The Still Inconvenient Truth
Glaciers have lost nine trillion tonnes of ice since 1961. Just think about that for a second: nine trillion tonnes in less than six decades. Alaska lost the most at three trillion tonnes of glacial ice, followed by Greenland with a loss of 1.23 trillion tonnes, and then the Southern Andes, which lost 1.2 trillion tonnes. Rounding out the top-five biggest losers are the Russian and Canadian Arctic regions, each of which lost over a trillion tonnes. This is according to a recent study that combined field observations on the ground with satellite data.
“While we can now offer clear information about how much ice each region with glaciers has lost, it is also important to note that the rate of loss has increased significantly over the last 30 years. We are currently losing a total of 335 billion tonnes of ice a year, corresponding to a rise in sea levels of almost 1 mm per year,” says Michael Zemp, leader researcher on the aforementioned study. Zemp and his team studied 19 regions around the world, along with 19,000 individual glaciers, to do the calculations estimating the changes in ice thickness.
In Antarctica, part of the world’s largest ice shelf, the Ross Ice Shelf – which is roughly the size of France and covers more than 500,800 km2, accounting for 32% of Antarctica’s total ice shelf area – is losing ice at an alarming rate. This melting is taking place 10 times faster than the melt rate for the whole structure.
The continent’s ice shelves play a crucial role in lessening sea level rises, albeit indirectly. The problem is not when a huge chunk of an ice shelf breaks off that sea levels rise, as ice shelves – being always connected to a landmass and sitting afloat in the ocean – have already displaced this amount of water. The problem is that they slow the flow rate of the ice streams and glaciers that feed them. Basically, they are very, very large stoppers.
Data used in a recently released study was collected over the course of four years using a range of instruments, including a custommade radar system and sensors positioned along a wire anchored to the ocean floor below the Ross Ice Shelf. The data revealed some fascinating facts, including that relatively warm water was flowing into a cavity situated beneath the shelf – the reason it was losing ice at roughly three times the normal rate for the region during summer months. This unusually warm water was coming from a nearby stretch of open ocean that is relatively free of sea ice, but which was being heated by the sun. Although the Ross Ice Shelf is not likely to collapse any time soon, scientists are very concerned about its stability over the long term.
A similarly concerning phenomenon has been seen in West Antarctica with the Thwaites Glacier, which is roughly the size of Florida. A giant cavity observed beneath this glacier is already 300 m deep and approximately two-thirds the size of Manhattan, and is eating away at the glacier’s ice.
As part of NASA'S ongoing Operation Icebridge, specialised planes fly over this frozen landscape several times each year, taking radar readings of the ice below. While scientists expect a certain amount of ice loss, the latest results surprised them. The radar revealed the extensive size of the aforementioned enormous cavity beneath the glacier – big enough to have once contained 14 billion tons of ice, the majority of which melted away in just three years. Starting in late 2019, early 2020, the International Thwaites Glacier Collaboration will embark on a field study on the ground, in the hopes of finding out more about what is happening here.
Himalayan glaciers are in trouble too. Researchers have found that these glaciers have been melting twice as fast this century as they did during the last quarter of the 20th century. A research team led by Columbia University has analysed satellite data of 650 glaciers in Bhutan, China, India and Nepal, gathered over four decades. (Interestingly, the older images were taken by US spy satellites during the seventies and eighties, and were only recently declassified.) By comparing overlapping photos of the same areas and mapping the differences between them, researchers managed to build threedimensional models of the mountains in the images. Using these, they could then measure the changes in elevation of the ice. This data was then compared to more recent shots from sophisticated satellites, which are able to directly measure elevation changes.
When the periods of 1975 to 2000, and 2000 to current were compared, a very stark difference was found. Since 2000, Himalayan ice seems to have melted at twice the average rate of the previous 25 years. Between 1975 and 2000, the average loss of glacial ice was around 25 cm per year. During the 21st century, this doubled to 50 cm per year. And warming air temperatures are to blame.
Sea levels have risen and fallen considerably over Earth’s 4.6-billion-year history. However, the recent rate of global sea level rise is frighteningly different to the average rate of the past 2,000 to 3,000 years, scientists say, and is rising faster. A continuance – or worse, acceleration – of this trend will potentially cause massive changes in the world’s coastlines. According to the American Museum of Natural History, sea levels would rise about 70 m if all the land ice on the planet were to melt. That’s enough to forever flood all of the world’s coastal cities.