From UPSC perspective, the following things are important :
Prelims level : Icesat 2, Cryosat 2
Mains level : Paper 1- Arctic ice and how it influence climate change
This article is about Polarstern, is an icebreaker, which traversed the Arctic Ocean to study the aspects related to ice there. Here, we will look at some of these aspects. These aspects are-monitoring of the ice, difficulty in measuring the thickness, rate of melting of ice and relations with cloud formation.
Arctic: A recorder and driver of climate change
How is it a recorder of climate change?
- It is a recorder because of two co-related factors, these are-
- 1) The visible difference between ice and water.
- 2) The obvious relationship between global temperatures and the amount of ice around.
- Two factors together shows in an easily graspable way how things are changing.
- The extent of the Arctic sea ice in summer has declined by 30% in the past 30 years, and that loss is accelerating (see chart).
How is it a driver of climate change?
- The Arctic is also a driver of climate change, because the whiteness of ice means it reflects sunlight back into space, thus cooling Earth.
- Whereas the darkness of open water means it absorbs that light.
- The less of the reflection of sunlight and the more absorption of light will result in a faster rise in global temperatures.
Monitoring the Arctic’s ice
- At the moment this is monitored mainly by satellite.
- Measuring the extent of the Arctic’s ice from space is easy.
- Measuring its thickness is trickier.
- From orbit, this is done by a mixture of radar and laser beam.
- Icesat 2, an American craft, provides laser-altimeter data that record the height above sea level of the top of the snow that overlies the ice.
- Cryosat 2, a European one, uses radar to penetrate the snow and measure the height of the top of the ice itself.
- The thickness of the ice in a particular place can then be calculated by applying Archimedes’ principle of floating bodies to the mixture of ice and snow, and subtracting the thickness of the snow.
- But there is a view that the data collected by these two satellites may be inaccurate, leading to an overestimation of the ice’s thickness.
Let’s understand why the data about thickness could be inaccurate
- When all is working perfectly, the return signal for Cryosat 2 comes exactly from the boundary between the ice and any overlying snow.
- But, that this is not always what happens.
- Variables such as layering within the snow, along with its temperature and salinity, might affect the returning radar signal by changing the snow’s structure and density.
- This could cause the signal to be reflected from inside the snow layer, rather than from the boundary where it meets the ice.
- If that were happening, it would create the illusion that the ice beneath the snow is thicker than is actually the case.
How topography of Arctic ice matters
- Though sea ice is solid, it is not rigid.
- It forms but a thin skin on the ocean—varying in depth from around 30cm in summer to a couple of metres in winter—so is readily moved by wind and current.
- As the ice moves it stretches and cracks in some places.
- Large cracks formed in this way are called leads, because they are wide enough to “lead” a ship.
- In other places, by contrast, movement makes the ice thicker.
- As individual panes of ice butt up against each other, they create ridges that can be metres high.
- But even from the ship’s deck one can watch leads opening and ridges forming around the vessel.
- Observations suggest that winter the ice has been particularly mobile—and has thus become particularly rough, with a surprising number of ridges.
So, how these ridges affect the rate at which ice melts?
- These ridges may affect the rate at which the ice melts—but to complicate matters, this could happen in two opposing ways.
- Ridges make ice thicker, and thicker ice melts more slowly.
- On the other hand, a ridge projects down into the sea as well as up into the air (Archimedes, again), so it may stir up water from below the surface.
- Deep water is warmer than the surface layer, so this stirring would serve to increase melt rates.
- Moreover, to add to the confusion, ridges are prone to having pieces of ice fall off them into the sea, to form small blocks known as brash.
- This brash, having more surface area per unit volume than unbroken ice, melts faster.
How cloud formation is affected by cracks in Arctic ice
- On most parts of Earth clouds form as droplets of water condense around “seeds” of dust or organic molecules.
- In the Arctic, there is little dust.
- Biological activity, too, is in short supply compared with elsewhere—and is, moreover, conducted mainly below the barrier of the sea ice.
- It might, therefore, be expected that there would be few seeds present for clouds to form around.
- And yet, clouds are present.
- Cloud seeds there tended to be compounds containing sulphur, nitrogen, chlorine, bromine or iodine.
- Presence of these molecules suggests their link with cracks in the ice sheets.
- This means that more cracks in the ice sheet could lead to more clouds in the Arctic.
- What overall effect that might have on the climate is unclear.
- Summer clouds would reflect sunlight back into space, cooling the planet.
- Those formed in winter, when the sun is below the horizon, would serve as insulation, warming it.
- Two opposite outcomes are possible—or perhaps the net effect will be that they cancel each other out.
Properly disentangling the interactions between Arctic ice, atmosphere and ocean life will require data collected across a full year—for the contrast between winter and summer at the poles is greater than anywhere else on the planet.