Satellite laser measurements show change in environment for the first time
The accelerating loss from the world's two great land-based ice sheets means a rise in sea levels is likely to happen even more quickly than UN scientists suggested only two years ago, the findings by British scientists suggest.
Although floating ice, such as that in the Arctic Ocean, does not add to sea-level rise when it melts as it is already displacing its own mass in the water, melting ice from the land raises the global sea level directly. At present it is thought that land-based ice melt accounts for about 1.8mm of the current annual sea level rise of 3.2mm – the rest is coming from the fact that water expands in volume as it warms. But the new findings, published online today in the journal Nature, imply that this rate is likely to increase.
High-resolution satellite laser measurements have shown that along both the Greenland and Antarctic coastlines, the glaciers and ice streams which for thousands of years have slowly carried ice into the sea are now rapidly thinning, meaning they are speeding up in their flow. In both cases, the increased flow rate is extending back far into the ice sheets' interior.
This is happening all the way around Greenland, even at the high northern latitudes, and around much of Antarctica, especially in West Antarctica and around the Antarctic Peninsula.
Areas around the Greenland coast are hotspots of glacier thinning – in some cases the glacier surface level is dropping at a rate of half a metre per year, while in others it is a remarkable rate of a metre and a half.
It is the first time that a comprehensive view of the rate of thinning – and thus ice loss – all the way around the coast has been made possible. It has been put together by Hamish Pritchard and his colleagues from the British Antarctic Survey and the University of Bristol, by analysing millions of measurements from Nasa's high-resolution ICESat (Ice, Cloud and Land Elevation Satellite).
Launched in January 2003, ICESat examines changes in the world's ice and land masses. The satellite's lasers have measured the surface elevation of the Earth's ice sheets with unprecedented accuracy – and thus picked up how they are changing.
"The fact that the changes are so large is alarming, and you wonder how far they will go," Dr Pritchard said. "The thinning effect must be relatively recent, as it is so strong that it could not have been sustained previously without the glaciers melting away."
The scientists compared the rates of change in elevation of both fast-flowing and slow-flowing ice. In Greenland, they studied 111 fast-moving glaciers and found 81 thinning at rates twice that of slow-flowing ice at the same altitude. They found that ice loss from many glaciers in both Antarctica and Greenland is greater than the rate of snowfall further inland.
In Antarctica, some of the fastest thinning glaciers are in the west, where the Pine Island, Smith and Thwaites Glaciers are thinning by up to nine metres per year.
"We were surprised to see such a strong pattern of thinning glaciers across such large areas of coastline – it's widespread and in some cases thinning extends hundreds of kilometres inland," Dr Pritchard said. "This kind of ice loss is so poorly understood that it remains the most unpredictable part of future sea level rise."
"Uchunguzi wa kwanza wa laser unaonyesha mabadiliko ya mazingira kwa mara ya kwanza, ""kuongezeka kwa kasi ya kupoteza kwa barafu mbili kubwa za dunia inamaanisha kuongezeka kwa viwango vya bahari kunaweza kutokea hata kwa kasi zaidi kuliko wanasayansi wa Umoja wa Mataifa walipendekeza miaka miwili tu iliyopita.""" Ingawa barafu inayoelea, kama ile ya Bahari ya Aktiki, haiongezi juu ya usawa wa bahari wakati inapoyeyuka, kwani tayari inabadilisha wingi wake mwenyewe katika maji, barafu inayeyuka kutoka nchi kavu huinua usawa wa bahari ya kimataifa moja kwa moja. Kwa sasa, inadhaniwa kwamba barafu inayeyuka kutoka kwenye ardhi huchangia karibu 1.8mm ya ongezeko la kiwango cha bahari la kila mwaka la 3.2mm, na wengine hutokana na ukweli kwamba maji huongezeka kwa kiasi wakati wa joto. Utafiti huo, uliochapishwa leo katika jarida la Nature, unaonyesha kuwa idadi hiyo ya watu itazidi kuongezeka. Uchunguzi wa laser ya satelaiti ya azimio la juu umeonyesha kwamba kando ya pwani ya Greenland na Antarctica, barafu na mito ya barafu ambayo kwa maelfu ya miaka imebeba barafu polepole baharini sasa inazidi kupungua, ikimaanisha kuwa inaongeza kasi ya mtiririko wake. Katika visa vyote viwili, mwendo ulioongezeka wa mtiririko huo unaenea mbali sana ndani ya tabaka za barafu. Hii ni kweli karibu na Greenland, hata katika latitudo za juu za kaskazini, na karibu na sehemu kubwa ya Antaktika, hasa katika Antaktika Magharibi na karibu na Peninsula ya Antaktika. Maeneo karibu na pwani ya Greenland ni hotspots ya thinning barafu - katika baadhi ya kesi glacier uso ngazi ni kushuka kwa kiwango cha nusu mita kwa mwaka, wakati katika wengine ni kiwango cha ajabu ya mita moja na nusu. Ni mara ya kwanza kwamba mtazamo wa kina wa kiwango cha thinning <unk> na hivyo kupoteza barafu <unk> njia yote kuzunguka pwani imekuwa inawezekana. "Hamish Pritchard, mtafiti wa utafiti wa anga ya kati wa British Antarctic Survey na Chuo Kikuu cha Bristol, alisema: ""Tumekuwa tukijaribu kufanya uchunguzi wa anga ya juu zaidi kwa kutumia satellite ya NASA inayoitwa ICESAT (Ice, Cloud and Land Elevation Satellite)." ICESAT ilizinduliwa mnamo Januari 2003 na inachunguza mabadiliko ya barafu na ardhi duniani. Lasers za satelaiti zimepima urefu wa uso wa tabaka za barafu za Dunia kwa usahihi usio wa kawaida <unk> na hivyo kugundua jinsi zinavyobadilika. "Ukweli kwamba mabadiliko hayo ni makubwa sana ni wenye kuogopesha, na unajiuliza yataenda mbali kadiri gani", alisema Dakt. Pritchard. "Ni lazima athari ya kupungua kwa barafu iwe ya hivi karibuni, kwa kuwa ni yenye nguvu sana hivi kwamba haingeweza kudumu kabla ya barafu hizo kuyeyuka.""" Wanasayansi hao walilinganisha viwango vya mabadiliko ya mwinuko wa barafu inayotiririka haraka na ile inayotiririka polepole. Katika Greenland, watafiti walichunguza barafu 111 zinazosonga kwa kasi na kugundua kuwa 81 zinapungua kwa kasi mara mbili ya barafu inayosonga polepole kwenye urefu huo huo. Kwa mfano, katika barafu za Antarctica na Greenland, theluji inapotea mara nyingi kuliko theluji inapoanguka. Katika Antarctica, baadhi ya barafu zinazopungua kwa kasi zaidi ziko magharibi, ambapo barafu za Pine Island, Smith na Thwaites zinapungua kwa hadi mita tisa kwa mwaka. "Tuli kushangazwa kuona muundo mkubwa wa barafu zinazopungua katika maeneo makubwa ya pwani - ni ya kuenea na katika visa vingine kupungua kunapanuka mamia ya kilomita ndani ya nchi, "alisema Dk Pritchard." "Aafu, aina hii ya kupoteza barafu haijulikani vizuri sana hivi kwamba bado ni sehemu isiyoweza kutabiriwa zaidi ya ongezeko la kiwango cha bahari katika siku zijazo."""