Oceans levels have always varied, despite today’s rising tide

The IPCC assumes sea levels have barely changed over the past two millennia, setting today’s rate of 2 to 3 millimeters per year in stark contrast.

But some scientists are questioning that simplification. Ocean levels, it seems, have never been stagnant. Glaciers and ice sheets have come and gone. Land masses have moved course. The Earth’s crust has rebounded following glacial melt from the last Ice Age, and that’s changed the volume of water in the oceans too.

Understanding the variability is key to predicting future sea level rise, write W. Roland Gehrels from the University of Plymouth, UK and colleagues in a paper published in the most recent issue of Eos, a publication of the American Geophysical Union.

They say that falling sea levels have been the overall natural trend over the last 1,000 years, according to proxy data, making today’s rising seas a “significant departure” from the past. But reconstructions also show differences in geography. The West Antarctic ice sheet thinned during the late Holocene “Little Ice Age” (1500-1800 BC) while Greenland and many Northern Hemisphere ice sheets grew.

Elsewhere along the coast of Israel, fossil records show the sea rose to its present level about 2,000 years ago, reaching a high point of 0.5 meters higher than today during the years 300-600 BC, only to drop again to as much as 0.8 meters below today during the 13th Century. Since the late 18th Century, sea level on the Israeli coast has risen by at least 0.2 meters.

These regional changes are caused by ocean dynamics that are affected by such patterns as El Niño–Southern Oscillation, the authors write.

The scientists argue that regional variations and changes caused by global influences must be incorporated into climate models if they are to be accurate.