Environment, GS-3, Uncategorized

Environment Digest – 22 May – 28 May 2016

[1]More extreme rainfall on land masses predicted

Issue

  • According to a recent study published in the journal Nature Climate Change, extreme rainfall events can be expected to increase in both dry and wet land regions of the world in the current global warming scenario.

Key points

  • The water vapour concentration increase in the atmosphere per degree Celsius rise in temperature is 7 per cent according to the Clausius-Clapeyron theory.
  • Over the ocean, where evaporation is greater than precipitation (rainfall), such as in dry areas, the atmosphere will get drier with increased global warming as the evaporated water, in the form of vapour, is carried away by winds, leaving behind a dry atmosphere. And where the precipitation is greater than evaporation such as in wet areas the areas will get wetter.
  • But over the land masses it has been less clear as to how the rainfall patterns change with global warming.
  • Based on models and observations it has been found that global average precipitation increases only about 2 per cent.
  • In some way the atmosphere produces less rain.
  • One way that this could happen is if rain increases at the Clausius–Clapeyron rate when it does fall, but falls less often, making precipitation events more extreme.
  • This is what is seen typically in general circulation models (GCM), the most detailed models of the climate system.
  • Another point is that the heat released by the condensation of water itself tends to pull more moisture into a precipitating system.
  • This suggests that intense rain might instead increase with warming at even higher rates than the Clausius–Clapeyron rate— perhaps twice as fast, as some observations for short timescales (minutes to hours) seem to show.
  • On the other hand, as the atmosphere’s capacity to evaporate moisture from arid regions and to transport it away will increase at the Clausius–Clapeyron rate, arid regions are expected to become drier still, and it seems plausible that this would reduce all precipitation (from light to heavy) in these regions.

Limitations of the study

  • The study, though significant for its findings has the drawback that the tropics were poorly covered. The global warming effects are most severely felt in the tropics where complex physical interactions make prediction hardest.
  • Also, extreme rainfall data is widely available only on the daily timescale.
  • It cannot be known how it will change on shorter and longer timescales, which will indicate the flood risk in different places.

[2] Water staircases in seas

Issue

  • Water staircases.

Water Staircases

  • Water staircases are steplike variations of density of water due to steplike changes in temperature and salinity.
  • Though internal waves exist where the density gradually increases with depth, they cannot propagate where the density is uniform, for instance, within the steps of the staircase.
  • This suggests a possible mechanism by which the upper layers of the Arctic Ocean warm up, causing the ice to melt.
  • Ocean warming
  • The Arctic Ocean has inflows coming from the Pacific Ocean and Atlantic Ocean.
  • In this, the top layers consist of cooler and less saline water and below that is a layer of water coming from the Atlantic Ocean which is more saline and warmer, too.
  • The effect of salinity wins over that of temperature and so, though the water below is warmer, it is heavier than the cooler, less saline layer on top.
  • Warm, but salty water — ultimately originating from the Atlantic Ocean resides near the bottom of the Arctic Ocean.
  • If turbulence could somehow mix this water with that above, then, eventually, the surface could warm more rapidly, and this would increase the rate of sea-ice melt.
  • One mechanism for mixing is the result of breaking internal waves.
  • In a staircase-like formation, though the density is constant within the step, there is a jump in density from one step to another.
  • Hence, the wave’s energy can be transmitted from one interface to another.

Selective filtering

  • So the scenario is that when an internal wave strikes a density staircase, a part of its energy may be transmitted through the staircase.
  • In other words, density staircases in the ocean can act to reflect short wavelength internal waves and transmit longer wavelength waves.
  • This is analogous to the selective transparency of glass windows on modern buildings that have multilayered coatings designed to reflect red light (long wavelength light) and allow green-blue (shorter wavelength) light through.
  • On reaching the ocean floor, the long-wavelength waves which have been transmitted cause turbulence and mix up the water.
  • The warm waters then rise to the top and warm the top layers.

[3] Why sea ice cover around Antarctica is rising

Issue

  • Why has the sea ice cover surrounding Antarctica been increasing slightly, in sharp contrast to the drastic loss of sea ice occurring in the Arctic Ocean?

Study by NASA

  • A new NASA-led study has found the geology of Antarctica and the Southern Ocean is responsible for this phenomenon.
  • The researchers used satellite radar, sea surface temperature, land form and bathymetry (ocean depth) data to study the physical processes and properties affecting Antarctic sea ice.
  • They found that two persistent geological factors — the topography of Antarctica and the depth of the ocean surrounding it — are influencing winds and ocean currents, respectively, to drive the formation and evolution of Antarctica’s sea ice cover and help sustain it.
  • They focused on the 2008 growth season, a year of exceptional seasonal variability in Antarctic sea ice coverage.

Analyses

  • Their analyses revealed that as sea ice forms and builds up early in the sea ice growth season, it gets pushed offshore and northward by winds, forming a protective shield of older, thicker ice that circulates around the continent.
  • The persistent winds, which flow down slope off the continent and are shaped by Antarctica’s topography, pile ice up against the massive ice shield, enhancing its thickness.
  • This band of ice, which varies in width from roughly 100 to 1,000 km, encapsulates and protects younger, thinner ice in the ice pack behind it from being reduced by winds and waves.
  • Older, thicker sea ice returns a stronger radar signal than younger, thinner ice does.
  • They found the sea ice within the protective shield was older and rougher (due to longer exposure to wind and waves), and thicker (due to more snow accumulation).
  • As the sea ice cover expands and ice drifts away from the continent, areas of open water form behind it on the sea surface, creating “ice factories” conducive to rapid sea ice growth.

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