larsen c iceberg

Should we be worried about surging Antarctic ice melt and sea level rise

Short answer: maybe

— 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.

global mean sea level
Global mean sea level data from the Colorado University Sea Level Research Group, with 4-to-5-year linear trends shown in black and red. Illustration: Dana Nuccitelli


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.

antarctica's ice loss
The amount of ice loss across Antarctica in total (purple), and in West Antarctica (green), East Antarctica (yellow) and the Antarctic Peninsula (red). Illustration: Shepherd et al. (2018), Nature


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.

by Dana Nuccitelli | The Guardian