Should we be worried about surging Antarctic ice melt and sea level rise?
Dana Nuccitelli, The Guardian, 18 Jun 2018
There’s recently been a spate of sea level rise denial in the conservative media, but in reality, sea level rise is accelerating and melting ice is playing an increasingly large role. In the first half of the 20th Century, average global sea level rose by about 1.4 millimeters per year (mm/yr). Since 1993, that rate has more than doubled to 3.2 mm/yr. And since 2012, it’s jumped to 4.5 mm/yr.
Thermal expansion (ocean water expanding as it warms) continues to play the biggest role in sea level rise, but its contribution of about 1.3 mm/yr is now responsible for a smaller proportion of total sea level rise (30% in recent years) than its contribution since the 1990s (40% of the total). That’s because of the acceleration in melting ice.
Glacier melt is accelerating, recently contributing about 0.75 mm/yr to sea level rise, up from 0.65 mm/yr since the 1990s. But the biggest jumps have come from ice in Greenland and Antarctica. Greenland had been responsible for about 0.48 mm/yr sea level rise since 1990, but in recent years is up to 0.78 mm/yr. A recent study in Nature Climate Change found that Greenland contributed about 5% to sea level rise in 1993 and 25% in 2014.
Antarctica is a huge question mark with warning signs
A new study published in Nature using data from a range of satellites found that Antarctica’s contribution has tripled from about 0.2 mm/yr since the 1990s to 0.6 mm/yr since 2012, during which time global sea level rise also spiked. Accelerated ice melt from Antarctica, Greenland, and glaciers have all played a role in the faster recent sea level rise. The question is whether it’s a temporary jump, or if we need to worry about a continued acceleration in Antarctic ice loss.
Another recent paper published in Earth’s Future found that rapid losses from Antarctic ice are plausible. The study found that in moderate to high carbon-emission scenarios, an average expected sea level rise of 2 to 2.5 feet by 2100 could actually become 3 to 5 feet once Antarctic ice sheet dynamics are taken into account.
The vast majority of Antarctica’s current ice loss is coming from West Antarctica, where about 75% of the glaciers are located below sea level. In East Antarctica, which has so far remained stable, only about 35% of the glaciers are below sea level. Warming ocean waters are melting the Antarctic ice from below, which is particularly problematic for that low-lying ice in West Antarctica. Research suggests that the collapse of the Western Antarctic ice sheet is already unstoppable.
Should we be worried?
Short term variations in sea level rise do happen. Sea level actually briefly fell in 2010 due to a strong La Niña cycle, which typically results in an increase of rain and snow falling over land. This resulted in a number of epic deluges and flooding across the globe; more water on land temporarily meant less in the ocean.
However, Antarctica and Greenland could potentially cause rapid sea level rise. As James Hansen explains in the video below, there have been periods in the not-so-distant past when sea levels rose at an average rate of 1 meter every 20 years.
In past eras when temperatures and atmospheric carbon dioxide levels were similar to those today and to the Paris climate targets, like in the last interglaciation and the Pliocene, sea levels were about 20 to 80 feet higher. Unless we manage to actually cool global temperatures, we’re certainly due for significantly more sea level rise. The large ice sheets on Greenland and Antarctica will continue to melt for as long as 1,000 years. That’s why sea levels were so much higher in past eras whose climates remained at hot temperatures like today’s for thousands of years.
It takes time for ice to melt. The question is, how fast will it happen? Sea level rise unquestionably poses a long-term threat, but how much of a short-term threat largely depends on just how stable the Antarctic ice sheet turns out to be. The recent acceleration of Antarctic ice loss, while not yet definitive, is certainly cause for concern.
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Scientists have used evidence to reconstruct sea-level rise around America’s northeast coast over the last 10,000 years.
New Jersey going back 10,000 years in research newly published in the Journal of Quaternary Science. To do this, they collected sediment cores drilled tens of meters below ground from coastal marshes, then examined the sediment back in a lab for microscopic organisms that only exist at specific depths below sea level. Salt marsh grasses also fossilized within the sediment were used to radiocarbon-date the samples.
The 10 maps contained in the GIF below show the movement of sea level at 1,000-year intervals leading up today:
“If we keep burning fossil fuels indefinitely, global warming will eventually melt all the ice at the poles and on mountaintops, raising sea level by 216 feet. Explore what the world’s new coastlines would look like.
“The maps here show the world as it is now, with only one difference: All the ice on land has melted and drained into the sea, raising it 216 feet and creating new shorelines for our continents and inland seas.
There are more than five million cubic miles of ice on Earth, and some scientists say it would take more than 5,000 years to melt it all. If we continue adding carbon to the atmosphere, we’ll very likely create an ice-free planet, with an average temperature of perhaps 80 degrees Fahrenheit instead of the current 58.”
from National Geographic Magazine, What the World Would Look Like if All the Ice Melted
The entire Atlantic seaboard would vanish, along with Florida and the Gulf Coast. In California, San Francisco’s hills would become a cluster of islands and the Central Valley a giant bay. The Gulf of California would stretch north past the latitude of San Diego—not that there’d be a San Diego.
The Amazon Basin in the north and the Paraguay River Basin in the south would become Atlantic inlets, wiping out Buenos Aires, coastal Uruguay, and most of Paraguay. Mountainous stretches would survive along the Caribbean coast and in Central America.
London? A memory. Venice? Reclaimed by the Adriatic Sea. Thousands of years from now, in this catastrophic scenario, the Netherlands will have long since surrendered to the sea, and most of Denmark will be gone too. Meanwhile, the Mediterranean’s expanding waters will also have swelled the Black and Caspian Seas.
Land now inhabited by 600 million Chinese would flood, as would all of Bangladesh, population 160 million, and much of coastal India. The inundation of the Mekong Delta would leave Cambodia’s Cardamom Mountains stranded as an island.
East Antarctica: The East Antarctica ice sheet is so large—it contains four-fifths of all the ice on Earth—that it might seem unmeltable. It survived earlier warm periods intact. Lately it seems to be thickening slightly—because of global warming. The warmer atmosphere holds more water vapor, which falls as snow on East Antarctica. But even this behemoth is unlikely to survive a return to an Eocene Climate.
West Antarctica: Like the Greenland ice sheet, the West Antarctic one was apparently much smaller during earlier warm periods. It’s vulnerable because most of it sits on bedrock that’s below sea level.The warming ocean is melting the floating ice sheet itself from below, causing it to collapse. Since 1992 it has averaged a net loss of 65 million metric tons of ice a year.
All maps by: Jason Treat, Matthew Twombly, Web Barr, Maggie Smith, NGM Staff. Art Kees Veenebos. From Sept. 2013 National Geographic Society