"This area has suffered a considerable increase in water and air temperatures since the 1970s as well as a rapid rise in the sea levels since the 1990s," said the study by the Spanish Oceanographic Institute.

It said the Mediterranean has risen "between 2.5 and 10 millimetres (0.1 and 0.4 inches) per year since the 1990s, "which implies that, if this trend continues, the water levels will rise between 12.5 centimetres (five inches) and half a metre (20 inches) in around 50 years."

"This would have very serious consequences in low-lying coastal areas even in the case of a small rise, and catastrophic consequences if a half-metre increase occurs," the institute, a unit of the science ministry, said.

Water temperatures in the Mediterranean have gone up by 0.12 to 0.50 degrees C since the 1970s, said the study, Climate Change in the Spanish Mediterranean.

"This increase can seem small, but what must be taken into account is that ... small increases in temperature mean the sea absorbs huge quantities of heat," it said.

The rise in salinity has resulted in part from the reduced rainfall in the Mediterranean, the study said.

It noted that these trends are global phenomena.

The Nobel-winning Intergovernmental Panel on Climate Change (IPCC) reported last year that sea levels rose by between 10-20 centimetres (four to eight inches) from 1900 to 2006.

It forecast a rise of at least 18 centimetres (17.2 inches) by 2100, mainly as a result of thermal expansion, for water expands when it warms. The IPCC declined to set an upper figure to this estimate specifically because of uncertainty about icemelt from Antarctica and Greenland.