This rise-rate is twice as fast as the one measured by the tide gauges for the past 50 years. And in addition, it is not uniform: in some regions (such as West Pacific and North Atlantic) the sea-level rose three to four times faster than the 1993's average.
Oceans expand while ice sheets melt...
Experts have hardly any doubt that the present sea-level is due to the global warming.
Thanks to temperature soundings of the ocean carried out by boats and more recently by the profiling floats of the Argos international system, the oceanographers discovered that the ocean is much warmer than some thirty years ago. The sea is thus expanding and rising.
This phenomenon can explain 1/3 of the sea-level rise measured by the altimetric satellite for the last 18 years.
The 2/3 left can be explained by the continental ice melting, equally distributed between two effects:
- almost generalized melting of the mountain glaciers
- ice loss in the ice polar caps.
Before the early 1990's, the mass-balance of the Greenland and Antarctica's ice caps was poorly known. Since this date, several space observations measured the mass variations of these two ice caps: radar interferometry, space gravimetry with GRACE and, of course, altimetry with Topex-Poseidon and then Jason-1 and 2. Since the beginning of the 2000's, the peripheral regions of the Greenland and West Antarctica each lost on average 150 billion tons of ice per year (i.e. an approximately 1 mm rise per year if considering the total of the two ice caps).
This important loss in ice-mass increased for the past few years and is principally due to the very fast run-off to the sea of the coastal glaciers, which became unstable because of the adjacent oceanic water warming.
The sea-level rise is not uniform
In some regions (such as West Pacific and North Atlantic) the sea-level rose three to four times faster than the 1993's average.
But what about tomorrow?
The ice caps' dynamics is complex and remains poorly understood. We still do not know if the phenomena which are being observed for the past few years are going to mitigate or overspeed. If they melt entirely, the ice currently stocked in the Greenland and West Antarctica's ice caps (the most unstable part of the continent) would make the sea-level rise approximately 12 m! It is a major uncertainty for the climate patterns which are trying to simulate the future evolution of the sea-level.
The sea-level rise is a worrying threat for many low coastal regions, which are often heavily populated. In addition to the rise resulting from the global warming and its important regional variability, other non-climatic phenomena such as the soil sinking also play their role. This sinking may be due to the sediment load in the great river deltas or even to the underground water pumping and gas and oil extraction.
The sustained and global measurement of the sea-level evolution by space altimetry is a critical issue which must be continued in the long term. Over and above their value for a possible acceleration of the phenomenon, these observations will be used to validate the climate patterns used to calculate the coming sea-level rise and study its impact in low and heavily populated coastal regions of the planet.
- Oceanography program scientist at CNES: Juliette Lambin