Why does sea level change?
The sea level budget
Thermal expansion is producing about half of the current 3mm/year increase in global sea
level. This contribution has increased from around 0.5 mm/year over the second half of the
20th century to around 1.6 mm/year over the last 12-14 years. This contribution is expected
to continue at at least this level over the next century or more due to greenhouse-gas-induced
warming of the atmosphere and ocean.
Due to the very patchy and sparse (especially as we go back in time) body of ocean temperature
data that is available to estimate longer-term contributions the contribution over most of the
20th century is hard to estimate reliably.
A major contribution to recent sea level rise is from the melting of glaciers, and
contributions from the Greenland (both surface melting and iceberg calving and
the Antarctic ice sheet (iceberg calving only). This is believed to produce about one third or
more of the current 3mm/year annual increase in global sea level. The contribution from the ice
sheets is poorly understood at the moment and is an active area of research. The melting of
the Greenland ice sheet alone could increase global mean sea level by around 7 metres. This
would probably take around 1,000 years, but it is believed that ice melt from Greenland could
still contribute significantly to sea level rise over the next 50-100 years.