(Weekly Organ of the Communist Party of India (Marxist)
April 18, 2010
European Satellite to Measure Global Warming Impact
EARLIER articles in these columns have shown how an odd careless remark, a few errors in judgement, a lapse in scientific rigour and peer review procedures, as happened over the clearly erroneous prediction that Himalayan glaciers would melt by 2035, are pounced upon by climate sceptics and corporate lobbies to undermine the scientific understanding of societally induced climate change and hence undercut efforts to bring about policy changes to avert the impending crisis. This despite massive and mounting amounts of evidence to show that, if anything, the problem is even more serious than the extant scientific consensus says it is. For instance, many recent studies on polar ice have tended to show that ice cover at both poles is shrinking faster, and that consequently sea levels would rise even more, and more quickly, than previously thought.
Diminishing ice cover is often referred to as clear evidence of global warming. Further, since the large quantity of terrestrial ice is known to play an important role not only with regard to sea level but also to regulating global and regional climate, changes in ice cover are closely watched and carefully studied. For many years, satellites have been mapping the area covered by ice and this data has formed the basis of much prediction on the extent and impact of global warming. For instance, the Fourth (and latest) Assessment Report of the IPCC (IPCC/AR4) in 2007 noted that �satellite data since 1978 show that annual average Arctic sea-ice extent has shrunk by 2.7 per cent per decade� and based on that projected a rise in sea levels by 28-43 cm by 2100. But a subsequent authoritative report titled Antarctic Climate Change and the Environment released in 2009 by the highly regarded Scientific Committee on Antarctic Research projected a significantly higher sea-level rise of 1.4 metres by the next century!
polar ice cover is most dramatically seen in the Arctic and especially
There is thus little doubt that ice cover is shrinking. The real question is how much? So far we have been talking only about the area covered by ice, not about the total volume or quantity of ice. To get from area to volume, one needs to measure the thickness of the ice. The Cryosat-2 satellite (so named because it seeks to study the cryosphere or parts of the earth covered by ice) was launched by the European Space Agency (ESA) on April 8 last week to do precisely this at a total mission cost of 180 million dollars (Rs 870 crore).
Much of the solar radiation reaching the earth�s surface is reflected back to the atmosphere and to outer space. Permanent ice-sheets, especially when covered by snow, have high �albedo� (literally whiteness or more accurately reflectivity) and reflect around 80 per cent of sunlight. This helps the ice to stay frozen and plays an important role in maintaining the heat balance of our planet. However, when ice cover begins to melt, the albedo effect gets diminished, thus reducing the proportion of solar radiation reflected back and hence increased absorption of heat by the ice, leading in turn to what is termed �positive feedback,� that is, increased ice melt and greater warming of the earth. Under normal seasonal variation, vast amounts of ice melt every summer and freeze again in the winter but, with global warming, melt rates are getting higher while the re-freezing is slowing down.
Polar ice is of two types: ice that covers land (icecaps, defined as less than 50,000 sq km in area, or ice-sheets, larger than that, although the former term is commonly used to describe both), and sea ice (formed by freezing seawater). These two forms of ice behave differently, have differing impact on climate and will affect the planet in diverse ways as they melt due to global warming and pose distinctive challenges for measuring their thickness.
changes of sea-ice are known to
have a significant influence on ocean circulation patterns called
circulation.� When ice melts, freshwater enters into the surrounding
reducing its salinity and therefore its density. In reverse, as
and sea ice forms, the salinity and density increases, causing the
waters to sink down. Continuous such action drives deep ocean currents
the equator and away from the polar regions, in response to which a
of warmer and less dense water is drawn towards the poles from higher
towards the equator. These ocean currents have a profound influence on
and weather. One of the more important such warm water currents is the
Stream from the Gulf of Mexico towards the Arctic which keeps Britain
degrees warmer than other northern European countries. With climate
rise in average temperatures globally, and volumes of sea-ice
Gulf Stream would become significantly weaker, leading to much colder
conditions in regions on both sides of the northern Atlantic (thus
�global warming� can be a misleading term!). There are even
at some particular point, the mechanism governing these important ocean
currents could get �switched off,� leading to catastrophe, the subject
It must be remembered, though, that melting of sea-ice has no impact on sea levels since the ice is already floating in water.
sea-ice which is only a few metres
thick, icecaps or ice-sheets over land, such as those that blanket
However, to better understand and make predictions about these and related phenomena, scientists need to know much more about the total volume of ice which in turn needs measurement of its thickness.
Sea-ice being thin, its thickness can be measured directly, such as by drilling into it from above, but this method can only provide localised information over a small area. Thickness of icecaps, on the other hand, needs to be estimated by measuring the height of its surface relative to the land below, not an easy task by any means.
ESA�s Cryosat-2 is the second
attempt at undertaking
this task, the first mission in October 2005 having failed right at
to a software problem affecting the rocket. This time, ESA used
The spacecraft orbit is the steepest hitherto, taking it as close to the poles as possible. NASA�s ICEsat, with a laser altimeter, flew in a high inclination orbit of 86� but Cryosat-2 goes even better with an orbital path of 88� north and south on each orbit, covering most of the Arctic and Greenland coastline of which only 10 per cent is covered by current satellites. This clearly non- sun-synchronous orbit requires that the spacecraft�s solar panels are tilted so that it can receive maximum possible sunlight and also that it carries newly-designed high capacity batteries.
The satellite carries as its primary payload an advanced SAR-Interferometric Radar Altimeter (SIRAL). SAR stands for Synthetic Aperture Radar which provides high-resolution and simulated 3-D images even with small antennae by creating a rapid sequence of images while in motion which are then computationally put together. (See People�s Democracy, May 3, 2009 for a more detailed explanation.) To complement the altimeter, the payload includes a radio receiver called DORIS (Doppler Orbit and Radio Positioning Integration by Satellite) and a laser retro-reflector. The International Laser Ranging Service or ILRS, a global network of laser ranging stations, will support the mission. To provide the datum or reference position of the satellite itself against which all other positional readings are read to obtain absolute data, the satellite relies on the oldest navigational method, namely the position of stars which it continuously monitors through three star-trackers.
The spacecraft�s instrumentation provides accurate data on sea-ice �freeboard� or height of floating ice above sea level, and on the elevation of ice sheets. The SAR technique enables high resolution data in the direction of movement of the satellite. In conventional radar altimeters, distance to the top of the ice would be measured by the radar echo off the nearest point on the surface, but on sloping surfaces such as on land and on the edges of ice caps, there is no reference point to indicate where on the slope this nearest point is. The SAR�s series of multiple images taken at 10 times quicker intervals than conventional radars enables determination of the position and height of the surface in the along-track direction while left and right echo positions are provided by the SAR-interferometry mode which provides the angle of the returning echo, thus all together giving a three dimensional picture and accurate measurement of the thickness of the ice.
As with all
remote sensing, error correction and calibration has to be done through
with ground data both before and after launch.
An interesting sidelight is that the launcher that put Cryosat-2 into orbit is a modified Soviet-era SS-II ballistic missile, only slightly modified for commercial use. And earlier in the week, NASA also launched a modified Global Hawk, the US Air Force�s most potent Unmanned Aerial Vehicle (UAV) spycraft that can fly at high altitudes of over 60,000 feet for a very long time, to monitor sea-ice and ice sheets. If countries so decide, all knowledge and technology can indeed be harnessed for the collective benefit of humankind. Talk about turning swords into ploughshares!