“Oh say can you see”… 20th Century Sea Level Changes, When Viewed in a Geological Perspective?

Over the past couple of decades, we have been bombarded with warnings about our coastlines being inundated by rising seas as the result of anthropogenic global warming.  Here’s an example from Greenpeace…

According to the world’s leading scientists, sea level rise is “arguably one of the most important potential impacts of global climate change.” 1 Global average sea level has been rising over the last 100 years, and global warming is expected to increase the annual rate of sea level rise by two to five times. By the year 2100, sea level is projected to be approximately 19 inches higher than it is today. 2 An increase of this magnitude could inundate coastal areas, erode beaches and exacerbate coastal flooding. The costs associated with protecting shorelines in the U.S. alone would be enormous… LINK

Wow! A 19-inch sea level rise by the year 2100! That’s a fott-and-a-half!  Almost half-a-meter!

The EPA has modeled the impact of sea level rise on US coastal areas.  The  following map shows coastal areas of the US Gulf Coast that would be vulnerable to rising sea level…

US Gulf Coast areas vulnerable to sea level rise (EPA).

The “red” areas on the map lie less than 1.5 meters (~5 feet) above sea level.  The “blue” areas are between 1.5 and 3.0 meters (5-10 feet) above sea level.  The areas adversely affected by a 0.5 meter sea level rise would be a small subset of the red areas on the EPA map.

Is there any reason to think that sea level could rise another 0.5, 1.5, 3.0 meters or more over the next few hundred to few thousand years?  Sure.  Sea level was 3 to 5 meters higher than it is now during the previous interglacial (Sangamonian/Eemian ~130 kya).  There’s no reason to expect the Holocene sea level to behave any differently than the Sangamonian sea level.

How about over the next few decades?  Is there any reason to think that sea level might rise 0.5 meters or more by the year 2100?  Well, to answer that question, we have to look at how sea level has been behaving over the last few decades to few centuries and see if it is behaving anomalously.


Since the launch of the the Topex/Poseison satellite mission in 1992, global ocean surface topography has been continuously measured with great precision.  These data are analyzed at the University of Colorado – Boulder to monitor global sea level changes.  Here is CU-Boulder’s record of global sea level change since 1993…

CU-Boulder Sea Level from Jason/Topex Satellite Data

I took the liberty of pasting a 6-inch ruler onto the chart so that we can keep recent sea level changes in perspective as we expand the time horizon.  Total sea level variation since 1993 has bee less than 3 inches.  The rate of sea level rise has been ~3.2mm/yr over that period.  However, if you look closely at the data, you’ll see that the slope changed back around 2003…

Sea level rise has slowed down since 2003.

Since 2003, the rate of sea level rise has declined by more than 30%.  Is this sort of sea level rise unusual?  Is the variation in rate unusual?  Unfortunately, the Jason/Topex data only go back to 1993… But, there is an excellent sea level reconstruction available that goes back to 1700.  Jerejeva et al., 2008  produced an isostatically corrected sea level reconstruction from 1700 AD to 2002 AD.  Here is the Jerejeva reconstruction with the CU-Boulder data tied in using a static shift…

Jerejeva et al., 2008 and CU-Boulder Jason/Topex Data.

It looks to me as if the post-1993 satellite data fit right into a trend of sea level rise that began in the 1700’s (during the warm up from the Little Ice Age).  If we take a closer look at the 20th Century portion of the Jerejeva reconstruction, we can see a pattern of alternating rates of sea level rise…

Jerejeva et al., 2008 – 1900-2002

It appears that the change in sea level over the last 20 years is not anomalous when compared to the last 100 years and it appears to be a continuation of a pattern established in the early 1800’s.

So, how does the last 300 years of sea level change fit into the “big picture”?

Let’s look at sea level change since 4000 BC by comparing Jerejeva to a couple of Holocene/Pleistocene sea level reconstructions…

Jerejeva compared to Lea and Bard.

When viewed in the context of the last 6,000 years, the sea level rise since the 1700’s becomes insignificant.

Now let’s keep expanding the time frame.   First stop: 20,000 years ago during the last Pleistocene glaciation…

Now, let’s go back a bit over 300,000 years to take in a couple of full glacial cycles…

That 6-inch ruler is now totally insignificant… But let’s keep the time machine rolling.  Now we’ll use Miller’s (2005) Phanerozoic Eon sea level reconstruction and go back to 2,000,000 BC and the Lower Pleistocene…

On to the Lower Miocene… 20 million years ago…

Now on to the Mid-Cretaceous… 100 million years ago…

While I was working on this blog post, this appeared over at Watts Up With That?…

IPCC sea level prediction – not scary enough

From the Niels Bohr Institute – Studies agree on a 1 meter rise in sea levels

New research from several international research groups, including the Niels Bohr Institute at the University of Copenhagen provides independent consensus that IPCC predictions of less than a half a meter rise in sea levels is around 3 times too low. The new estimates show that the sea will rise approximately 1 meter in the next 100 years in agreement with other recent studies. The results have been published in the scientific journal, Geophysical Research Letters.


So now… The alarmists are Chicken Little-ing about a 1 meter rise in sea level over the next 90-100 years.

That is flat out impossible… Barring Dr. Evil using a space “laser” to melt the Antarctic and Greenland ice sheets. Could sea level rise another 1, 3 or even 5 meters before the end of the current interglacial? Sure. It rose that high in the previous interglacial.

Could sea level rise another 1 meter over the next 90 to 100 years? No. There is no historical or geological evidence to support such a scenario.

Assuming Earth stays in the warm phase of the 1,500-yr cycle until at least 2100 (not a huge assumption), the maximum possible sea level rise by then will be a bit more than 0.25m above current MSL…

1/4 of a meter… A bit less than 1 foot… That’s it… That’s the worst case scenario that is actually possible in the real world.

The climate modelers make me think of an Aubrey McClendon (CEO of Chesapeake Energy) quote:

“That kind of analysis, I think, can only come at the dangerous intersection of Excel and PowerPoint. It can’t happen in reality.”

8 Responses to ““Oh say can you see”… 20th Century Sea Level Changes, When Viewed in a Geological Perspective?”

  1. Joseph Seymour Says:

    “A 19-inch sea level rise by the year 2100! That’s two and a half feet!”

    I agree with the sentiments and content of this page, but when I went to school, two and a half feet was 30 inches.

    I’d bet on less that one foot of sea level rise by 2100, but I won’t be around to collect my winnings.

  2. JohnH Says:

    Your first graph shows Jason satellite altimetry data, I think. But how reliable and fundamental is the datum against which this is measured? One might expect it to be derived from ground points of known altitude in tectonically stable areas below the satellite track. But the NOAA website seems to say that the datum is derived from Tide Guages. Does the selection of these introduce an inherently subjective element into the data? And aren’t Tide Guages (unlike ground points) subject to numerous vagaries – changes in land levels, destruction of guages by vessels, and possibly poorly controlled replacement, man-made or natural changes in local hydraulics, irregular or faulty reading, etc.? Can NOAA justify their system?

    • David Middleton Says:

      I believe it’s a combination of Jason and TOPEX.

      I don’t know how they are datumizing it.

      I don’t think they are applying any isostatic corrections.

      It does tie into Jerejeva’s isostatically corrected sea level reconstruction fairly well… So I think it’s a reasonable approximation of relative MSL changes. Or at least, it’s the most reasonable apporximation available.

  3. Michael D Smith Says:

    Really nice analysis David, I found this while looking for something else… I might need to refer to this later… Mike S.

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