania.

On tropical cyclones: Northern Australia is expected to get fewer cyclones in future – but their maximum wind speeds are expected to become stronger.

On rainfall: Scientists tend to be quite confident that climate change will be accompanied by an increase in extreme rainfall for most storms in future. One of the main reasons for this is that increased temperatures will cause increased evaporation. While the total amount of water held in the atmosphere will also increase slightly in future, the total amount of rain has to go up too.

Is it true you can’t attribute any particular storm to global warming?

Turnbull is correct. We cannot say for sure that a particular flooding rainfall event was solely “caused” by climate change, any more than we can say for certain that a particular car accident was solely caused by speeding (even if excessive speed was a likely or even major contributing factor).

Evidence for the effects of global warming on extreme rainfall events that have already occurred is currently equivocal for most regions.

According to a collection of studies published in 2015:

A number of this year’s studies indicate that human-caused climate change greatly increased the likelihood and intensity for extreme heat waves in 2014 over various regions. For other types of extreme events, such as droughts, heavy rains, and winter storms, a climate change influence was found in some instances and not in others.

One recent study in that report found:

evidence for a human-induced increase in extreme winter rainfall in the United Kingdom.

Verdict

Malcolm Turnbull was essentially correct on both points.

It’s true that scientists predict more frequent and intense storms for some parts of Australia as the climate changes. The evidence appears to be strong that extreme rainfall will increase. Some increases in extreme wind speeds are possible – but not in all regions or all seasons.

Turnbull was right to say you cannot attribute any particular storm to global warming. –Kevin Walsh


Review

This is a good FactCheck that summarises the broad conclusions from a range of studies examining the nature of current and likely future storms across Australia.

As the author points out, Australian storms range from tropical cyclones in the northern tropical regions to temperate east coast lows and mid-latitude cyclones.

The consensus regarding tropical cyclones is that they will generally decrease in frequency in the Australian region. In northeast Australia, they are forecast to experience the most dramatic decrease in frequency of any ocean basin globally. Some northern hemisphere ocean basins will see an increase in their frequency.

The intensity of these types of storms is expected to increase. This will not only involve higher wind speeds but also higher storm surges and floods. That will mean greater coastal impacts and damage to coastal developments and infrastructure.

So the prime minister’s statement about more frequent storms resulting from climate change does not apply to tropical cyclones – however, he was right to say that larger and more frequent storms are one of the predicted consequences of climate change. This consequence is predicted to apply to other storm categories, but not tropical cyclones.

And yes, climate scientists are hesitant to attribute the occurrence of any single storm to global warming. – Jonathan Nott


Have you ever seen a “fact” worth checking? The Conversation’s FactCheck asks academic experts to test claims and see how true they are. We then ask a second academic to review an anonymous copy of the article. You can request a check at checkit@theconversation.edu.au. Please include the statement you would like us to check, the date it was made, and a link if possible.

The Conversation

Kevin Walsh, Reader, School of Earth Sciences, University of Melbourne

This article was originally published on The Conversation. Read the original article.