Energy and Environment To truly grasp what we’re doing to the planet, you need to understand this gigantic measurement By Chris Mooney July 1, 2015 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:
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 “giga-ntic” — the kind of
unit that planets depend on.
In the International System of Units, the prefix “giga” means 109, or 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. In 2013, the Intergovernmental Panel on
Climate Change estimated that to have a good chance of keeping global warming under 2
degrees Celsisus above pre-industrial times — a widely embraced international goal —
humanity had, as of the year 2011, only about 1000 gigatons of CO2 left that it could emit.
Divide 1000 by 34 and you get maybe 3 decades more of emissions at current levels. And that
assumes that emissions merely stay constant, rather than rising along with rising economic
growth.
When it comes to our gigatons of carbon dioxide emitted each year, NOAA’s Earth System
Research Laboratory has done some fascinating math (which starts with a relatively simple
conversion of gigatons of carbon dioxide into gigatons of carbon):
In 2010 about 9 Giga-tons of Carbon (GtC) were emitted from burning fossil
fuels as 33 Giga-tons of CO2 gas.
How much is 9 Giga-ton? 9 billion tons or 9,000,000,000,000,000 grams, or
19,800,000,000,000 pounds.
Can you imagine…9 Giga-tons is the weight of about 132 billion people. The amount
of carbon we are putting into the atmosphere each year is equal to 20 times the
weight of the current world population.
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.