Think about how climate change affects earth and some obvious answers come to mind: Rainfall patterns will change, sea temperatures will rise, more corals will bleach, planetary wind systems will change, the number of extreme weather events will rise...
But a paper published last month in scientific journal ScienceAdvances, added another change to what we know. As ice-caps melt and the meltwater moves beyond the polar regions, it said, the earth is seeing a planet-wide redistribution of mass. This redistribution is significant enough to change the behaviour of the planet, the paper argued. According to it, melting ice – especially the rapid losses in Greenland – explains about 66% of the change in the shift of the Earth’s spin axis (its tilt) – in the last 12 years. One of the two authors of the piece is Surendra Adhikari, a geophysicist at National Aeronautics and Space Administration's Jet Propulsion Lab in Pasadena, California. Over email and phone, the NASA scientist answered questions from Scroll about how weight distribution would affect the planet.
Excerpts from the interview
Why does weight redistribution affect Earth’s pole motion?
Well, let me give you some background here. Imagine you have a “perfect” sphere which is rotating about a fixed spin axis. Now, take a chunk of material out of this sphere. You will see that the spin axis starts wobbling, in such a way that the spin axis tends to move toward the region where the mass is removed. The same idea is applied to wobbling of planetary bodies, such as Earth. Climate change redistributes mass on Earth’s surface (because of melting of glaciers, rise in sea level, change in land water storage patterns), which perturbs Earth’s inertia tensor, angular velocities, and hence, pole motion.
What are the major weight redistributions underway on the planet. There is water, like you say...
Ice/water would be the dominant mode of earth’s surface mass transport. There could be others. For instance, the transport of mass between atmosphere and oceans, but these do not really contribute to pole motion. Mass is also being transported within the interior of Earth. For example, there is a huge ongoing mass deficit around Canada (Hudson Bay), which is associated with the demise of the former Laurentide Ice Sheet.
Within water, I can think of three or four major redistributions – the polar icecaps are melting, the glaciers are melting, we are taking out groundwater faster than ever before. With all of them, water moves towards the seas. But is it mainly these three or are there other redistributions underway
You pretty much covered everything. We rely on GRACE (Gravity Recovery and Climate Experiment) monthly gravity solutions, which essentially measures the change in “water storage” in a given region. So, you could have many possible contributors to “water storage” – for example, intensive precipitation, excessive groundwater pumping et cetera. But, our analysis was not aimed at isolating what particular aspect of “water storage” is the dominant contributor to pole motion. (This is something I am currently investigating.)
The paper accords a very central position to the Greenland ice cap while explaining polar motion between 2003 and 2015. What makes Greenland so significant?
Simply because Greenland Ice Sheet has been the dominant source for Earth’s surface mass transport. It has been releasing about 280 gigaton of ice every single year over the past 1.5 decades.
We know some of the fallouts of this redistribution – rising sea levels, perhaps a cooling of ocean currents, et cetera. But what are its other impacts, especially when see through the prism of a change in weight distribution? The tilt is one aspect...
There could be many – a redistribution of mass alters gravity and hence sea-level distribution. Meaning, you do not expect “uniform” rise/fall in sea-level distribution. In fact, this is how sea-level must have been evolved over the past 1.5 decades (See this animation I prepared). Earth’s surface mass redistribution also affects length-of-day.
How does it affect the length of day?
If you look at the Earth, it is not a perfect sphere. It has a shorter radius towards the poles and a longer one along the equator. It is what we call the equatorial bulge. If ice sheets continue melting, for example, most of the meltwater will tend to move towards the equator. This ultimately affects Earth’s spin rate, and hence length-of-day. To understand what that means, think of a figure skater. If she stretches her arms out, her spin rate will slow. If she pulls them in, it will rise.
We have a fairly accurate record of length of days for more than 2,500 years. Ancient civilisations used to record timing and duration of solar and lunar eclipses. With those archaeological records, we can estimate length of days. And what we see is that the earth’s spin rate is generally slowing down. Meaning days are becoming slightly longer.
At what point would a shift in the tilt be considered significant enough to alter weather in what we conceive of as equatorial and polar regions? At the present rate of movement, how long would it take us to get there, as opposed to the older rate?
There is no magic number that determines when pole shift does really alter the global weather pattern. For now, I can only say this: We are talking about several inches per year of pole shift. There is no certainty that it will be heading in the same direction in the future. (Note: materials will also get transported within the interior of our planet toward counterbalancing the surface mass redistribution, although this process is very slow.) And, imagine the vastness of our planet.
The real significance lies in the fact that these changes could be increasingly manmade.
What are some of the major questions thrown up by this question of weight distribution? What progress are we making towards understanding those?
Well, in this paper, for the first time we have made a causal connection between global-scale climate variability and (decadal-scale) pole motion. Since we have a very accurate record of pole motion (since 1899), we should be able to utilise this pristine dataset to constrain the model of the past climate change. And, this will ultimately lead us toward more confidence predictions of global climate variability in the decades to come.