Why are you so confident that the world’s glaciers, including the polar ice sheets, will keep melting?
One way to understand where we’re heading in this warming world of ours is to run a climate model. The other way is to
look to the past and ask what the ice sheets did the last time we were
this warm or a little bit warmer. We’re currently in an interglacial—a
warm period between glacial cycles. If humans weren’t warming the
climate, Earth might be poised to enter into another Ice Age in the
future. The last interglacial prior to the present one was about 120,000
years ago. Of course, 120,000 years ago, humans weren’t having any
impact on climate. That was natural climatic variability.
What did the ice sheets do the last time the climate was this warm?
The last time we were as warm as we are today, the ice
sheets that we think of as the least stable disappeared, albeit over a
protracted period. So why should we expect that the issue is going to be
any different in the next few hundreds to thousands of years? There’s
no reason to believe it, unless we do something to reverse what we’re
doing.
OK. So we’d expect warming to cause ice sheets to melt and raise sea level. But what’s the evidence that we’re seeing that now?
The average sea level change in the 20th century was 1.2
millimeters per year. What we’ve seen in the last 20 years is an
average of three millimeters per year—that’s a factor of two-and-a-half
increase from the 20th century to now. So that’s a nice way to address
the skeptic’s argument that it hasn’t changed or that it’s not getting
worse. It’s already gotten worse. And if you look back thousands of
years, you have a wide range of tools at your disposal. One is eclipse
records, and one is the Roman fish tanks.
What do Roman fish tanks tell us about sea levels?
Wealthy Romans at the time of Augustus were building
fish holding tanks. The fishermen would come in with the fish, they’d
put them there so that the fish were fresh when they ate them—they
wanted to keep them alive for a few days or weeks or whatever. The
Romans were engineers, so they built these fish tanks at very precise
levels relative to sea level at the time. You didn’t want the walls to
be too low because at high tide the fish would swim out; you didn’t want
it to be too high because you wanted tides to refresh the water within
the tanks.
Kurt Lambeck, a professor at the Australian National
University, recognized that by looking at the present day elevation of
those fish tanks, we could say something about how sea level had changed
over the 2,500 years since then. If sea level over the last 2,500 years
was going up at the rate that it went up in the 20th century, those
fish tanks would be under 4 meters of water—12 feet of water—and I can
assure you they’re not. You can see them. You can walk along the coast,
they’re visible. What that tells you is that it is impossible that sea
level went up by the rates that we saw in the 20th century for any
extended period of time earlier than that. Sea level has not gone up
over the last 2,500 years like it has in the 20th century.
This is an entirely different way to show that ice sheets are melting.
What can records of Babylonian eclipses 2,500 years ago tell us about climate change?
When we look at eclipse records, we can say “here’s when
a Babylonian eclipse was recorded.” Now, I can do a calculation and ask
when that Babylonian eclipse should have occurred if the present
rotation rate of the Earth had stayed constant in the time between the
eclipse and present day. And you can do that for Greek, Arabic,
Babylonian, Chinese eclipses, and this is what a professor in the U.K.,
F. Richard Stephenson, did. He tabulated, as others did before him, a
large suite of such eclipses that show a clear slowing of the Earth’s
rotation rate over the last few thousand years. Say you have two clocks
synchronized 2,500 years ago. One kept time perfectly and the other was
connected to the Earth whose rotation rate was slowing. Over 2,500
years, they would go out of sync by about four hours. That’s kind of the
level of slowing. So what we know is that the Earth’s rotation rate has
slowed over the last 2,500 years. But the Earth’s slowing isn’t what we
would predict exactly.
Why would you expect the Earth’s rotation to slow at all?
I published
this paper in
Science Advances
on something called Munk’s Enigma. What we showed is that it comes from
three different effects. One is what’s known as “tidal dissipation.”
Tides crash into the shoreline and each time they do they dissipate
energy, and for a variety of reasons they slow the Earth’s rotation.
Another thing we talk about is that there is a very subtle coupling
between the core of the Earth, which is iron, and the rocky part of the
Earth, the mantle, which acts to change the Earth’s rotation rate we see
sitting on the surface of the planet.
Is it like the friction of the fluid in a car’s a
transmission; it has to do with how viscous the connection is between
the inner and outer parts of the planet?
It’s not friction, but it’s pretty darn close. It’s the
fact that you’ve got one fluid moving against another fluid that’s
moving at a different rate. If they come out of sync, their rates will
influence each other. But it is as you say, a connection.
So, this is another effect. We have the tides crashing
in and what geophysicists would call core-mantle coupling. We can
predict both of those pretty accurately, but you’re still left with a
difference and that difference is due to the ice age and we model that.
We’ve got tidal dissipation, core-mantle coupling, and now we add the
Ice Age Effect, which I’m the expert on. And lo and behold, when I add
that to these other two effects, I get precisely the four-hour slowing I
saw.
What is the Ice Age Effect?
The Earth is growing more spherical because 20,000 years
ago we had a lot more ice at the poles. When ice sheets were at the
poles they kind of squished the Earth from both poles and the Earth
flattened a little bit. When those ice sheets melted, that flattening
started to rebound and we’re becoming spherical, so our spin rate should
be increasing, like a ballerina or a figure skater. The ice age
correction is a speeding up of the rotation rate.
So these three factors—core-mantle coupling, post ice
rebounding of poles, and tidal dissipation—explain changes in the speed
of the Earth until the 20th century. What’s happening now?
We want to take that same ice age model and correct for
20th-century changes in Earth’s rotation. When we do, we get a
difference that we haven’t explained yet. So now we say; well, maybe
that’s due to polar ice sheet melting or polar glacier melting.
The way to do that is to go to the IPCC, their last
assessment report, and look at the calculation of mountain glacier
melting, because those tabulations suggest that the ice sheets weren’t
changing that much in the 20th century. Ice sheets have only really
started to melt in the last 20 years or so, but the glaciers were
popping off all through the 20th century. We take that glacier melting
that the IPCC tells us, compute its effect on rotation, and one effect
would be to slow the Earth’s rotation just like the figure skater, and
compare it to these ice-age corrected observations.
Is water moving off glaciers, slowing the Earth’s
rotation, this time analogous to a figure skater putting arms out?
Right. Glaciers are mostly near the axis. They’re near
the North and South Poles and the bulk of the ocean is not. In other
words, you’re taking glaciers from high latitudes like Alaska and
Patagonia, you’re melting them, they distribute around the globe, but in
general, that’s like a mass flux toward the equator because you’re
taking material from the poles and you’re moving it into the oceans.
That tends to move material closer to the equator than it once was.
So the melting mountain glaciers and polar caps are moving bulk toward the equator?
Yes. Of course, there is ocean everywhere, but if you’re
moving the ice from a high latitude and you’re sticking it over oceans,
in effect, you’re adding to mass in the equator and you’re taking mass
away from the polar areas and that’s going to slow the earth down.
That’s the calculation we did. We also computed how those glaciers would
affect the orientation of poles. In both cases, when you do that
calculation and you compare it to this ice age corrected satellite and
astronomical observations, you fit them precisely.
What we showed in this paper is that when you look at
the modern data on rotation and you correct for ice age, you have a
leftover, and that leftover is precisely what it should be if it were
due to the kind of melting that global change scientists believe
happened in the 20th century.
There are some things that you can explain, but as a
scientist you’re always going to face things that are counterintuitive.
With all those steps, it’s amazing that the calculations work out.
This is an entirely different way to show that ice
sheets are melting. It’s a very good way because if you’re looking at
Greenland and you say, “Oh, it’s melting in the southern sector, I can
see ice diminishing,” you don’t necessarily know what it’s doing in the
northern sector. You don’t get a good integrated view of what the
Greenland ice sheet is doing. But rotation doesn’t care about north vs.
south, it just cares about how much mass is moving from Greenland into
the oceans. And so rotation provides what a scientist would call a
really elegant integrated measure of the mass balance of polar ice
sheets.
What inspired you to become a scientist?
In my family, we had more discussions about Renaissance
history than we ever did about science. I’m the only scientist in my
family. I went into what’s called an engineering science or engineering
physics program. I took a course in plate tectonics in my third year,
and I thought, “Whoa!” And my first paper—it wasn’t my idea, it was my
advisor’s idea—about what caused the flooding of the western part of
North America 50 to 80 million years ago—that was quite a thrill. You’re
a few years into research graduate school, and you’ve just published a
paper that explains why North America was underwater, the western part.
What is the explanation?
Some said it was some ice effect, that ice volumes had
changed. More often people thought that it was linked to changes in the
rate at which tectonic plates were created. But in my work and that of
some colleagues we’ve shown that those sorts of events when continents
flood typically are due not to some global change in sea level. Rather,
it’s due to the vertical motion of the continent itself reacting to the
flow that’s driving plate tectonics and driving continents up and down.
So many of your results seem abstract and counterintuitive. Is that a coincidence?
There are so many interesting problems in our science
that you can see with your eyes. But your eyes can fool you. Richard
Feynman, the great physicist, used to start his physics lectures by
showing students their intuition could take them a long way. They could
do things just through intuition that would get them roughly the right
answer. Then he used to throw some counterintuitive examples at them.
Then he said, “This is why you need physics. You need to understand when
your intuition might go wrong.” I firmly am a Feynman acolyte. There
are some things that you can explain, but as a scientist you’re always
going to face things that are counterintuitive. You’re never going to
understand that water is falling near an ice sheet from your everyday
experiences of the bathtub. You need to bring in something more; in this
case, Newton’s second law of gravitation. You have to bring in physics;
otherwise, you’re never going to explain that.
Where do your “A-ha!” moments come from?
I think some scientists would disagree with me, but I
think you really do have to give yourself time to think. You need to
have some way in your life as a scientist to mull over what you’re
seeing. And I strongly encourage my graduate students to have other
interests, because the best way to have that time is to take a break
from science. I’ve had moments where I’ve seen something in my models
that I’d never seen before and I think, “Well, you know, a good
scientist is never going to walk away from that.” A good scientist at
that point sort of burrows in and says, “Why am I seeing that?” Because
to see the unexpected is the reward of science.
Daniel Grossman is a freelance science journalist and radio producer based in Boston.
This article was originally published on July
19, 2018, by Nautilus, and is republished here with permission.
