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Interactive: Short-Lived Pollutants and Sea Level Rise

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Research Report by Climate Central

The article, "Mitigation of short-lived climate pollutants slows sea level rise", by Hu et al., is a collaboration between scientists at the National Center for Atmospheric Research (NCAR), Scripps Institution of Oceanography, and Climate Central, and examines how much the rate and amount of global sea level rise can be reduced by cutting emissions of carbon dioxide (CO2) and four short-lived climate pollutants (SLCP) — methane, tropospheric ozone, hydrofluorocarbons, and black carbon — by mid-century (2050) and in the long term (2100). These results are compared to a "Business As Usual" scenario and to mitigating CO2 only.

Population Benefitting From Action
State SLCP-Only CO2-Only Combined
U.S.
FL
LA
VA
NY
CA
NJ
TX
SC
NC
MD
MA
GA
DE
WA
MS
CT
AL
RI
ME
OR
NH
PA
D.C.
1,780,280
1,389,946
80,051
56,173
55,695
50,886
39,600
24,657
20,914
19,347
10,438
10,079
5,505
3,900
2,949
2,866
2,578
1,186
943
926
859
386
249
150
559,798
396,902
31,758
19,203
22,668
16,563
20,082
17,088
7,442
7,929
4,579
4,549
2,353
2,562
1,407
1,498
1,179
533
439
387
355
166
98
58
2,014,042
1,476,910
112,835
67319
76,717
90,071
53,664
27,973
24,349
26,574
14,308
13,389
7,655
4,737
3,845
3,409
3,947
1,543
1,294
1,258
1,174
559
306
205

The most important findings of the study are as follows:

  • Immediate mitigation of SLCPs can reduce the rate of sea level rise by about 18 percent “with negligible effect from CO2 reduction before 2050.”  By 2100, cutting SLCPs can cut the annual rate of sea level rise by 24 percent, with CO2 providing an additional 24 percent reduction.
     
  • Combined SLCP and CO2 reductions can cut the rate of sea level rise by 18 percent by 2050 and nearly 50 percent (from 2.1 to 1.1 cm per year) by the end of the century, while reducing actual sea level rise by 31 percent.  SLCPs account for more than 70 percent of this reduction.
     
  • SLCP mitigation acts faster than CO2 mitigation on a time scale of decades to a century, but mitigating CO2 remains critical to limiting the ultimate level of warming and its impacts beyond those time scales.
     
  • Delaying SLCP mitigation until 2040 (25 years) will decrease the impact of CO2 and SLCP mitigation on sea level rise by a third (~30 percent), and will make it difficult if not impossible to keep warming under 2°C (3.6°F) by end of the century.

To estimate the state-level impact of these projected reductions in sea level rise, we took the central estimates of global sea level rise from the four scenarios — with the caveats that these have large uncertainties surrounding them — and applied them along the U.S. coastal topography to determine the risk of submersion under these four different futures.  We then computed differences in population at risk based on the  results.

By focusing on the differences under pairs of scenarios, one being business as usual, the other being one of the mitigation options, we highlight benefits of the different choices; curbing CO2 only, SLCP only, or both.

Florida has by far the greatest population at risk from sea level rise, and thus derives most of the benefit of any action to slow it.  The estimates reflect only the population at risk from ambient sea level rise; they do not take into account storm surges that in the future would launch from higher seas.

These numbers should be taken as an index of gain/benefits rather than at face value. As the paper discusses, the sea level rise values are projections, subject to uncertainties. Additionally, the actual computations along the U.S. coast rely on estimates of people or assets at risk that carry errors themselves. And perhaps most significantly, these numbers represent current demographics, not the future. Projections of future population growth all show an intensification of development and an increase in the population living along the U.S. coasts.

Comments

By Jon
on April 14th, 2013

I see that we’ve shed two states with significant coastlines.  Are we assuming Alaska and Hawaii will no longer be states at some point in the future?

Reply to this comment

By Fred Moolten (Canonsburg, PA 15317)
on April 15th, 2013

The study provides evidence that prompt adoption of measures to control both CO2 and short lived pollutants (SLCP) will offer greater protection against sea level rise than CO2 mitigation alone - i.e., it would be desirable not to delay curtailing the SLCP.  While technically accurate, this conclusion may unfortunately be interpreted by some to mean that if we promptly mitigate SLCP, we can afford to delay efforts to control CO2.  In fact, while a delay in SLCP mitigation would increase global warming consequences for intervals of years, a delay in curtailing CO2 would impose those consequences for centuries to millennia because of the very long atmospheric persistence of excess CO2.

The best option is of course to do both promptly, but if attention to one were used to justify delaying the other, the worst choice would be to delay curtailing CO2.  In my view, there is a serious danger that this may happen if political leaders seize on the benefits of reducing SLCP as an excuse to put off facing the more difficult challenge of reducing CO2.

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