Earlier articles in these columns have shown how the odd careless remark, a few errors in judgment, 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 skeptics 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 decade” and based on that projected a rise in sea levels by 28-43 cm (centimetres) 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!
Reduction in polar ice cover is most dramatically seen in the Arctic and especially around Greenland. Since 2000, the area of the Arctic Ocean covered by ice in the summer has reduced drastically, with September 2007 recording the minimum and 2008 and 2009 being similar. (see satellite images below)
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 (Rs.870 crores).
Problem and Challenge 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 percent 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 (ice-caps, 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 sea water). 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.
Seasonal changes of sea ice are known to have a significant influence on ocean circulation patterns called “thermohaline circulation”. When ice melts, fresh water enters into the surrounding ocean reducing its salinity and therefore its density. In reverse, as seawater cools and sea ice forms, the salinity and density increases, causing the surface waters to sink down. Continuous such action drives deep ocean currents towards the equator and away from the polar regions, in response to which a return flow of warmer and less dense water is drawn towards the poles from higher latitudes towards the equator. These ocean currents have a profound influence on climate and weather. One of the more important such warm water currents is the Gulf Stream from the Gulf of Mexico towards the Arctic which keeps Britain several degrees warmer than other northern European countries. With climate change and rise in average temperatures globally, and volumes of sea-ice declining, the Gulf Stream would become significantly weaker, leading to much colder conditions in regions on both sides of the northern Atlantic (thus showing that “global warming” can be a misleading term!). There are even apprehensions that at some particular point, the mechanism governing these important ocean currents could get “switched off” leading to catastrophe, the subject of a recent Hollywood movie!
It must be remembered, though, that melting of sea-ice has no impact on sea levels since the ice is already floating in water.
Unlike sea ice which is only a few metres thick, ice caps or ice sheets over land, such as those that blanket Antarctica or Greenland, are several kilometers thick. Till recently it was thought that ice caps were relatively stable, certainly in their interiors, but evidence now appears to have dashed this fond hope. Ice caps both in Antarctica and in Greenland are now known to be melting, especially from their base, due to the effect of warming oceans. When this huge quantity of ice melts, the water released into the oceans will undoubtedly contribute significantly to a rise in sea levels.
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 localized information over a small area. Thickness of ice caps, 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.
Cryosat-2 ESA’s Cryosat-2 is the second attempt at undertaking this task, the first mission in October 2005 having failed right at launch due to a software problem affecting the rocket. This time, ESA used Russia’s Dnepr launcher from the usual Soviet-era Baikonur station in Kazakhstan. The launcher was shot off the pad by a high-explosive charge with, unusually, its upper stage flying backwards pulling the satellite up rather than pushing it from below, the new configuration expected to enable greater accuracy in injecting the satellite into its designated orbit.
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 degrees but Cryosat2 goes even better with an orbital path of 88 degrees north and south on each orbit, covering most of the Arctic and Greenland coastline of which only 10 percent 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 (see image below) 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 PD, 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 provide 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 comparison with ground data both before and after launch. Arctic and Antarctic expeditions are already measuring snow density and behaviour relative to the ice below. After the launch, ground-truthing will be done to validate the radar data obtained and enable more accurate interpretation of the satellite data through the rest of the mission expected to last 3 years.
The Mission goal is to measure changes in ice thickness within 10% of the expected inter-annual variation which works out to a required accuracy of about 7mm per year averaging out variations between sea-ice and ice sheets. Data obtained so far shows that this goal will be met quite comfortably. Cryosat-2 will undoubtedly yield invaluable data on polar ice and considerably advance scientific knowledge of climate change and its impact, and also put at rest much speculative debate and scepticism.
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.