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Sea-level rise likely to swallow many coastal mangrove forests

Posted on 11 June 2020 by Guest Author

This is a re-post from Yale Climate Connections by new regular contributor Jeff Masters

Coastal mangrove forests aren’t adapting rapidly enough to escape rising sea levels, and many could disappear by 2050 in much of the tropics, according to recent research published in Science.

Authors of a study reported June 5 used sediment cores from 78 sites on five continents to determine when mangroves first appeared over the past 10,000 years, as sea-level rise had slowed once Earth fully emerged from the Ice Age. They found that mangrove ecosystems did not develop unless relative sea-level rise was less than 6 to 7 millimeters* per year. (The term “relative” is used because the rate of sea-level rise is determined by the increase in water volume of the oceans plus subsidence or uplift of coastal land).

Sea-level rise is accelerating

The global rate of sea-level rise has doubled from 1.8 millimeters per year over the 20th century to approximately 3.4 millimeters per year in recent years. In many coastal areas, the rate of relative sea-level rise is much higher as a result of subsidence resulting from human causes, such as groundwater pumping and fossil fuel extraction.

For example, the Mekong Delta of Vietnam is subsiding at a rate of 6 to 20 mm/year and the Ganges-Brahmaputra Delta by 1 to 7 mm/year. At the same time, sediment supply to the coast has declined as a result of damming of rivers and mining and export of sediment, further increasing the vulnerability of mangroves to sea-level rise.

Coastal wetlands act as natural levees against storms as a result of their ability to reduce water velocity and wave turbulence. Moreover, wetlands accumulate sediments that provide protection against rising sea levels and local subsidence. In the U.S., per square kilometer, wetlands save $1.8 million per year in storm damages.

A March 17, 2020, study in PNAS, Coastal wetlands reduce property damage during tropical cyclones, showed just how valuable wetlands are in reducing storm damage. The researchers analyzed property damage caused by 54 tropical storms and 34 hurricanes hitting the U.S. between 1996 and 2016. They found that counties with more wetland coverage experienced significantly less property damage: a 1% loss of coastal wetlands was associated with a 0.6% increase in property damage. (Side note: a 1% increase in wind increased damages by 7%, and counties on the storm path’s right side experienced 140% more damage than those on the left.)

The expected economic value of the protective effects of wetlands varied widely, averaging that $1.8 million per square kilometer. Wetlands conferred relatively more protection against weaker storms and in states with weaker building codes. Wetland losses of 2.8% in Florida between 1996 and 2016 are estimated to have increased property damage from the 2017 Hurricane Irma by $430 million.

Figure 1Figure 1. Annual county-level marginal value (the value gained from either consuming or producing one additional unit of a good or service) of coastal wetlands for storm protection in (A) northeastern coastal counties, (B and C) eastern and southeastern coastal counties, and (D) coastal counties from Texas to Florida. The highest-value wetlands were located in the Boston, Massachusetts area. Credit: Sun et al., Coastal wetlands reduce property damage during tropical cyclones, Proceedings of the National Academy of Sciences Mar 2020, 117 (11) 5719-5725; DOI: 10.1073/pnas.1915169117.

Mangroves in Louisiana and Texas in trouble

In the U.S., mangroves grow along much of the Florida coast and along large portions of the coasts of Louisiana and Texas. Most of the mangroves in those two states are experiencing rates of relative sea level that will threaten their existence. The NOAA Tides & Currents website indicates that six out of seven coastal tide gauges between Rockport, Texas, and New Orleans, Louisiana, over the past 40 to 50 years had rates of relative sea-level rise exceeding 5.8 mm/year:

Southeast coast mangrovesFigure 2. Distribution of mangrove forests in the U.S. Credit: Osland, M.J. N. Enwright, R.H. Day, and T.W. Doyle. 2013. Winter climate change and coastal wetland foundation species: salt marshes vs. mangrove forests in the southeastern United States. Global Change Biology 19:1482-1494

9.6 mm/yr: Eugene Island, Louisiana
9.1 mm/yr: Grand Island, Louisiana
6.6 mm/yr: Galveston, Texas
6.0 mm/yr: Sabine Pass, Texas
5.8 mm/yr: New Canal, New Orleans, Louisiana
5.8 mm/yr: Rockport, Texas

2017 analysis of wetlands change at 185 sites across the Mississippi Delta found even higher rates of sea-level rise since 2006: 13 ± 9 mm  per year. About 65% of these sites were able to keep up with this rate of sea-level rise over that relatively short period of time. The situation is more encouraging in Florida: The highest rate of relative sea-level rise at NOAA tide gauges along Florida’s mangrove forests is 3.8 mm per year, in the Florida Keys.

The results of the June study support the findings of a May 22, 2020, paper in Science Advances, Tipping points of Mississippi Delta marshes due to accelerated sea-level rise. That research presented an 8,500-year-long sediment record from 355 boreholes in the Mississippi Delta marshes of Louisiana. It showed that at rates of relative sea-level rise of 6-9 mm per year, marsh conversion into open water occurs in about 50 years. Even at slower rates of relative sea-level rise of 3 mm per year, the researchers found that marshes in 80% of cases transitioned to open ocean in a few centuries, and they concluded that drowning of the Mississippi Delta marshes is inevitable.

An excellent analysis of this paper at details provisions in Louisiana’s coastal master plan – a $50 billion effort to defend the coast against rising sea levels – to help deal with the threat.

Commentary: Think Scooby-Doo ‘Ruh-Roh’

Mangrove forests provide a U.S. coastal population of more than 200 million people with services like protection from intense storms and waves, reduction of coastal flooding, sequestering of carbon, improved water quality, and preservation of biodiversity and fisheries.

Some might say it’s something of a Scooby-Doo “Ruh-roh” moment to see science predicting the loss of much of the world’s mangroves in just 30 years. Human development and sea-level rise have already led to the loss of one-fifth of the world’s mangroves between 1980 and 2010; Tampa Bay, Florida, has lost almost half of its mangroves in the past century.

Nonetheless, the Trump administration has torpedoed a major Obama administration regulation protecting wetlands: On June 22, new EPA regulations substantially reducing the number of water bodies and wetlands protected by the Clean Water Act are to take effect.

Orrin Pilkey: ‘… we can walk away methodically, or we can flee in panic.’

Sea-level rise is expected to cause massive upheavals to civilization in coming decades:

– forcing millions of people to abandon the coast as rising seas inundate populated areas and major cities;
– opening the way for climate change-amplified hurricanes to drive higher storm surges farther inland;
– knocking out transportation systems and sewage treatment plants;
– swallowing prime agricultural land and barrier islands; and
– infiltrating aquifers with salt water.

But the impacts of sea-level rise are not limited to future decades—they’re happening right now. Hurricane Sandy’s storm surge in New York City in 2012 caused an extra $2 billion in damage as a result of higher water levels the city experienced from 20th century sea-level rise. “Nuisance” flooding has become a growing problem in places like Miami Beach, Norfolk, and San Francisco. And in Maryland, for instance, both Annapolis and Baltimore now get more than nine times the number of flood days they experienced in the 1960s. (I review many more examples in a December 2017 review of Jeff Goodell’s must-read book on sea-level rise, “The Water Will Come: Rising Seas, Sinking Cities, and the Remaking of the Civilized World.”)

In response to news that a long-predicted acceleration of sea level is already underway, one does well to heed the words of Duke University sea-level rise expert Dr. Orrin Pilkey and co-authors in their 2016 book, “Retreat From a Rising Sea: Hard Choices in an Age of Climate Change”:

Like it or not, we will retreat from most of the world’s non-urban shorelines in the not very distant future. Our retreat options can be characterized as either difficult or catastrophic. We can plan now and retreat in a strategic and calculated fashion, or we can worry about it later and retreat in tactical disarray in response to devastating storms. In other words, we can walk away methodically, or we can flee in panic.

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Comments 1 to 6:

  1. Would be good to expand on the value of mangroves a little more. It is mentioned, but their main value is not just protecting against damage to coastal properties.

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  2. And what is causing this sea level rise exactly? Because it can't be "global warming" or CO2 emissions. There just isn't the energy available to do this as I have calculated here -

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    Moderator Response:

    [DB]  Please refrain from sloganeering.  Human activities are the dominant contribution to SLR since 1970.

    Per Slangen et al 2016,

    Anthropogenic forcing dominates global mean sea-level rise since 1970

    "the anthropogenic forcing (primarily a balance between a positive sea-level contribution from GHGs and a partially offsetting component from anthropogenic aerosols) explains only 15 ± 55% of the observations before 1950, but increases to become the dominant contribution to sea-level rise after 1970 (69 ± 31%), reaching 72 ± 39% in 2000 (37 ± 38% over the period 1900–2005)"


    1. Although natural variations in radiative forcing affect decadal trends, they have little effect over the twentieth century as a whole

    2. In 1900, sea level was not in equilibrium with the twentieth-century climate, and there is a continuing, but diminishing, contribution to sea-level change from this historic variability

    3. The anthropogenic contribution increases during the twentieth century, and becomes the dominant contribution by the end of the century. Our twentieth-century number of 37 ± 38% confirms the anthropogenic lower limit of 45%

    4. This would increase even further if increased ice-sheet dynamics were considered to be a consequence of increased anthropogenic forcing (to 83% in 2000) and if reservoir storage and groundwater extraction were included (to 94% in 2000)

    5. Our results clearly show that the anthropogenic influence is not just present in some of the individual contributors to sea-level change, but actually dominates total sea-level change after 1970

    From Cazenave et al 2018 we know that land-based ice sheet mass losses comprise the biggest component of measured SLR, with that contribution increasing:


    A continuance of violating this site's Comments Policy will result in further moderation, up to and including a revocation of commenting privileges.

  3. Slarty Bartfast @2,

    You are evidently not the real Slartybartfast, the designer of planets from Hitchhiker's Guide the the Galaxy because he would not make the profound mathematical error and logical error which you make on your blog post.

    Firstly, in your calculations, you use the 'linear' coefft of thermal expansion which is the linear expansion of a solid, something which involves expansion in all three directions. The value for water is given in tables solely to allow the easy calculation of the differential rate of expansion when a volume of water is held in a container. Thus your 0.66mm/yr SLR (from the linear coefft of 69e-6/deg C) due to 0.9Wm^-2 global energy imbalance should be 2mm/year (from the volumetric coefft of 207e-6/deg C although note this coefft does vary with temperature, pressure and salinity). As most of the energy imbalance does end up warming the oceans, the actual thermal expansion component of SLR isn't greatly lower than that value (as the graphic in the Moderator Response shows).

    Secondly, you fail to make the explicit point (although you do manage to demonstrate it) that the melting of ice is a far far more thermally efficient means of raising sea level. Thus if the 0.9Wm^-2 global energy imbalance were solely applied to melting ice, it would result in 130mm SLR pa. We are saved from this SLR as the global energy imbalance is spread over the whole world while glaciers and ice caps are concentrated in a few particular regions. As the world around the planet's ice warms, that ice does attract an increasing percentage of that imbalance, resulting in SLR far in excess of the limit you impose using solely the land area of ice fields.

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  4. MA Rodger @3 , there are several "profound logical errors" in Slarty Bartfast's own blog.   I am not planning to go into them here, for they are mentioned (at least some of them) on another thread = 2020 News Roundup #25.

    And as yet, I have seen only part of his blog.

    I can say that he demonstrates admirable skills in mathematical analysis ~ but he seems not to realize that he has built his edifice on a base which is simply unphysical.

    ( In science, can there be any crueller word than "unphysical" ? )

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  5. Eclectic @4,

    I fear we will be wasting out breath trying to put Slarty Bartfast straight. His grand work on SLR which he set out on a webpage in his blog and presented @2 disappeared following the criticism of it here.

    But it is now reappeared with its silly errors unchanged.

    Perhaps the silliest part of his blog is his own cedentials which in the circumstances he should be presenting to the world. Yet he hides behind his pseudonym and the description "Physicist, socialist and environmentalist (not necessarily in that order)." His physics has the feel of a school-book regurgitated, giving the impression of somebody who thinks he is a man-on-a-mission when, if you read his blog, he is actually a deluded fool in freefall. Despite all the words and equations, he still manages to say nothing of any interest.

    Beyond him presenting himself here at SkS, I would give him and his silly misconceptions no heed.

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  6. MA Rodger @5 . . . on the contrary (speaking for myself) ~ it ain't climatology, but it's psychologically interesting to give some inspection to these forms of intellectual pathology !

    Perhaps Slarty will modify his blog self-label to :- physicist, socialist and environmentalist . . . and denialist (not necessarily in that order).

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