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Research: Sea-Level Rise in Response to Warming Climate

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From the top, I want to include important context for the research results I am presenting.  This research is based on peak warming of only either 1.5°C or 2°C.  It is my educated opinion that such goals are unrealistic.  Prevention of warming past 2°C is no longer a viable option based on the globe’s history of burning carbon-intensive fossil fuels as well as the medium- to long-term future, which doesn’t promise much of a difference.  Furthermore, as I have stated numerous times in the past year, policy discussion would be better served if scientists would conduct research on developments that are much likelier to occur and not the world they want to see (i.e., higher vs. lower emissions scenarios).  That said, this research fulfills an important role in the overall discussion because I think some of the results can be used as a “floor” – conditions are likely to reach higher magnitudes than those found in this and similar papers.

Michiel Schaeffer, William Hare, Stefan Rahmstorf & Martin Vermeer’s Nature paper was published on June 24, 2012.  They examined sea-level rise in response to warming scenarios using a semi-empirical model.  By 2100, global sea-level rise would be ~60cm above the 2000 level if global GHG emissions were zeroed by 2016.  This is an obvious fantasy world, but it provides a useful benchmark for other scenarios the scientists examined.  The reason sea-level rise would continue through the 21st century even if we haled emissions completely in the next 3-4 years is the response of the climate system to the anthropogenic forcing imparted on it through the 20th and early 21st centuries.  If 1.5°C or 2°C warming is not exceeded, global sea-level rise would be 75-80cm above the 2000 level.  The authors also report that unmitigated emissions could result in 100cm rise above 2000 levels.  It is important to note that 20th century sea level rise has been estimated to be ~20cm.  It doesn’t require much thought to realize that the rate of sea-level rise has increased throughout the 20th century and continues to do so in the 21st.  Moreover, it is clear that since we will most likely warm beyond 2°C, the 75-100cm projection can be viewed as a reasonable estimate for a “floor”: actual sea-level rise could be greater than this.

The authors go on to estimate global sea-level rise by 2300.  Since the world won’t end in 2100, nor our civilizations (though they will be forced to adapt to a changing world), projections out to 2300 are useful to gain an understanding of the long-term effects of our actions.  By 2300 a 1.5°C scenario could result in peak sea level at a median estimate of 1.5m above the 2000 level. The 50% probability scenario for 2°C warming would see sea level reaching 2.7m above the 2000 level and still rising at about double the present-day rate.

That last sentence is important to understand because of 2 factors.  The first is the 50% probability descriptor and the second is the extreme difficulty in keeping warming at or below 2°C.  Even if we were to keep warming at or below 2°C, there are realistic scenarios in which sea-level rise exceed 2.7m in the year 2300.  Beyond that, 2°C acts more as a floor in the real world than a ceiling in the fantasy world that continues to garner research focus.  Our historical emissions values were closer to the higher end of the IPCC AR4’s range than the lower – or put another way, 4°C looks likelier to occur by 2100 than 2°C at this point because our emissions more closely resemble the A1B or A2 scenario.  The following graph shows the AR4’s global surface warming projections by scenario.  Note that the A2 scenario didn’t run all the way to the year 2300, but the B1 and A1B scenarios were.  The equilibrium temperature under the A2 scenario is obviously higher than that of the A1B scenario, but left unprojected in previous work.


Figure 1. IPCC AR4 WGI global surface warming observations (20th century; black) and projections based on SRES scenarios (21st – 24th centuries; color).  The average warming is indicated by bold lines in each color and the range of projections per scenario are the lighter-shaded envelope surrounding the bold lines.  The number of individual proxy datasets and model projections are located under the curves (e.g., 17 A2 runs, 21 A1B runs between 2000 and 2100, etc.)

The Schaeffer paper includes the following graph for sea-level rise (SLR) through the 21st century:


Figure 2. Rate of sea-level rise (SLR; left) and SLR projections (right) between 2000 and 2100.  The rate of SLR after zeroed emissions is shown by the gray dashed line (Zero 2016).  The black solid line and accompanying 90% uncertainty range represents the authors’ results from least mitigation while the blue line and accompanying 90% uncertainty range represents the results of highest mitigation.

And here is the paper’s graph for SLR from model year 1000 through 2300:


Figure 3. Rate of SLR (left) and SLR projections (right) from the year 1000 through the year 2300.  90% uncertainty ranges are shown for only two scenarios, focusing on the lowest and highest temperature-goal scenarios.  Error bars on the right-hand side as in Fig 2.

The right-hand-side of Figure 3 shows that for the lowest temperature-goal scenario, sea-level begins to stabilize around the year 2300 at +150cm from the 2000 level.  Unfortunately, the highest temperature-goal scenario does not result in stabilized sea level by the same point in time.  Indeed, SLR continues well into the future.  I will point out again that we are likely to exceed the authors’ temperature-goal scenario by a significant margin.  What does that mean?  The globe is likely to experience increasing SLR rates, resulting in higher sea-level values sooner than what is projected here and in other similar studies.

A few words on what this means in a societal context.  Put simply, most modern, developed societies are not prepared to handle the sea-level rise that has already occurred or that which is occurring today and in the near future.  The costs of doing so are relatively high and are becoming higher every year (at least in the US) given our lack of long-term planning when zoning along coastal regions.  Populations have exploded along US coasts during the 2nd half of the 20th century, and as the country has gotten increasingly wealthy, more “stuff” that is worth more money is situated in areas that will be increasingly affected by SLR in coming decades.  Developing countries also have large populations near coastlines, but SLR will impact them differently.  Those populations’ direct livelihoods are threatened, which will likely result in mass migration as the ocean swallows miles of land.

There are a very small number of countries who are doing anything meaningful about this situation, in my opinion.  Denmark and England are two that come to mind most readily, although I’m sure other examples exist.  And really, it is communities, towns, and a few cities that are preparing, not their national governments.

Since we are ill-equipped to handle today’s SLR rate, it follows that we are even less prepared to deal with tomorrow’s SLR rates and subsequent sea-level.  Mitigation is not going to happen anytime soon, so adaptation strategies will move to the fore.  How communities plan and execute responses will go a long way in determining who ends up affected and how much cost will fall to local populations vs. national populations.  Given the right-wing’s insistence on austerity and their “you’re on your own” attitude, I think we will witness hardships encountered by individuals and their communities for a while before these situations are addressed at national levels.  I also think countries with weak national governments will see the greatest potential for geopolitical strife.  Lifestyles in developed countries will have to change, but probably to a lesser degree.

As we move into the future, this conversation would be better informed with projections resulting from emissions scenarios that more closely resemble historical values and likely future pathways.  Of course, there are many aspects of models that deserve simultaneous improvement (e.g., all first-order forcings and feedbacks) so that real-world processes are more accurately represented.  But I think policy discussions will benefit in the short-term from more realistic emissions pathways as a starting point.


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