Posts Tagged ‘sea level rise’

The Ever Rising Seas

Tuesday, February 23rd, 2016

Since 1970 more than 90% of the extra heat trapped by the Earth as the climate warms has been absorbed by the oceans. The oceans respond more much more steadily than the terrestrial surface climate: they give us the real, underlying picture of the warming of the planet.

We have known for a long time that as the sea warms, sea levels rise – partly from melting ice sheets and glaciers, partly from thermal expansion, and partly for other reasons. People living in coastal regions know too this all too well they experience tidal flooding increasingly often.

Global mean sea level rise from 1970 until now has been at about 2.8 mm per year, but this graph hides the huge regional variation that exists. (noaa.gov)

Global sea level rise from 1970 until now has been at about 2.8 mm per year, but this graph hides the huge regional variation that exists.(noaa.gov)

We also know that sea levels are rising at surprisingly different rates around the globe. We’re starting to understand why.

A new study, using remarkable satellite radar altimetry and satellite gravimetry data starting in 2002, teases apart the various drivers of sea level. And it contains surprises.

In addition to thermal expansion (the ‘steric’ component), and meltwater from Greenland, Antarctica and land based glaciers, other contributors to sea level changes are hydrology (involving the water cycle of evaporation, precipitation and run-off), glacial isostatic adjustments (GIA), and a component called ocean bottom pressure (OBP)

Ocean bottom pressure refers to the weight of all the water on the bottom, ocean’s version of atmospheric pressure. It varies with tides, currents, winds, and water entering and leaving basins, and it changes over oceanic regions both seasonally and annually. It is difficult to incorporate into the picture, but the satellite gravimetry makes it possible, and it is an important driver.

The various contributing components to the rise of sea levels from 2002-2014. The lines are offset for clarity: the critical information lies in their slopes (pnas.org)

The various contributing components to the rise of sea levels from 2002-2014. The lines are offset for clarity: the critical information lies in their slopes (pnas.org)

The overall view? From 2002 to 2014, ocean levels rose globally by an average of 2.74 mm/yr (the steepest, thicker grey line). Of that, 1.38 mm was the result of the thermal expansion of the warming water (the top orange line), and 1.37 mm the result of melting glaciers and ice sheets (the green, light blue and darker blue lines). Other contributors such as OBP and GIA appear on average to be variable and less important, while hydrology (the bottom green line), slopes downward, indicating a loss of water from the oceans to the land due to more rainfall, floods, groundwater loss, irrigation, and reservoirs, and so less runoff back into the oceans.

But those are global means, of little help for warning or advising coastal regions of what’s ahead.

This new study goes a lot further. It shows us not only how great is the regional variation in sea level rise, but also how much the various drivers vary in their contribution. The biggest surprise is that thermal expansion, the steric component, is about twice as great as previous estimates: sea levels are rising faster than we had thought.

The relative contribution of the drivers of sea level change can be depicted in pie charts. The same colors indicate the various contributors: steric=orange, meltwater=2 blues and light green, Glacial Isostatic Adjustment (GIA)=purple, ocean bottom pressure (OBP)= light brown, hydrology=dark green. (pnas.org)

The relative contribution of the drivers of sea level change can be depicted in pie charts. The same colors indicate the various contributors: steric=orange, meltwater=2 blues and light green, Glacial Isostatic Adjustment (GIA)=purple, ocean bottom pressure (OBP)= light brown, hydrology=dark green. (pnas.org)

The variation around the world is striking, to say the least. Look at the Philippines with mean sea level rise of a whopping 14.7 mm/yr, and Indonesia with 8.2 mm/yr. Thermal expansion of the warming water is the overwhelming contributor.

Components of sea level rise around Asian coasts The boxed number on each pie chart is the mean sea level rise, mm/year.(pnas.org)

Components of sea level rise around Asian coasts. The boxed number on each pie chart is the mean sea level rise, mm/yr.(pnas.org)

The Northwest Atlantic (eg the east coast of the US) is the other region with the most rapidly rising sea level at 9.1 mm/yr) where glacial isostatic adjustments (GIA) still play a strong role: the land continues to subside from the elastic rebound following its release from glaciation. In contrast, along the west coast of the Americas and unlike most of the rest of the world, the sea level isn’t changing in any significant way.

Contributers to sea levels around the Americas. (pnas.org)

Contributers to sea levels around the Americas. (pnas.org)

Obviously seasonal, annual and decadal variation in winds and currents play a critical role in modifying steric and OBP contributions, but the data appear to be robust and the general picture is likely to hold: the global average rise in seal level is accelerating, the steric component is greater than we expected, and global variation in rates of sea level rise are remarkable.

It is this last statement that is perhaps most important for the coastal countries of the world to absorb. Rising sea levels are already affecting some regions far more quickly than others.

What can we do about this? We cannot at this stage reverse the rise, and we probably won’t not be able to slow it for a very long time. warming sea water will continue to expand, polar ice sheets and glaciers will continue to melt.

What’s left is for us to adapt as intelligently as we can, pulling back from our current threatened shorelines.

Current hotel construction on Clearwater Beach, Tampa Bay, west coast of Florida (tampabay.com)

Current hotel construction on Clearwater Beach, Tampa Bay, west coast of Florida (tampabay.com)

Or we can continue to be really short-sighted, greedy and ignorant.

Old Dominion Leads the Way

Saturday, March 21st, 2015

Old Dominion University is in Norfolk, Virginia, a small city right on the edge of the entrance to Chesapeake Bay. It is part of a metropolitan area of almost 2 million people called Hampton Roads that also includes Newport News and Virginia Beach.

Hampton Roads is one of the two most vulnerable metropolitan areas in the US to rapid sea level rise (the other is New Orleans).

Sea level is rising at about twice the rate of the global average along the coast north of Cape Hatteras, centered on Chesapeake Bay (sciencenews.org)

Sea level is rising at about twice the rate of the global average along the coast north of Cape Hatteras, centered on Chesapeake Bay (sciencenews.org)

Hampton Roads , a complex metropolitan region at the mouth of Chesapeake Bay (hamptonroadsof.org)

Hampton Roads , a complex metropolitan region at the mouth of Chesapeake Bay (hamptonroadsof.org)

Global sea levels rise as a result of the melting land-based glaciers of Greenland and the West Antarctic Peninsula as well as the thermal expansion of warming waters – an average of 22 cm (8 in) since 1930. What makes Hampton Roads of special interest is that sea levels there are rising twice as fast as the average.

Old Dominion University has established the Center of Sea Level Rise and the Mitigation and Adaptation Research Institute (MARI). It has chosen to be in the thick of it all.

Why such rapid sea level rise? And why there?

Partly it is because the land in that region is also sinking – the mile thick glaciers of the last glaciation did not reach so far south, but they compressed the land they did cover, forcing the land beyond them to bulge up. Since the glaciers withdrew, the land they compressed has risen again, while the bulge to their south is still falling back to its pre-glaciation state. Along with subsidence of the land from extraction of groundwater, this accounts for about half of the current rapid rise of sea level.

Sea level rise north of Cape Hatteras is about half due to recent climate change, and about half due to the land level readjustments following the retreat of the glaciers (americanroads.us

Sea level rise north of Cape Hatteras is about half due to recent climate change, and about half due to the land level readjustments following the retreat of the glaciers (americanroads.us

So Hampton Roads has immediate challenges, finding ways to adapt to the sea level rise sooner than most coastlines elsewhere. Coastal beaches and wetlands will certainly deteriorate, and the low lying parts of the coastal cities will be flooded. Norfolk is especially vulnerable. Pretty well everyone living there now knows this.

Old Dominion has taken the lead in a pilot project aimed at developing a comprehensive government and community cooperation in preparing for further sea level rise in Hampton Roads. In the past couple of weeks MARI has hosted seminars involving residents and state officials, focusing on resilience and environmental engineering and on perceptions of climate change and sea level rise, encouraging a willingness to address change.

In the past year it held a Rising to the Challenge Conference on sea level rise with strong bipartisan support from Congressional ans State politicians – in itself a rare and extraordinary event.

And everything, in the context of preparedness and resiliency, is on the table: tide gates, levees, flood walls, raised buildings and roads, marshes created to absorb storm surge, abandonment of low lying areas, elimination of subsidized flood insurance – the list is very real and very serious. The cities of Washington,D.C., Baltimore and Philadelphia all have reason to be watching closely.

Part of the US navy of 2012 at Norfolk Naval Base - which covers 4 miles of coastline and has 7 miles of piers (wikipedia.org).

Part of the US navy of 2012 at Norfolk Naval Base – which covers 4 miles of coastline and has 7 miles of piers (wikipedia.org).

And then there is the military. Nearby is the Norfolk Naval Base, the world’s largest naval base. Old Dominion has also recently hosted discussions by the military on how to prepare the naval base for the tidal flooding and extreme storm surges associated with sea level rise, while contemplateing the immense upheaval of having to move.

Meanwhile, home owners in the lowest parts of Norfolk can find no buyers for their homes, and as one pastor says
“I don’t know many churches that have to put the tide chart on their Web site so people know whether they can get to church.”

So: Go, Old Dominion. The whole world isn’t watching, but probably should be.

(iawrestle.com)

(iawrestle.com)