Oceans

‘Astounding’ Ocean Temperatures in 2023 Intensified Extreme Weather 0124

'Concerning' - Marine Heat Waves Increasing, Especially near Australia 0418

Global Ocean Heat Content - WEB

     Sea level has varied greatly over the course of geological history, from perhaps 400 feet higher than today to ~ 420 feet lower than today.  (Levels far higher than today, very long ago, were augmented by different vertical distribution of land mass, due to many-million year plate tectonic movements.)   Sea level since 35 million years ago has been no more than 240 feet higher than today, but sea level was 400 feet lower than today as recently as 20,000 years ago.

     Worldwide, sea level has risen perhaps 1 inch per century in the last 3,000 years, but perhaps 2 inches per century in the 3,000 years before that.  Local sea level rise varies modestly from the global average - depending on land subsidence, any glacial rebound, and uplift from tectonic plate movements.  Global sea level rose 8 inches during the 20th century.
    In 2011 sea level rise was 13 inches per century.  About 1/2 of that is due to thermal expansion of warmer water.  Melting ice in Greenland and Antarctia contributes almost ~1/4.  ~1/4 comes from glaciers in Asia, North America, Europe, and elsewhere.  ~1/20 comes from mined groundwater.

     From 15,000 to 8,000 years ago, seas rose 1.5 meters (5 feet) per century, 4.5 times today's rate, including 4 meters (14 feet) per century for one 700-year period.

     Over the past 800,000 years, as shown below, sea level changes have followed changes in atmospheric CO2 (and CH4) pretty closely.  The difference between minima and maxima was ~110 meters, or 360 feet.

During Meltwater Pulse 1A, sea level rose 4+ meters per century, for 7 centuries.

Sea Level Rise Acceleration 0922 - Rate doubled in last 20 years.

20th Century Sea Level Rise Was Fastest in at Least Last 2,700 Years 0216

Sea Level Rise over Past Century Unmatched in 6,000 Years 1014

Note: Atlantic Ocean relative to North Carolina only.  Its coast may have subsided.

     At first glance, sea level might rise 65-73 meters (214-240 feet).  61-69 meters come from melting ice: 5-7.5 from Greenland and 3.3-6 from West Antarctica, ~0.5 meters from "mountain" glaciers, and 52-66 from East Antarctica.  (While Wikipedia estimates that Antarctic ice has 70 meters of sea level rise equivalent (SLRE) in water, the USGS in 2013 cut its estimate from 73 to 55.3 meters SLRE, according to Phil Blum at www.theatlanticwire.com/politics/2013/11/map-sea-level-rise-probably-wrong-its-too-optimistic/71246/?goback=%2Egde_119460_member_5803866787263037442#%21.

     4-8 meters comes from sea water's thermal expansion.  Warming the world ocean by 5°C would raise sea level perhaps 4 meters, while 10°C (which prevailed 130-40 million years ago) would raise it 8 meters.  Warming the entire ocean by 5°C would take ~ 2,500 years, at the current heating and expansion rate of 1.6 mm/year.  But it would take less as heating speeds up, with more CO2 in the air.

     However, based on past sea levels over 60 million years, the upper bound on sea level rise is ~75 meters.  Almost enough to make Memphis a seaport.  Burlington and Brattleboro, in Vermont, might become seaports.  98% of New York City would lie beneath the waves.

     The life expectancy for Greenland's ice, based on 1992-2011 ice loss rates, is ~11,000 years.  For West Antarctica, ~29,000 years.  But ice loss rates have continued accelerating.  (Greenland's rate recently rose 7-fold in 17 years, Antarctica’s doubled in 4.)

     Until about 2010, East Antarctica was gaining ice, but more slowly than West Antarctica is losing ice.  More recent data suggest East Antarctica (on the whole, some areas gain, others lose) is losing ice, but the amount is smaller than the uncertainty band.  That is, its % net annual volume change is modest compared to the “noise”.  Modeling suggests that melting all of East Antarctica's ice could take several million years.  But studies of East Antarctic glaciers show that, in recent decades, they receded when temperatures there warmed (Stokes 2013).  Morever, when atmospheric CO2 levels were this high (almost 400 ppm) 4 million years ago (and slightly higher 15-20 million years ago), much higher sea levels indicate that East Antarctica lost 10-40% of its ice (6 to 23 meters of SLRE).  The question is how FAST its ice melts.

     Since the last glacial maximum, sea level rise rates from melting ice peaked at 4 meters per century 14-15,000 years ago (during Meltwater Pulse 1A), or 1+ inch per year.  Temperatures now are rising 30 time as fast as then, but only 1/3 as much ice is left.  It remains to be seen how fast sea level will rise this time.  But sea level rise will accelerate from current levels.  1 inch/year is a reasonable, if high, estimate by 2100: just below the 1.1 inch/year upper bound for 2050-2100 in the draft 2013 US National Climate Assessment (next graph).

     Sea level might rise 5 feet (60 inches) or more by 2100.  See city sea level rise maps below.  A 12-foot rise might well happen by 2200.  A 25-foot rise appears unlikely till after 2300.  But the uncertainty is large.  The dates depend on our emissions and Mother Nature's, plus our success in removing carbon from the air (with some help from Mother Nature).  Our knowledge about the dates also depends on how well we understand ice loss dynamics from Greenland and Antarctica.  We are learning that our models have underestimated some rates and failed to acount for, or inadequately accounted for, some ice dynamics.  (See Ice Dynamics section on Ice page.)

What Sea Level Rise Looks Like for 24 US Cities 0413

What Sea Level Rise Looks Like for 24 US Cities 0413 - WEB

The 3 maps and text, about ocean CaCO3 concentrations, are from the US EPA.

     "Corals require the combination of temperature, light, and the presence of calcium carbonate, which they use to build skeletons.  As atmospheric CO2 levels rise, some of the excess CO2 dissolves into ocean water, reducing its calcium carbonate saturation.

     "As the maps indicate, calcium carbonate saturation has already been reduced considerably from its pre-industrial level.  Model projections suggets much greater reductions in the future.  The blue dots indicate current coral reefs."  EPA

Global Warming Could Deplete the Oceans’ Oxygen, with Severe Consequences 0416

Deoxgenation due to climate change is already detectable in some parts of the ocean.

New research from NCAR finds that it will likely become widespread between 2030 and 2040.

Other parts of the ocean, shown in gray, will not have detectable loss of oxygen due to climate change even by 2100.

(Image courtesy Matthew Long, NCAR.)

Why Our Oceans Being the Warmest in Recorded History Is So Concerning 0823

     Marine life struggles with heat, its resulting oxygen loss, and acidification.  Coral is especially hard hit.  It also, via the warming-melting effects on Greenland and Antarctic ice, slows down Atlantic Ocean circulation, especially the AMOC and Gulf Stream.  The AMOC may collapse by mid-century, give or take a few decades and possibly as soon as 2025.  The warming also melts sea ice, a major feedback that further accelerates global warming.

Scientists Spot Warning Signs of Gulf Stream Collapse 0821 - take 1 on study

Section Map: Water