Measuring impact of climate change from space: Gravity measurements shed light on key questions

These questions are being addressed by scientists from the Institute of Geodesy at the University of Stuttgart, Germany, and the Department of Spatial Science at the Curtin University of Technology in Perth, Australia. For this purpose, the German-Australian team has been investigating space-borne gravity measurements provided by the GRACE satellite mission.

As a result, they have found out that the Greenland glaciers shrunk continuously in the last few years; above all, they estimated the changes not to be linear in time but accelerating. On average, recent Greenland ice-mass decline caused an annual sea-level rise of about 0.5 millimetres.

For the first time ever, the GRACE satellite mission has allowed the determination of global mass variations -- such as ice melting in the polar areas -- from changes in Earth's gravitational pull. The underlying measurement principle is simple: it is based on the fact that the redistribution of masses on the Earth surface can be mapped in terms of changes of the terrestrial gravity field. Hence, scientists can measure the spatio-temporal variations of Earth's gravitational attraction on a test mass in space, namely the GRACE spacecraft. From these observations they can derive surface mass-variation patterns.

The satellite data clearly reveal that the Greenland area exhibits the main dominant mass shrinkage over the whole globe. It is predominantly caused by the persistent melting of the Greenland glaciers. Presently, the Arctic island loses between 165 and 189 cubic kilometres ice a year. This estimate is considerably higher than the results derived from geometric satellite measurements conducted in the 1990s.

Deglaciation causes melt water influx into the oceans. Furthermore, globally increasing atmospheric temperatures involve thermal expansion of seawater. Both effects substantially contribute to sea-level rise. Most notably, contrary to common reasoning, sea level does not change uniformly -- that is, in terms of a constant layer over the world's oceans. In fact, the global redistribution of masses causes the sea level to vary differently from location to location. As a basic principle, ice melting in the Northern Hemisphere translates into sea-level rise in the Southern oceans. On the other hand, deglaciation over the Antarctica causes sea-level rise in the Northern oceans.

Such effects are alarming, especially considering the fact that millions of people settle down in coastal and near-coastal areas. Based on current findings, extrapolations to the end of the 21st century forecast the Greenland ablation impact on mean sea-level rise to be in the range of five centimetres. This estimate is highly conservative, neglecting both accelerated deglaciation and mass balance over Antarctica, Alaska and further ice-covered regions. Taking progression effects into account, potential sea level rise of 50 centimetres within the next 100 years becomes realistic. Continuing satellite measurements will manifest the reliability of short-, medium- and long-term predictions.