BOB MCNABB IS one of
the keepers of a number
that even he can’t fully
comprehend: 267 billion
metric tons of water. To
explain a quantity of this size, the
Ulster University glaciologist must
use analogies.
Depending on how you look at
it, 267 billion metric tons of water is
roughly half the volume of Lake Erie.
Or six months of discharge from the
Mississippi River. Or all the water in
a 10-foot-deep swimming pool that’s
the size of Ireland. “It’s really hard to
grok,” McNabb tells me.
If we could imagine all that water—a
pool as big as Ireland—could we also
imagine that much ice melting every
year for the past two decades?
Ultimately the enormous number
that McNabb helped derive is a speed:
the speed at which Earth’s more than
200,000 glaciers are being undone.
From 2000 to 2019, glaciers other
than the Greenland and Antarctic ice
sheets lost an average of 267 billion
metric tons of water each year, give or
take 16 billion metric tons, according
to research McNabb co-authored in
the journal Nature. Melting accelerated
over that span, from 227 billion metric
tons a year in the early 2000s to 292 bil-
lion metric tons a year from 2015 to 2018.
It’s one thing to try to reckon with
the speed of this loss, and another toSINCE 2000 THE WORLD’S
GLACIERS HAVE SHED MORE THAN
5.3 TRILLION METRIC TONS OF
WATER, REMAKING LANDSCAPES
AND RAISING SEA LEVELS.SPEED
B
BY
MICHAEL GRESHKO