United States Geological Survey har netop udgivet en autoritativ forskningsbaseret rapport, som i sine konklusioner vurderer, at vi kan vente havniveaustigninger på mellem 0,8 og 2,0 meter i år 2100, med mindre der gennemføres drastiske ændringer i CO2-udledningerne. Klimapanelet siger som bekendt maksimalt 0,59 centimeters havniveaustigninger under et business as usual scenarie.
Rapporten baserer sig på nyere forskning end klimapanelet. Om isafsmeltning og havniveau hedder det:
Recent rapid changes at the edges of the Greenland and West Antarctic ice sheets show acceleration of flow and thinning, with the velocity of some glaciers increasing more than twofold. Glacier accelerations causing this imbalance have been related to enhanced surface meltwater production penetrating to the bed to lubricate glacier motion, and to ice-shelf removal, ice-front retreat, and glacier ungrounding that reduce resistance to flow. The present generation of models does not capture these processes.these adjustments will very likely become more frequent in a warmer climate. The regions likely to experience future rapid changes in ice volume are those where ice is grounded well below sea level such as the West Antarctic Ice Sheet or large glaciers in Greenland like the Jakobshavn Isbrae that flow into the sea through a deep channel reaching far inland. Inclusion of these processes in models will likely lead to sea-level projections for the end of the 21st century that substantially exceed the projections presented in the IPCC AR4 report (0.28 ± 0.10 m to 0.42 ± 0.16 m rise).
Rapportens hovedtema er ikke isoleret set havniveaustigninger, men fænomenet pludselige klimaændringer, som kendes fra studier af fortidens klima. Rapporten advarer om, at en fortsat stigende CO2-udledning også kan fremprovokere noget lignende i fremtiden med alvorlige konsekvenser til følge:
Four types of abrupt change in the geologic record stand out as being so rapid and large in their impact that, if they were to recur, they would pose clear risks to society in terms of our ability to adapt:
- Rapid change in glaciers, ice sheets, and hence sea level.
- Widespread and sustained changes to the hydrologic cycle, including drought and flooding.
- Abrupt change in the Atlantic Meridional Overturning Circulation, a critical component of global climate, characterized by the northward flow of warm, salty water in the upper layers of the Atlantic Ocean.
- Rapid release to the atmosphere of methane, a powerful greenhouse gas, trapped in permafrost and in ocean sediments.