Researchers at the Hong Kong Polytechnic University produced the first thirty-year analysis (1993-2022) of change in mass of the oceansobtained through high-precision satellite data: the study, published in Proceedings of the National Academy of Sciencesmeasured a sea level rise of approx 90 millimeters over the last 30 years, but above all he discovered that the rate of sea rise is increasing due toacceleration in the melting of ice in the polar ice caps. This factor, according to the analysis conducted by the Hong Kong team, contributes to 60% to the actual increase in the mass of water in the oceans.
The causes of sea rise and ice melting in Greenland
The rise in mean sea level is mainly due to two factors. The first is thethermal expansion of water: with global warming the temperatures of the oceans also increase, and consequently the water increases in volume, causing the sea level to rise without there being an increase in the quantity of water.
The second factor, however, is theincrease in ocean masscaused by the melting of ice, which introduces new water into the oceans and increasingly contributes significantly to the rise of sea levels.
These two mechanisms act simultaneously, but while once the main factor was the increase in volume, from 2005 – according to the analysis conducted by the Hong Kong Polytechnic – the increase in ocean mass due to the increasingly rapid melting of ice has become the predominant factor. In particular, about the 60% of the total rise of sea level is due to the addition of new water, and of this portion approximately80% comes mainly from accelerated ice melt in Greenland. This phenomenon explains much of the increase observed since 2005.
How the study was conducted: the laser ranging satellite technique
The analysis was conducted with the technique of laser ranging satellite. In practice, laser pulses are sent from ground stations towards satellites equipped with retroreflectors: by measuring the time taken by the laser beam to return back, it is possible to calculate the distance between earth and satellite with extreme precision. This measurement allows us to detect subtle variations in Earth’s gravity and, indirectly, in the movements and mass distribution of the oceans. This technique, developed already in the 1960s and 1970s, is now consolidated in space geodesy, but its use to directly reconstruct the variations in ocean mass over a thirty-year period represents a truly innovative approach.
