In elementary school, we learn that jagged mountains like the Himalayas are young and growing, while smoother mountains like the Appalachians are older, eroding, and shrinking. And most of the time this is true. But we are entering a very special part period of geologic history. If we play the correct semantic games, we may be able to coerce the mountains that are ‘growing’, to ‘shrink’ for a few years, after which time they will reverse course and continue to grow again.
The game we have to play involves looking at how we define the height of a mountain. The height of a mountain is traditionally defined as the height of the peak compared to sea level. This seems like a no brainer, except that the concept of sea level is quite complex. Bear with me for a simplified explanation.
First off, the oceans are not level, even though they are all connected. This is not intuitive, as water in a container like a measuring cup will tend to normalize itself even if the container is tipped; the water level stays constant. However, the earth is a big place, and because it is spinning, it is not spherical; the equator puckers out slightly more than the poles (the earth can be approximated as an oblate spheroid). This asymmetry leads to unequal gravitational forces at different places on the surface of the earth, and as result the equilibrium ‘sea level’ varies. Furthermore, density of the earth is not constant, and one has to take into account details such as the thickness of the mantle in various places, and surface aberrations, like mountains and canyons, that further affect gravitational forces and the earth’s center of mass. The geoid, which models where sea level should be based solely on the earth’s gravitational effects, takes into account these phenomena. According to the geoid, sea level should be lower in areas where gravity is stronger, and higher where gravity is weaker. The geoid is a model of the global mean sea level (MSL).
Actual local measurements of sea level can deviate starkly from the geoid. The obvious suspects are tides caused by celestial bodies such as the moon, and to a weaker extent, the sun. However, these effects can be averaged out by taking measurements over a couple decades, and are ignored when calculating the geoid as they are temporally dependent.
But now less likely culprits that manipulate sea level start to show up. For instance, the thermal expansion of water is a large contributor to sea level rise. The warmer the ambient temperature of the earth, the more water expands and the greater the actual MSL becomes. Of course, this is affected by factors such as salinity, further complicating matters.
Finally, increased global temperatures are inducing the ice caps to melt. This extra water can in turn undergo thermal expansion, which yields an ever growing MSL. And the ice caps are melting at a faster and faster rate: the less ice there is, the less sunlight is reflected back into space, which means that more light and heat is absorbed and melts glaciers more quickly. This is referred to as the ice-albedo feedback and means that the sea level is rising at an increasing rate. Interestingly, if a snow cap on a mountain melts, the peak height of the mountain decreases, while simultaneously contributing to sea level rise. This serves to shrink the mountain from both ends. On shorter time scales, effects like El Niño, floods, and large earthquakes can change the relative heights between water and land on a local scale.
Basically, defining mean sea level is incredibly difficult as there are a lot of dynamic components that all must be taken into account, and can change over time. But MSL is the standard one has to use when measuring the height of a mountain, And the MSL is anything but standard. But back to the point at hand, how can a growing mountain temporarily shrink?
Briefly, mountains like the Himalayas are growing because tectonic plates are pushing against each other and undergoing subduction (one plate is forced underneath another, pushing the top plate skywards). The Himalayas are growing at about 2 centimeters per year (or slightly less than an inch).
So, in order for the Himalayas to ‘stop’ growing, all that has to happen is for the MSL to increase at a rate faster than that of continental subduction. It is not very likely (one hopes) that sea level rise will be an inch per year, but there are other slower growing mountain ranges, such as the Saint Elias mountains in Alaska and the Yukon, that are growing 3 to 4 millimeters per year. This is on par with current sea level rise of 3.2 millimeters per year since the early 1990s, and is only expected to increase. With major melting of glaciers and thermal expansion of water due to global warming and climate change, it is quite possible that the rate of sea level rise will outpace the skyward rise of many growing mountains, for a decade or a few until the MSL finds a new equilibrium.
So for the next few decades to a century or two, we might see many of our great, young mountains actually shrink, and then finally begin to grow again.