How our intuition deceives us in matters of global warming
Geologist explains that climate change is not limited to a simple increase in average sea level throughout the planet
Jerry Mitrovica [Jerry Mitrovica] has been engaged in a refutation of conventional wisdom for several decades. As a specialist in geophysics from Harvard, he studies the internal structure of the Earth and the processes taking place in it that affect areas such as climatology, human migration, and even the search for life on other planets. At the dawn of a career, he and his colleagues demonstrated that the tectonic plates of the Earth are moving not only from side to side, which leads to continental driftbut also up and down. By redirecting attention from the horizontal of modern geophysics to the vertical, he helped discover what he calls postmodern geophysics. Recently, Mitrovica revived and breathed new life into old ideas about factors seriously changing sea level, the consequences of which are very important for studying climate change as glaciers and ice sheets.
We met with Mitrovitsa in his spacious office near the famous Harvard mineral collection. Although he has a lot of experience in public speaking and many awards, in a normal setting he speaks softly and rejects compliments. He often talks about how his colleagues, graduate students and teachers inspired him to work and contributed to it.
This may seem counterintuitive, but the melting of glaciers can lead to a drop in sea level in one place and increase in another, more distant.
Some of your recent work explores the attraction of ocean water and ice sheets. This was unexpected.
This is simply Newton's law of attraction as applied to the Earth. An ice sheet, like the Sun and the Moon, exerts a gravitational pull on the water surrounding it. There is no doubt about it.
What happens when a large glacier melts like the Greenland ice sheet?
Three things happen. First, all this melted water ends up in the ocean. Therefore, the total mass of the ocean will definitely grow if the ice sheets melt - as it does today. The second is the gravitational attraction exerted by the ice shield on the surrounding waters, decreases. As a result, water moves away from the ice sheet. Third, with the melting of ice, the ground rises beneath it; there is a return.
What is the combined effect of melting the ice sheet, water flow and reducing gravity?
The effect of gravity is great. When the ice sheet melts, near its sea level falls. It is not intuitive. The question is, how far should we move away from the ice sheet in order for the effects of decreasing gravity and raising the crust to become so small that sea level starts to rise? This is also not intuitive. It is about 2000 km from the ice sheet. Therefore, if the ice of Greenland disappeared tomorrow, the sea level in Iceland, Newfoundland, Sweden, Norway - and all of them are within a radius of 2000 km from Greenland - would have fallen. On the coast of Greenland, the fall would be 30-50 m. But the farther from Greenland, the stronger the reckoning. If the Greenland ice sheet melts, the sea level in the southern hemisphere will increase by 30% more than the average. This is a lot.
What happens after the ice melts in Antarctica?
If the ice sheet of the Antarctic melts, then the sea level will close to it. But it will grow more than one would expect in the northern hemisphere. This scheme is known as sea level traces, since each ice sheet has its own geometry. Greenland gives one change in sea level, and Antarctica gives another. Mountain glaciers have their own tracks. This explains the variability of sea level. This is also an important opportunity. If someone denies climate change due to geographic variations in sea level change — that is, it does not grow the same everywhere — you can say: “This is not so because melting ice sheets give a geographically varying change in sea level.” This variability can be used to calculate how many percent comes from Greenland, how much from Antarctica, how much from mountain glaciers. You can determine the source of melting.
Why is the source of melting so important?
If you live on the east coast of the United States or in Holland, you do not need to worry about where the global sea level goes. A few years ago I was in Holland, and tried to convince the locals that they need to worry less about melting the ice sheet in Greenland than in Antarctica. But this is not perceived. When I give lectures, people just shake their heads. They do not believe when I show these circles around the melting ice sheet of Greenland, denoting the area in which sea level drops. Our intuition is based on walks along the shore or using water taps. It is not built on thinking about what will happen when one of the main ice sheets melts.
Melting ice sheet affects sea level in two ways. A decrease in gravitational attraction lowers sea level near the shield. At the same time, the water flowing into the ocean lifts it. So, if the Greenland ice sheet fell into the sea, the melted water would greatly raise the sea level. But nearby countries would record a drop in level.
Why are you sure that the glaciers of the planet, including the polar caps, will continue to melt?
One way to understand where our warming world is heading is to drive out the climate model. Another is to look into the past and ask what the ice sheets did last time when the temperature was the same or slightly higher. Now we are in a warm intermediate period between the ice cycles. If people did not warm up the climate, the Earth would have to prepare for the entrance to the next ice age in the future. The last interglacial period before this was about 120,000 years ago. Of course, 120,000 years ago, people had no effect on climate. It was a natural climate variability.
How did the ice sheets behave the last time the climate was so warm?
The last time, when it was as warm as it is now, the ice sheets, which we consider to be stable, disappeared, although not quickly. So why will we expect something else in the next few hundred or thousands of years? There is no reason for this, unless we do something to reverse the process.
Okay, let's say we expect that the warming will melt the ice sheets and raise the sea level. But where is the evidence that we are seeing this process today?
The average change in sea level in the 20th century was 1.2 mm per year. Over the past 20 years, we see an average change of 3 mm per year - an increase of 2.5 times relative to the XX century. A very good argument for skeptics who say that nothing changes or that nothing gets worse. Already it got worse. And if you look thousands of years into the past, you will find many handy tools. Records of eclipses, or Roman aquariums.
What can Roman aquariums tell us about sea level?
In the time of Octavian Augustusrich Romans built fish tanks. Fishermen came with a catch, and placed it there to keep the fish fresh when they were going to eat it - the Romans wanted to keep the fish alive for several days or weeks. The Romans were engineers, so they built these tanks in very precise accordance with the sea level. It is necessary that the walls are not very low, because at high tide the fish will be able to swim away, and not very high so that the waves refresh the water in the aquariums.
Kurt Lambek, a professor at the Australian National University, realized that studying the current sea level compared to the height of the walls of these aquariums, we can say how the sea level has changed over the past 2500 years. If the sea level in the last 2500 years would rise at the speed with which it rose in the XX century, these aquariums would be under 4 meters under water - and I can assure you that this is not so. They can be seen. You can walk along the shore, and you can see them from there. This suggests that the sea level could not rise at such a speed that we saw in the 20th century for a long time. Sea level over the past 2500 years has not risen as much as in the 20th century.
What do the Babylonian records of 2500 years ago eclipses tell us about climate change?
You can study these records and say exactly at what point the eclipse was recorded in Babylon. Then you can make calculations and say when this eclipse should have happened, if the current speed of the Earth’s rotation had not changed since that moment. And this can be done for Greek, Arab, Babylonian, Chinese records of eclipses - as did the British professor F. Richard Stevenson. He built a table, like other scientists before him, with a large set of similar eclipses, and showed a clearly visible slowdown in the speed of rotation of the Earth over the past few thousand years. Suppose, 2500 years ago, you synchronized two hours. Some counted the time accurately, while others were connected to the Earth, slowing down the rotation. For 2500 years, they would be out of sync for 4 hours. Here is this slowdown. Therefore we know that the Earth’s rotational speed has slowed down over the past 2500 years. But we would predict not a slowdown of the Earth.
And why should the rotation of the Earth in general slow down?
I recently published in Science Advances a paper about the Munch Puzzle . We have shown that this is due to three different reasons. One is “tidal scattering.” Tides beat on the coast, dissipating energy, and for many reasons slow down the rotation of the Earth. Another reason is the rather subtle interaction between the iron core and the rocky mantle of the Earth, which works to slow down the rotational speed, which we observe on the surface of the planet.
Is it something like friction fluid in a car gearbox? Is this due to the viscous interaction of the inner and outer parts of the planet?
This is not friction, but very close to that. The fact is that with us one fluid moves around another fluid, only at a different speed. If they are out of sync, their speeds affect each other. But, yes, you rightly say that there is a connection between them.
This is the second effect. There are tides on the shore, and what geophysicists call the mating of the core and the mantle. Both effects can be predicted quite accurately, but there remains one more factor - it is associated with the ice age, and we also model it. That is, we get tidal scattering, mating of the nucleus and the crust, and add the effect of the ice age, in which I speak as an expert. And, have a look: add all three of these effects together, and calculate exactly the four-hour slowdown, which we got in reality.
What is the effect of the ice age?
The earth is getting closer to the sphere. 20,000 years ago there was much more ice at the poles. When there are ice caps at the poles, they squeeze the Earth from both poles, and it flattens out. When the caps melted, the oblate planet began to regain its shape, become closer to the sphere, so our rotation speed should increase - like that of a ballerina or a figure skater. Correction from the glacial period provides an increase in rotational speed.
It turns out that these three factors - mating of the core and mantle, the restoration of the poles after ice and tidal dispersion - explain the changes in the speed of the Earth before the 20th century. What is happening today?
It is necessary to take the same model of the ice age and correct it, taking into account the rotation of the Earth in the 20th century. Having done this, we get a difference that we cannot yet explain. Therefore, we say - well, perhaps this is due to the melting of polar caps or glaciers.
We need to take the latest report of the Intergovernmental Panel on Climate Change ( IPCC), and look at the calculations for the melting of mountain glaciers. They say that in the 20th century the ice sheets did not change much. They began to actively thaw only in the last 20 years, but glaciers, in principle, disappeared during the entire XX century. We take melting calculations from the IPCC, calculate their effect on the rotation - they should slow down the Earth's rotation, as in the example with the skater - and compare them with the observations corrected taking into account the ice age.
It turns out that the water flows from the glaciers, and slows down the rotation of the Earth, as if the skater was spreading his hands to the sides?
Yes. Glaciers are mainly located near the axis. They are located near the North and South Poles, but most of the ocean water is not. In other words, we take glaciers at high latitudes, like Alaska and Patagonia, we melt, distribute around the planet. In general, water flows to the equator, since the material from the poles moves into the oceans.
That is, the melting of glaciers and polar caps moves a lot of water to the equator?
Yes. Of course, the ocean is everywhere, but if you move the ice from high latitudes to the ocean, you add masses at the equator and take it from the polar regions, and this should slow down the rotation. We carried out such calculations. We also calculated how these glaciers will affect the orientation of the poles. In both cases, our calculations exactly coincide with astronomical and satellite observations, as amended for the Ice Age.
In a recent paper, we demonstrated that the current data on rotation after the correction for the glacial period remains one discrepancy, and it is exactly what it should be, if you believe the opinion of scientists about how the ice melted in the XX century.
Taking into account such a number of stages, it is generally surprising that the calculations came together.
This is a completely different way of demonstrating the melting of ice sheets. And very good, because if you look at Greenland and say: “Oh, ice melts in the southern sector, you can see a decrease in its number”, then it is not known what happens in the northern sector. It’s impossible to build a good holistic picture of the whole Greenland ice sheet. But the rotation doesn’t matter, the south is there or the north, it depends only on how much mass moves from Greenland to the oceans. Therefore, rotation gives, as scientists say, an elegant integral measure of the mass balance of polar ice caps.
What inspired you to the path of the scientist?
I have always had more talk in the family about the history of the Renaissance than about science. I am the only scientist in the family. I took up engineering, an engineering physics program. In the third year, I took a course on plate tectonics and thought: “Wow!” And my first job — that was my curator’s idea — was an article about the causes of flooding in western North America from 50 to 80 million years ago. It was very interesting. You study at the institute, and you are already publishing a work explaining why North America was under water, more precisely, its western part.
Some say that all because of the ice, because of the change in its volume. Most often, people believe that this is due to changes in the rate of occurrence of tectonic plates. But in my work, which I wrote with my colleagues, we showed that such flooding of the continents usually does not occur because of changes in sea level. This is the result of the continent’s vertical movement, a reaction to the forces that control plate tectonics and move the continents up and down.
Many of the results of your work seem abstract and counterintuitive. Did it happen by chance?
There are many interesting problems in our science that you can see with your own eyes. But eyes can deceive. Richard Feynman, a great physicist, sometimes began his lectures on physics, demonstrating how much can be done on one intuition. They could do some things purely intuitively, and get roughly the right answer. And then he gave them several counterintuitive examples. And he said: “That's why we need physics. You need to understand when your intuition may not work. ” I am a follower of Feynman. Some things can be explained, but the scientist will always be confronted with things that do not match intuition. Based on the daily experience of using the bath, you will not understand that the water level near the glaciers is going down. You need to attract more; in this case - the second backwater of Newton. Need to attract physics,
How do you come up with unexpected guesses?
I think some scientists will disagree with me, but I think that you just need to give yourself time to think. A scientist needs to have some opportunity to ponder the facts. I highly recommend my students to acquire some other interests, because the best way to free up time to think is to take a break in science. I used to have such that in my models I saw something that I hadn’t met before, and I thought: “Well, a good scientist will never just leave that behind.” A good scientist bites into it at such moments and asks questions like: “Why do I see it?” Seeing something unexpected is one of the rewards of doing science.