Environment

    

A warming world could slow the circulation of the Atlantic Ocean, potentially triggering African droughts and more rapid sea level rise around Europe. If it happens, it won’t be the first time the Atlantic has been disrupted during a warm period.

Water in the Atlantic is constantly on the move. In the icy north, cool and dense surface water sinks and flows south, forming the North Atlantic Deep Water. The NADW then encourages warm surface water in the south to flow north, creating the Gulf Stream.

In theory, this “conveyor belt” could weaken as a result of climate change. A hugely exaggerated version of this proposal was the premise for the film The Day After Tomorrow. But until now the evidence from warmer periods in Earth’s past suggested that temperature rises would not affect the circulation. A new study indicates otherwise.

Eirik Vinje Galaasen at the University of Bergen, Norway, and his colleagues looked at deep-sea sediments from a site off the southern tip of Greenland. Sediment builds up so rapidly there that 3.5 centimetres are deposited each century, meaning that important but short-lived climate shifts show up clearly.

Shifting waters

The team focused on sediments from the last interglacial, a warm period between 130,000 and 115,000 years ago, before the last ice age. The ratio of carbon isotopes in fossilised microbes from this time showed several sudden shifts, each indicating an abrupt change in environmental conditions and probably in the NADW.

Some members of the team had seen a similar isotope ratio shift before, in marine sediments from 8200 years ago. In 2007 they showed that the shift occurred when a vast North American lake burst, sending 100,000 cubic kilometres of fresh water into the North Atlantic, and briefly reducing formation of the NADW. Galaasen says NADW reductions were common in the last interglacial, when the North Atlantic was warmer.

According to Galaasen, the NADW reductions have been missed because sediment builds up slowly at most deep-sea sites, making such brief events hard to spot. At most sites the shifts would be recorded in just a few millimetres of sediment, rather than a few centimetres as in the Greenland site.

“The deep Atlantic is not as stable as previously thought,” says Galaasen. “Perhaps especially so when the North Atlantic is warmer and fresher, which may be the case again in the near future.”

Coming soon?

If the NADW conveyor did slow, we would certainly feel it. For starters, sea levels would rise faster around Europe, because the change in ocean currents would see more water ending up there.

What’s more, north Africa might suffer severe droughts, says Galaasen, because the altered ocean currents could affect atmospheric circulation in the subtropics. Climate records from the past 57,000 years support such a link (Paleoceanography, doi.org/d92bk9).

Less NADW could also mean that the planet would warm more than expected. “NADW formation is an important process by which anthropogenic carbon dioxide enters the deep ocean, thus helping to slow down the rise in atmospheric CO₂ levels,” says David Thornalley at University College London in the UK. An NADW reduction could reduce the size of this carbon sink, leading to even more warming.

However, it is far too early to say whether reductions in NADW are likely in the short term, says Carl Wunsch at the Massachusetts Institute of Technology. We still know little about how ocean circulation varies over millennia. “This is the sort of paper that really needs to be evaluated over the coming years.”

The relevance to today’s climate will need further investigation, agrees Thomas Stocker at the University of Bern in Switzerland. The study suggests NADW reductions are particularly likely after an influx of fresh water from melting ice, and it is by no means clear that will happen. “Today much less terrestrial ice surrounds the North Atlantic than at the early stages of the past interglacial,” he says. The NADW weakening would only happen if a lot of fresh water came off the Greenland ice sheet. Stocker says that is a wild card.

“Death Spiral” Continues

Bad Astronomy

The entire universe in blog form

Feb. 20 2014 7:45 AM

Old Arctic Ice Is Disappearing and Taking the Rest of the Ice With It

By Phil Plait

Photo by the NOAA, from the video

The National Oceanic and Atmospheric Administration recently released a video that shows compellingly just how bad things are getting at the top of the world. The animation displays Arctic ice over time not just by how much area it covers, but also by age, with white being the oldest ice (nine years or older):

It’s not hard to see that over the past few years, the oldest ice has melted away, and over time the ice gets younger. That’s not good: Older ice is thicker and tends to hang around longer; young ice is generally thinner and melts away every summer. That means that the year-round amount of ice is dropping, and dropping rapidly. As the Arctic warms, its ability not just to form ice but to keep it wanes.

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It’s not just area, either, it’s volume. Yes, the ice is covering less area of the sea, but it’s also thinning. That means it will melt even faster in the summer. This is very bad, because as far as we can tell this is a runaway process. Ice is white and reflective, while the water under it is darker. When the ice goes away it exposes the darker water which absorbs sunlight more efficiently, raising the temperature further. That’s one of the reasons we’re seeing the ice dwindling in the Arctic with alarming rapidity.

This is fact, pure and simple. Yet I still see some global warming deniers claiming the Arctic ice has “rebounded” from its low in 2012. That is 100 percent pure grade-A fertilizer (and I’d use far stronger words if I didn’t want to keep this blog at least semi-family-friendly). There were several reasons the ice hit a record low in 2012—including an overall thinning for years that made it prone to break up in big weather events. There was just such a storm in August 2012. But that thinning is due to increasing temperatures in the Arctic, and that’s due to global warming.

This video shows how bad it is:

As you can see, the little bump there in 2013 doesn’t go very far in mitigating the huge and stunning drop in Arctic sea ice we’ve been seeing for many years now. “Recovery,” my ice. That’s denier smoke-and-mirrors, an attempt to distract people from what’s really going on (just like the completely false claim that Antarctic ice growth compensates for Arctic loss). Ironically, even the major oil companies are admitting Arctic ice is going away. In some ways, they’re counting on it, since it makes drilling for oil up there easier.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! Follow him on Twitter.

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Incidentally, Arctic ice is right now at a record low for this time of year. It’s far too early to be predicting how low it will get during the spring and summer thaw—the current amount is not lower than the previous February minimum by a statistically significant amount—but it’s definitely worth keeping our eyes on it. The warmer-than-usual temperatures up north are certainly to blame for this.

We don’t know how long it will be before we see our first ice-free Arctic summer, but it may be as soon as 30 years. Most likely it will be somewhat longer; I hope so. But the bottom line is that the ice is going away due to global warming, and as it does we’ll see worse and worse effects from it. The time to stick our heads in the sand about this is long, long gone.

Urgent need to recycle rare metals

Rare earth metals are important components in green energy products such as wind turbines and eco-cars. But the scarcity of these metals is worrying the EU.

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Rising sea levels are typically written about as a “threat to future generations” – something to worry about by 2050 or 2100, not now. But if you want to see why even relatively small increases in sea levels matter, come to Darwin.

Australia’s top end is a global hotspot for rising sea levels. In Darwin and the World Heritage-listed floodplains of Kakadu National Park, we’re seeing how the combination of gradual sea level rise and “normal” weather events – such as storms and king tides – can have surprisingly big impacts.

Small changes adding up to big damage

Storms and heavy rain are not unusual in the Darwin wet season. But recent weather has been spectacular, as monsoonal onshore winds coincided with king tides to batter the shoreline. Crowds gathered to see waves crashing over cliffs and jetties that usually overlook calm seas. Tragically, two people got into trouble in these rough seas, losing their lives, and a young boy drowned in a flooded stormwater drain.

Sea levels around Darwin, which abuts the warm, shallow Arafura Sea, have risen by about 17 centimetres over the past 20 years. As the CSIRO noted in its last State of the Climate report, the rates of sea-level rise to the north and northwest of Australia have been 7 to 11 millimetres per year, which is two to three times the global average. Along the eastern and southern coasts of Australia, rates of sea-level rise are around the global average.

Click to enlarge

Seventeen centimetres may not seem much, especially with a 7 to 8 metre daily tidal range. However, raising the underlying base makes a big difference, not just to the ultimate penetration of big tides and storm surges, but also in the everyday hydrodynamic fluxes on beaches, estuaries and floodplains.

The impact of recent Darwin weather on infrastructure — both built and natural — has profound implications for coastal planning, design, management and regulation. The recent confluence of 8-metre king tides with strong onshore winds after weeks of wet monsoonal weather was unusual, but well short of being even a Category 1 cyclone.

By Darwin standards, there has been nothing exceptional about this wet season’s wind or tides. There was heavier than average rain last month – but even that has been a long way short of the records, or even a 1-in-10 year event.

Click to enlarge

Yet the damage we are seeing in Darwin has been considerable. Near where we live, a significant stretch of the city’s most popular bike path (right) was washed away. Further north, a large casuarina tree, which 10 years ago stood atop the landward side of two dunes, toppled into the surf. A blowhole emerged where waves had undercut the cliffs.

As the City of Darwin has acknowledged for years, eroding coastlines are a growing problem for Darwin.

And as global maps in a recent article in the journal Nature showed, Darwin is just one of many cities – including heavily populated centres such as New York City, Kolkata and Shanghai – at growing risk of coastal flooding, in part due to accelerating sea-level rise.

How can we manage change better?

In Darwin, like other low-lying coastal settlements, we essentially have three options: start managing our retreat from the sea; try to engineer coastal defences; or get used to much more volatile and risky life on the edge, and modify our systems, policies and behaviour accordingly.

Of course, we could simply do nothing. But we contend that is the least credible and potentially most expensive option in the long run.

The other three options of managed retreat, investment in coastal defences, and accepting greater risk are not mutually exclusive. They can be blended within a well-conceived long-term strategy.

Managed retreat is the most confronting option, which some communities are already facing. Some low-lying coastal areas simply cannot be defended cost-effectively, and even the best adaptation strategies may be inadequate.

But there are also significant opportunities to reconfigure coastal settlement in ways that minimise social disruption.

In places with valuable assets, such as parts of some cities or Kakadu, we can improve coastal defences, natural and/or engineered.

After our recent storms, Darwin’s coasts were more intact in sections where mangroves, trees and shrubs protected the soil. While the shoreline did retreat, damage was less than in cleared sections. We need to be replanting the dunes we want to keep, and retaining or restoring mangroves in estuarine and low-lying areas.

The North Australian Biodiversity Hub is working with Kakadu Traditional Owners to look at options for managing the impacts of weeds and sea level rise on the floodplains that are so important for food for local people, and more broadly for Top End fishing and tourism experiences.

In Darwin, hard protection of foreshore made some difference. But even rock-walled sections were disassembled in places, with the rocks dragged back into the sea or thrown, with astonishing force, onto the tops of cliffs.

If expensive hard protection is going to be used, it needs to be done at a scale that is engineered to last for decades and withstand extreme weather events, taking into account projected future sea levels.

The latest climate science suggests that northern Australia may have less frequent cyclones in future, but a higher proportion of extremely intense (Category 5 or worse) tropical cyclones.

Thirdly, the construction of new residential or tourism infrastructure in exposed zones of the coastal environment is inherently risky. At the very least, coastal planning must take into account the amplified risks from continuing sea-level rise.

Prepare now, or pay later

What we are seeing now in Darwin is a taste of things to come in many coastal areas of the world if we don’t take preventative and adaptive measures.

This has major implications for residents, investors, insurers, planners and policymakers. It also promises to create fertile grounds for litigation in the future, if people approving developments are not seen to be basing their decisions on the best available information.

Recent events in Darwin underline that sea level, especially in the monsoonal north, is rising fast, and old assumptions should no longer hold.

So we need to think long-term about which bits of coastal infrastructure we want to try to keep, and for how long, while steadily moving essential services to more secure places.

And we should remember that recent storms have been mild compared to the cyclone that will likely whack Darwin again sooner or later.