Most of you have heard sea levels are rising, mainly due to human activity. Warming is the underpinning of those rises.
I’m using "rises," plural, because sea level increases are uneven around the globe; they're happening faster in some places and slower in others. The warming is almost entirely due to the burning of fossil fuels and deforestation leading to more carbon dioxide in the atmosphere, along with increased methane and a few other greenhouse gases for which we are mainly responsible.
The sea level rises come through two mechanisms: One, meltwater from rapidly melting glaciers, including the massive Greenland ice cap, and two, the expansion of sea water due to more heat absorption from the atmosphere. Water expands when it gets warmer.
Rising sea levels are bad enough news for coastal cities and their populations. The effects increase during high-tide cycles and especially when storm surges enter the picture. New York City is currently not only repairing damage from Sandy’s major flooding, but is attempting to build more resistance against inevitable big surges in the future.
If you add subsiding land to rising sea levels, you can imagine how the impacts will increase more rapidly.
Subsidence is occurring in many places around the globe. The cities at greatest risk are those 30 feet or less above sea level, and many of those cities are where populations are growing. Quite a few are experiencing subsidence that is occurring more rapidly than the sea level is rising. Soil around cities gets drained, compacts and sinks more rapidly.
Many cities are on river deltas, which house more than 500 million people globally. These cities sit in vast agricultural surroundings, and some have nearby refineries producing more hydrocarbons. The refineries and drilling extract more fluids and gases from the soil, speeding compaction and subsidence.
Unlike global warming, in which humanity is the dominant force, some of the forcing mechanisms behind land subsidence are also natural. When aided and abetted by humankind, the subsidence can speed up dramatically. In addition to the factors I listed above, levees and flood-control embankments themselves can prevent the deposition of sediments downstream where those sediments could compensate for the compaction that is occurring.
That is what occurs in New Orleans. A University of Miami study done several years ago estimated that much of New Orleans subsides at a rate of 6 millimeters per year, but parts of that partially sub-sea-level city sink at 29 mm per year. Surveys of the levees show some of the fastest rates of subsidence occurred where levees failed after Katrina, probably weakening the levees beyond their projected design for such stress. Some parts of New Orleans will sink 3 feet over 40 years.
Speaking of Miami, that metropolitan area has vast problems due to rising sea levels without much land subsidence. Significant tidal flooding has already been occurring more regularly. The porous limestone beneath the region isn’t so prone to subsidence, but its porous nature is allowing seawater to progressively infiltrate the groundwater. The average elevation of Miami is less than 10 feet. The outlook for the long-term viability of Dade County is very grim in the scientific literature, but that’s another topic.
As coastal cities grow, demand on groundwater supplies such as aquifers grow with them. The reduced pressures in these heavily stressed aquifers lead to more compaction and subsidence. The most extreme example of this phenomenon is in China’s Huanghe Delta, where groundwater removal for coastal aquaculture has increased subsidence to 250 mm per year. That’s just under 10 inches.
Of course, when we start thinking of sinking, Venice comes quickly to mind. This LiveScience article covers the technology involved in tracking the subsidence near this Italian city with greater precision.
Advanced satellite and aircraft radars are now supplying the most precise measurements yet in gauging this problem, even as it worsens.
The bad news is, there are no magic switches to turn off either land subsidence or rising sea levels. The latest studies run on sophisticated models demonstrate that even if there were a magic way to turn off additional human greenhouse gas emissions, the long shelf life of carbon dioxide in the atmosphere would continue to produce more warming on a global average for many decades and possibly a century or more to come.
Even shorter-lived gases of methane produce warming (much more efficiently than carbon dioxide), and while the gases might theoretically disappear in this hypothetical scenario, the heating they produced would not.
Oceans are the biggest heat sink on the globe. The heat they absorb tends to eventually travel deep due to vertical ocean circulations. These deep waters, once heated, stay heated for an incredibly long time.
None of this is to suggest human efforts to mitigate these impacts are futile. It is important, however, for us to have some proper perspective on what the future holds, and what we probably can and can’t do about it.