There is up to 70 meters of sea level rise locked up in the world's iceD1. The glaciers started shrinking many decades ago, while the ice sheets of Greenland and Antarctica began losing significant mass in the last 15 to 20 years. NASA lecture on ice sheets:
This condition is for an average global warming of 0.8oC since 1880D3: 0.5oC of which happened since 1960.We know the temperature will increase, and that ice responds dynamically to warmth. The revised forecasts for 21st Century sea level rise are now 1 meterL to well over 2 metersD4 b c. [Background ]
A reasonable expectation is that the eventual sea level rise for a 2oC global warming compared to 1880 would be approximately 20 meters [+/- 5m]. While the timing is several centuries, this outcome is likely given that is where sea levels were in the past for those temperatures.
Indeed, the level of CO2 already in the air is already consistent with periods when sea level was 25 meters higher than present some three to five million years ago. CO2 is the core driver of average global temperature, and temperature drives ice mass and sea levelD5.
The current rate of warming is more than 15 times faster than the emergence from the last ice age - a period when sea levels rose over 1.5 meters and sometimes 5 meters per centuryL. During this transition global average temperature rose by 4oC to 5oC over 8,000 years and sea levels rose 120 meters over 11,000 years to approximate to present day levelsD6.
The projected rate of warming [4oC+] for the 21st Century under business as usual, assuming no ice sheet collpase, is 60 times faster than the rate of warming during the 8,000 year emergence from the last ice age. That's a 1oC rise in global average temperatures every 26 years in the 21st Century. In the exit from the last ice age it was a gradual 1oC rate every 1,600 years. This is an unprecedented and abrupt change in global climate meaning accelerated change in the cryosphere, and sea levels. NASA Ice Viewer
Sea level has been relatively stable for thousands of years, until warming from elevated greenhouse gas levels fed through. It began an abnormal rise in the 19th and 20th Centuries as glaciers responded to the 0.8oC temperature increase since 1880. The oceans also expanded causing much of the rise. Sea levels rose by up to 18cm in the 20th CenturyD7.
Ice Sheets & Sea Level
The Greenland & West Antarctica Ice Sheets which contain 15m of sea level rise potential only formed substantially less than three million years ago once global average temperatures were no more than 1 to 1.3oC warmer than present and atmospheric CO2 levels well below 350ppm. We know they are highly temperature sensitive as sea levels were 4m to 6m higherL [perhaps even 9m] 125,000 years ago for a global average climate no more than 0.6oC warmer than presentL.
The source of more recent sea level rise is thermal expansion of sea water [30%], and melt from Glaciers [30%], and the ice sheets of GreenlandD8 [22%], and AntarcticaD9 b [18%]. Greenland and Antarctica each hold 6.5 meters and 63 meters, in sea level rise potential, respectively.
Much of the West Antarctic and East Antarctic ice is grounded below sea level. Enough for over 25 meters of sea level rise, explaining why multi-meter sea level rise per century has happened in paleoclimate for warmings 60 times slower than that forecast in the 21st Century. Warm ocean currents can rapidly erode the ice if given the chance.
GreenlandsD10 annual melt is now 170 billion metric tons of ice: equivalent to an annual 0.5mm of sea level riseL. Antarctica is losing 130 billion metric tons of ice a year. The glaciers are losing 230 billion tons a year. A billion tons is equal to 1 cubic kilometre of ice. The measurements are taken by the NASA Grace satellite. NASA is the only organisation to operate such a piece of equipment L L.
Should the rate of ice mass loss from the Greenland and Antarctic ice sheets double every ten years, which measurements suggest it could be doingD11, a 5 meter sea level rise in the 21st century under the Business as Usual emission path is possibleL.
It is evident that the permanence of these ice sheets can no longer be assumeddl dl. There are dynamic tipping points beyond which significant ice sheet loss is irreversible. It is now recognized as conclusive that this threshold will be crossed at a greater than 1.2oC average global warming since 1880. The ice sheets have never been as large as present day for CO2 levels above 300ppm or temperatures warmer than present. Hence the call to reduce atmospheric CO2 to well below 350ppm as soon as possibledl.
"When CO2 levels were last similar to modern values [that is, greater than 350 to 400 ppm], there was little glacial ice on land or sea ice in the Arctic, and a marine-based ice mass on Antarctica was not viable. A sea ice cap on the Arctic Ocean and a large permanent ice sheet were maintained on East Antarctica when CO2 values fell below this threshold. Lower levels were necessary for the growth of large ice masses on West Antarctica [250 to 300 ppm] and Greenland [220 to 260 ppm]". Quote from a research paper with a detailed account of last 20m years: Link.
Arctic summer sea ice area is shrinking 12% every ten years. It's volume is shrinking even faster, with 70% lost already. An ice free Arctic Ocean during the summer is likely by 2025/2040. The last time this may have happened was 6 to 7,000 years ago for a 0.1oC warmer climate than present, and 125,000 years ago for just a 0.7oC warmer average global climate than present.
When ice is gone it exposes darker surfaces such as the ocean, vegetation, or rock. Dark landscapes absorb infrared radiation (part of sunlight) instead of reflecting 90% of it back into Space, thus warming their surroundingsL. This crucial feedback effect amplifies warming and accelerates regional melt of ice and snow, both on water and on land. Science calls this an Albedo reduction and it is a core part of the Polar Amplification process. [Remember the Poles receive 24 hour sunlight for six months of the yearw. ]
Water with a cover of 30% sea ice. The Extent/Area for September 6th for 1979 [left] 2012 [right]
Since the 1950s the thickness of the year round Arctic sea ice has easily fallen by over 50%D12 [57% by 2012], and the extent of the summer ice has dropped by 35%D13 [57% in 2012] since the 1970's, which means its volume has easily fallen by about 67% [76% in 2012]. The trend in the Arctic is clearD14, and tracks the 0.15 to 0.20oC average global warming per decade since the 1970's . We are crossing a Tipping Point and not just Santa needs a new homeD15 L. Recent satellite data shows an 18% drop in volume so far this century.
The importance of Arctic sea ice for climate is difficult to understate. Losing it is for the Earth like us taking off a base ball cap on a sunny dayL. The amount of energy the Earth then absorbs into the oceans increases enormously, accelerating the polar amplification processP and global warming. This effect is called an Albedo flip. It is clear we are on the threshold of such an eventD16 b. This means that the 2oC target as defined by the 450ppm peak of atmospheric CO2 cannot be successful and is doomed to fail.
Polar AmplificationL, of which sea ice is a key component, is a self-reinforcing process of temperature change 3 to 4 times stronger than average global temperatureL change. The other main elements of the process are i) greenhouse gas feedback emissions from permafrost loss and biosphere change, and ii) a reduction in land based snow cover also reducing the amount of light reflected back to Space. [Seasonal videos on albedo change ]
Antarctic Sea Ice
On the other hand while the Antarctic ice sheet at the South Pole is losing mass due to warming, the surrounding sea ice, though suffering large losses in traditional areas, has seen a slight increase overall in the dark winter [1% per decade as opposed to a 10% loss per decade in the Arctic summer]. One reason for this is that the Stratospheric Ozone hole over the Antarctic has dampened the initial extent of the warming in this region. Ozone is a greenhouse gas as well as being a filter for ultra violet lightD17. The albedo effect of the Antarctic sea ice increase on the Earth's energy balance is unimportant as it takes place in the dark winter.
We also know that the Antarctic ice sheet began to form 34m years ago at higher levels (450ppm to 550ppm) of Atmospheric CO2 and average global temperature in paleoclimate history than the Arctic sea ice and the Greenland ice sheet (260ppm to 320ppm) 2.8m years ago. So the chronology of ice sheet melt to date is both logical and straight forward. (See the section on PaleoclimateP)
The cold surface Antarctic circumpolar current turns clockwise and maintains the ice sheet. The predominant winds are also clockwise westerlies. The Coriolis effect in the southern hemisphere means the wind wishes to turn left i.e. out from the center. This natural tendency allows the deep warmer sub polar current to upwell onto the Continental Shelf. Warmer waters from the north have become entrained in these deep currents, meaning increased melt when upwelling occurs. Furthermore, as the planet has warmed the wind speeds around Antarctica have got stronger strengthening the upwelling. In this way ice melt has been accelerated. 7 of the 12 Antarctic ice shelves have already disintegrated. Their role was to plug the glaciers routes to the sea and take many thousands of years to form. Since collapse several glaciers have sped up by several hundreds of percent.
Even if global greenhouse gas emissions were reduced in line with the 2oC goal of the 2009 Copenhagen Accorddl Arctic summer sea ice will disappear, probably in our lifetimes. The abundance of the polar bear and other Arctic species dependent on sea ice, such as walrus ands seals, would crash.
Dozens of species, including the blue whalew and penguinsw, are dependent upon KrillD18 as it is the next step up the food chain from plankton. Krill abundance has declined by 80% since 1970 in areas of the Antarctic where sea ice has shrunk sufficiently. It requires ice for its juvenile growth stage. The follow on consequence has been a 50% decline in penguin abundancedl in the populations affected, as parents cannot feed their chicks. While over fishing and ocean acidification may play a role, this serves as an early example of future impacts.
Paleoclimate & the Carbon Cycle
The inescapable conclusion, once Earth's climate history is consideredD19 b c d and the carbon cycleD20 understood, is that all emission scenariosD21 b currently being considered by governments will result in significant collapse of ice sheets, large sea level rises and the extinction of many species. Indeed, our current "business as usual"D22 emission path would lead to an almost ice free planet with several tens of meters of sea level rise in record time [geological time]. A condition Earth last experienced 38m years agoD23 b c. Even the 2oC 450ppm overshoot scenario would put us well on our way once slow feedbacks are included: link.
Amazingly it is the next 40 years of CO2 emissions which will cause this irreversible outcome in the centuries ahead. Why? See the sections on CO2 and Paleoclimate. To see what governments are up to, refer to the section titled "status of UN negotiations" . Link to 100 videos on equilibrium sea level rise for 100 cities from the next 20 to 40 years CO2 emissions.
The surface carbon reservoirs (ocean, permafrost, soils, forests) easily contain 50 times the sum of all carbon in the atmosphere. 100ppm of atmospheric CO2 is equal to less than 0.5% of the surface reservoirs stores of carbon. They must only release 2% to 7% of their carbon stores to create a net 100ppm increase in CO2 by the year 3100 when compared to that in the 21st Century. It is warming which triggers the release of carbon from the biosphere. Given the pace of change, regrowth will not be able to compensate and these calculations may be overly optimistic. It could be much worse and it will certainly be no better. 1000 years of sustained warming 50 times faster than ever before on Earth is plenty of time for the ice sheets to melt.
Incremental temperature & CO2 change 2020 - 2098 from the UK Committee on Climate Change median A1B representation of the IPCC scenario
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