Archive for the ‘Hansen’ Category

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Let’s Build a Seawall

May 17, 2009

I read with great surprise and pleasure that ClimateSanity has been added to the blogroll at  WattsUpWithThat.  Anthony, thank you, it is an honor!

The thing that brought this on was one of my previous posts about the effect of sea level rise on Boston, which Anthony was kind enough to share with his huge readership.

Just to stir the pot a little, here is a simple back of the envelope calculation that may serve as a response to some of the comments at WUWT:

Click to enlarge

Click to enlarge

Suppose you felt the need to build a sea wall three meters high and three meters thick to protect the Boston area from the rising sea.  Lets say you built it from Lynn in the North to Quincy in the south.  These two communities are about 25 kilometers apart in a straight line.  But let’s say when you account for the curviness of the coastline its really 100 km.  So,this sea wall will be 3 meters high, 3 meters thick, and 100 kilometers long.  That means the volume of the sea wall will be 900,000 cubic meters, or about 1,000,000 cubic meters. 

Since the ocean is rising quite slowly, we can allow ourselves 100 years to build this sea wall.

The population of the greater Boston area is about 4 million. Let’s assume that the population remains unchanged for the next hundred years (of course, it will probably grow a lot).

Then the volume of seawall that needs to be built per person per year is:  0.0025 cubic meters per person per year.  Oh my,  how will you every do it?

But if you manage to get it done in 100 years you will be able to look at the accomplishment with pride.  And assuming that the sea level keeps rising at its current rate of about 2.5 mm per year, you will know that the 3 meter tall seawall will keep you safe for another 1000 years.

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Boston Underwater?

March 24, 2009

Boston, you have been warned. Sea levels are rising , and if one of the IPCC’s five scenarios is correct, the world’s oceans will rise somewhere between 18 and 59 cm (7 to 23 inches) by 2100. If that isn’t terrifying enough for the people living on the New England coast, the Boston Globe now tells us that the ocean near Boston will rise 8 inches more than the world average. How will the hapless rubes of Boston cope with this onslaught of Atlantic water?

I wouldn’t lose to much sleep worrying about the folks in Boston when it comes to pushing back against the ocean. Excerpts from the following maps were used to make an animation of the changing coastline in Boston:

  • A 1775 map showing the Boston area with the rebel military works. Note especially the isthmus, known as Boston Neck< that connects the town of Boston to the mainland.
  • An 1838 George W. Boynton engraving of Boston area from a Thomas G. Bradford atlas.
  • USGS map of Boston area.
  • A 2009 satellite image from Google Earth

The top of the animation shows the maps after photoshopping to make the land and water more obvious. The bottom of the animation shows the unaltered excerpts of the maps or images.

animation-5I

The panic prone will argue that our Bostonian ancestors dealt with a static ocean, not a rising ocean. Not so fast. Check out the NOAA graph below (click inside graph to see it in context at NOAA site). It shows a sea level rise rate of 2.63 mm/yr for the last 100 years in Boston. At that rate it will rise 23.9 cm (9.4 inches) by 2100.

Boston sea level rise

Boston sea level rise data from NOAA. Click in image fro view in context.

Anyone who panics over the IPCCs 100 year projections of rising sea levels does not understand the perseverance and ingenuity of free people. Then there are others, like James Hansen, who enjoy the feeling of panic so much that that they exagerate the probable sea level rise for this century to get their thrills. But that is a story for another day…

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The “Collapse” of the Wilkins ice shelf

April 1, 2008

A few quick calculations put the size and effect of latest broken piece of Wilkins ice into perspective

The recent “collapse” of the Wilkins ice shelf is causing quite a stir in the blogosphere.  The issue of disintegrating ice shelves is often entangled with the issue of sea level rise.  The Los Angeles Times carried an AP story on March 25th that reported:

…the western peninsula, which includes the Wilkins Ice Shelf, juts out into the ocean and is warming.  Scientists are most concerned about melting ice in this part of the continent triggering a rise in sea level.

The next day, CNN reported on the Wilkins ice shelf, saying:

…the poles will be the leading edge of what’s happening in the rest of the world as global warming continues.  Even though they seem far away, changes in the polar regions could have an impact on both hemispheres, with sea level rise and changes in climate patterns.

Although most reports do admit that this floating ice will not raise the sea level at all, they paint an ominous picture of land bound glaciers rapidly sliding into the sea.  In fact, the Wilkins ice shelf, like other ice shelves, is the product of a land glacier or ice sheet flowing over the coast and onto the water.

The piece of the ice shelf that broke off over the last month is reported to be 160 square miles (about 400 square kilometers).   It is “up to” 650 feet (200 meters) thick according to the Times Online.  A BBC video report corroborates the thickness by saying “Those cliffs are about 60 feet high,” when referring to the floating ice, which indicates that the total thickness is about 10 times that (because most of it is underwater), or about 600 feet (180 meters).  So, lets say the ice is about 0.2 kilometers thick (200 meters).  Then the total volume of the piece that broke off is about

400 km²  x  0.2 km  = 80 km³

One km³ of water will raise the sea level by a miniscule 2.78 microns (less than 3 millionths of a meter).  So, over the course of time that it took this 80 km³ volume of ice to move from the land to the sea it contributed to the sea level by:

80 km³  x  2.78 microns/km³  =  220 microns  =  0.22 millimeters  =  0.009 inches

That’s not very much, considering that it took many years. 

In general, it takes 360 km³ of water to raise the sea level by 1 mm.  In order for the Antarctic peninsula to contribute 12 inches (about 300 mm) to the sea level in 100 years, it would have to drop 1,080 km³ of ice into the ocean  (more really, because the density of the ice is less than the density of water) EVERY SINGLE YEAR FOR 100 YEARS!!  If the ice at the grounding line (where the ice leaves the land) were 0.33 km thick on average, then more than 3000 km² of ice would have to move into the ocean every single year.  Of course, this estimate is based on the unrealistic assumption that there would be no new ice accumulation on land from precipitation to offset the sea level rise.  The difference in the amount of ice sliding into the sea and the amount of ice building up on land due to snowfall is call the mass balance.

Typical estimates for the ice mass balance in the Antarctic Peninsula are nowhere near the 1,080 km³ (roughly 1,080 Gt).  The mass balance for the entire Antarctic continent doesn’t even come close.  Estimates for the entire continent vary greatly and have huge uncertainties.  Vilaconga and Wahr (2006) estimate a net ice loss of “152 ± 80 cubic kilometers of ice per year, which is equivalent to 0.4 ± 0.2 millimeters of global sea-level rise per year.”  Davis (2005) estimates a net increase in Antarctic ice, which would cause a net drop in sea levels.  Either way, the Antarctic is a very, very long way from any kind of catastrophic meltdown.

Then there is Greenland.  Luthcke (2006) estimates the mass balance for Greenland at a loss of 101 Gigatonnes per year.  This translates into a puny sea level rise of only 0.28 mm per year.

While we are at it, let’s consider James Hansen’s estimate of a 15 foot sea level rise this century. 

On the average, a 15 foot sea level rise in a hundred years translates into 46 millimeters per year, requiring 16,500 km³ of additional water per year!  This is about 65 times the current rate of ice melt, if we accept the mass balances of Vilaconga and Wahr for the Antarctic and Luthcke for Greenland.  If the ice sliding into the ocean is a third of a kilometer thick, then Hansen’s doomsday scenario would require 50,000 square kilometers of ice to move from land to ocean every single year!!!!

The bottom line

Pictures of huge chunks of ice and making scary comparisons like “Seven times the size of Manhattan” may get people excited, but they are not very enlightening.

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Davis, C., et. al., Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise, Science Vol. 308. no. 5730, pp. 1898 – 1901, 2005  Get copy here

Luthcke, et. al., Recent Greenland Ice Mass Loss by Drainage System from Satellite Gravity Observations, Science, Vol. 314. no. 5803, pp. 1286 – 1289, 2006   Get copy here

Velicogna, I. and Wahr, J., Measurements of Time-Variable Gravity Show Mass Loss in Antarctica, Science, Vol. 311. no. 5768, pp. 1754 – 1756, 2007  Get copy here