Measurement shows what we’re doing to Earth

There’s been a lot of news lately about the losses of ice from the planet’s two gigantic ice sheets, Greenland and Antarctica. And not surprisingly, some people have found it very stunning — simply because the volumes of ice involved sound so huge.

For instance, after I wrote about a new study showing a disturbing increase in ice loss from one relatively small part of the Antarctic ice sheet, I saw the following tweet:

Antarctic currently losing 56,000,000,000 tons of ice every year. goodbye, humanity.

Actually, that’s just the losses for one region of Antarctica, the Southern Antarctic Peninsula. Total losses are much larger. But the tweet — and the general magnitude of the kinds of changes that we’re seeing — got me thinking. It can be hard to comprehend the scale of these events, so it would be helpful to break it down into more comprehensible pieces.

Scientists often measure the loss of ice from the planet’s two gigantic ice sheets, Greenland and Antarctica, in a particular unit called a “gigaton,” which is sometimes also spelled “gigatonne.” Either way, it’s not something you encounter in your ordinary life, because it’s incredibly “gigantic” — the kind of unit that planets depend on.

In the International System of Units, the prefix “giga” means one billion (1,000,000,000). Hence terms like “gigawatt” or “gigahertz.” Thus, a gigaton is equivalent to a billion metric tons.

A male African elephant might weigh, at most, 6.8 metric tons, according to the San Diego Zoo. So a gigaton is well over a hundred million African elephants. As for sea life, the blue whale can weigh as much as 146 metric tons, according to NOAA. So a gigaton is more than 6 million blue whales.

Or for another analogy, consider how Meredith Nettles of the Lamont-Doherty Earth Observatory at Columbia University recently described a gigaton-sized piece of ice to me:

“If you took the whole National Mall, and covered it up with ice, to a height about four times as high as the Washington monument,” says Nettles, you’d have about a gigaton of ice. “All the way down from the Capitol steps to the Lincoln Memorial.”

With this in mind, let’s now look at how much ice Antarctica is losing, before moving on to the other sources of major ice loss, like Greenland.

A recent scientific paper estimated the total annual loss for Antarctica at 159 gigatons (plus or minus 48 gigatons, since these measurements are subject to considerable uncertainty). Other scientific estimates vary — Jonathan Bamber, an Antarctic expert at the University of Bristol, recently gave me a lower estimate — 130 gigatons per year. Yet another, even lower recent estimate is 92 gigatons per year. Whatever the exact number, Antarctica is clearly losing billions of African elephants worth of ice each year.

Climate change doubters, faced with such numbers, sometimes like to point out that East Antarctica is gaining some ice — but there is still a net loss of ice to the ocean because of massive losses in West Antarctica, the most vulnerable part of the Antarctic continent. And it’s not as if we should be exactly content about the status of East Antarctica either. The gigantic Totten Glacier of East Antarctica is losing 70 gigatons of ice per year, “enough to fill Sydney Harbour every two and a half days,” as one scientist with the Australian Antarctic Climate and Ecosystems Cooperative Research Center puts it.

“It is common to see broad stroke comparisons between East and West Antarctica for the sake of convenience, however, doing so risks diminishing the fact that there is net mass loss occurring in a large area of East Antarctica, as well,” Jamin Greenbaum, lead author of a recent study of Totten Glacier, told me by email. “It is well established that the region encompassing the Aurora Subglacial Basin and Totten Glacier is losing mass and contributing to sea level at a rate that is large compared to variability in snowfall indicating that the losses are due to processes beyond snowfall (e.g. enhanced ocean melting).”

Moreover, the rate of ice loss from Antarctica has been increasing, and scientists keep finding new sources.

So what does it all mean? It takes 360 gigatons of ice to raise the global sea level 1 millimeter, Bamber told me. So right now, Antarctica is doing that about once every three years. Globally, though, sea level is going up at an increasing rate of 2.6 to 2.9 millimeters per year, according to the latest research, because Antarctica isn’t the only contributor.

The gigantic Greenland ice sheet, which is facing rapidly rising Arctic temperatures, is also contributing, and at least for now, appears to be throwing off much more ice than Antarctica. A recent study put the loss at 378 gigatons per year for the years between 2009 and 2012. That’s a millimeter per year right there. Indeed, recent research suggests that some glacial iceberg calvings — from, say, Greenland’s Helheim Glacier — can be as large as a gigaton at a single time.

Glaciers around the world are also contributing — and at the scale of many gigatons. Indeed, recent research suggests that the glaciers of Alaska alone are now contributing 75 gigatons per year, which is very large number, considering that they only constitute about 11 percent of the world’s glaciers.

Warm ocean water also expands, so in addition to ice loss, the oceans are also growing in volume due to their temperature. And for those who don’t think 2 or 3 millimeters per year of ocean rise is a big deal —the real concern, with both Greenland and Antarctica (and also Alaska), is that these rates are increasing.

And why might that be happening?

It’s because of the 34 gigatons annually of carbon dioxide that humanity is currently putting in the atmosphere.

All of which further underscores that the gigaton is the unit that really explains to you how we’re altering the planet — changing its atmosphere, and changing its oceans, at a scale that’s hard for humans to conceive of. Net gigatons of CO2 are going into the atmosphere and net gigatons of H2O are going into the ocean. And if you wanted to reshape a planet, it’s hard to think of a better recipe than that.