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Research: Updated Projections of Future Sea Level Rise

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An international team of climate scientists led by Anders Levermann wrote a paper than appeared in the Proceedings of the National Academy of Sciences (PNAS) of the US that described long-term (2,000y) sea level changes in response to different stabilized temperature thresholds.  You can find a short Reuters summary of the paper here.  I will provide more detail and share some observations of this paper.  The paper garnered a good amount of attention in climate activist circles since publication.

First, a little historical perspective.  Global mean sea levels rose in the 20th and 21st centuries: about 0.2m.  Prior to research conducted in the past five years, projections of additional 21st century sea-level rise ranged from another 0.2m to 2.0m.  These projections did not, in general, consider feedbacks; the parent simulations did not consider cryosphere processes (i.e., melting glaciers and the land-based Greenland and Antarctic ice sheets).  More recent research included more feedbacks and cryosphere processes, but their treatment remains immature.  Additionally, recent research started to examine projections based on realistic emissions scenarios, after researchers began to accept the fact that policymakers are unlikely to enact meaningful climate policy any time soon.  As such, sea-level projection ranges increased.  Which is where this latest paper comes in.

Levermann et al. reported a 2.3 m/°C sea-level rise projection in the next two thousand years.  Benjamin Strauss’s PNAS paper put this into context:

[W]e have already committed to a long-term future sea level >1.3 or 1.9m higher than today and are adding about 0.32 m/decade to the total:10 times the rate of observed contemporary sea-level rise

Thus, if global temperatures rise only 1°C and stabilize there (an extremely unlikely scenario), sea-levels two thousand years from now could be 2.3 m higher.  This might not sound like much, but an additional 7.5 feet of sea level rise would inundate 1.5 million U.S. peoples’ homes at high tide.  With 2°C, sea levels could rise 4.6m.  On our current emissions pathway, global mean temperatures would rise 4°C, which would result in an additional 9.2 m of sea level rise.  That’s 30 feet higher than today!  Levermann notes that these higher sea level projections are supported by sea level heights that occurred in the distant past (paleoclimate), even with their associated uncertainties.

According to Strauss’s analysis, such a rise would threaten more than 1,400 municipalities – and those are just U.S. municipalities that exist today, not tomorrow.  Globally, billions of people would be adversely affected.  What social stresses would billions of people moving inland exert?  The U.S. experienced its own small glimpse into this future post-Hurricane Katrina as a few thousand people permanently abandoned their below-sea level neighborhoods.

And now a couple of important points.  Some of the processes Levermann utilized involved linear change.  In complex systems like the Earth’s climate, very few changes are linear.  Many more are exponential.  When I discuss linear and exponential change with my students, I include a number of different examples because our species doesn’t easily understand exponential change.  We typically severely underestimate the final value of something that changes exponentially.  If changes within the climate system occur exponentially, the Levermann projections probably won’t be valid.  But their estimate will likely be an underestimate.

A USA Today article on the Levermann paper has this quote:

Looking at such a distant tomorrow “could scare people about something that might not happen for centuries,” says Jayantha Obeysekera of the South Florida Water Management District, a regional government agency. He says such long-term projections may not be helpful to U.S. planners who tend to focus on the next few decades.

Should people be scared that their communities will be underwater in 2000 years?  I don’t think they will be, number one.  Few people pay attention to trends that will affect them in 2 or 20 years.  200 or 2000 are well beyond anybody’s individual concern.  But I think society as a whole should examine this updated projection.  Do we want to condemn one-third of Florida to the bottom of the Atlantic Ocean?  Do we want to condemn thousands of towns and cities to that fate, even if the time horizon may be well beyond our lifetime?

Moving beyond the inevitable question regarding fear, what do these results mean?  We need to include results like these in planning processes.  If nothing else, planners and policymakers have more realistic estimates of likely future sea level in hand.  Those estimates will continue to change (hopefully for the better) with additional research.  But decision-making shouldn’t stop with the expectation that some future projection might be perfect because it won’t be.  The decisions we make today will have profound effects on the eventual level of the sea in the distant future.  There will also be countless effects on the climate, societies, and ecosystems until we reach that level.   That is what today’s decision-making needs to address.

Climate Central has a useful map to investigate how potential thresholds implicate sea level rise with respect to US states at different points in the future.


One thought on “Research: Updated Projections of Future Sea Level Rise

  1. Pingback: Research: Timing Sea-Level Rise Projections | Weatherdem's Weblog

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