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Climate Crisis Sea Level Rise

An Existential Threat
Part 1

We face a direct existential threat… Extreme heatwaves, wildfires, storms and floods are leaving a trail of death and devastation… We are careering towards the edge of the abyss.

UN, 2018, UN Secretary-General’s press encounter on climate change.1 

Despite repeated warnings about climate change, civilisation has chosen not to be precautionary. Instead it chooses to be reckless and hold climate science in contempt by endorsing politicians who fail to act, and to indulge in oil, coal and gas. And so our climate radically changes along with Earth’s land and ocean, on vast scales at extreme rates. Civilisation chooses indifference, leaving a nightmarish burden for young people.

Greta Thunberg’s speech at the World Economic Forum in Geneva, Jan 2019. The video looks dim but this was the only full version available.

Climate change is now an existential threat to civilisation, not only because of changes due to the warming from our GHG emissions, but because of feedbacks. Feedbacks act on different timescales, so some are fast and others are slow, and they are either negative feedbacks that reduce an impact, or positive feedbacks that amplify. Our emissions have already set in play fast amplifying feedbacks in Earth’s climate system, and are now triggering additional further amplifying feedbacks associated with Earth’s ice-sheets that may take the climate system past tipping points. These are points of no return, with consequences beyond our control.

The current rate of global warming could raise sea levels by “several meters” over the coming century, rendering most of the world’s coastal cities uninhabitable and helping unleash devastating storms, according to a paper published by James Hansen, the former Nasa scientist who is considered the father of modern climate change awareness.

The Guardian, 2016, Climate guru James Hansen warns of much worse than expected sea level rise.2

The abundant empirical evidence of the unprecedented rate and global scale of impact of human influence on the Earth System has led many scientists to call for an acknowledgement that the Earth has entered a new geological epoch: the Anthropocene. The rise in global CO2 concentration since 2000 is about 20 ppm per decade, which is up to 10 times faster than any sustained rise in CO2 during the past 800,000 years. AR5 (the IPCC fifth assessment report)3 found that the last geological epoch with similar atmospheric CO2 concentration was the Pliocene, 3.3 to 3.0 Ma (million years ago). Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, compared to a long-term decline over the past 7,000 years at a baseline rate of 0.01°C per century. These global-level rates of human-driven change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past; even abrupt geophysical events do not approach current rates of human-driven change.

IPCC (2018), Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways.4

Humanity’s CO2 emissions: (i) are currently trapping two thirds of the energy causing global warming, (ii) are the only rapidly increasing contributor, (iii) almost solely determine our long term warming commitment, and (iv) continue to grow with no peak in sight. Half of all CO2 ever emitted has been emitted recently and almost all by the world’s energy system.5

The World Meteorological Organisation (WMO) points out that COannual rates of change since preindustrial times have been, and still are, about 10 times faster than the fastest natural change, and that levels of atmospheric CO2 concentration are now similar to climates million of years in the past when seas were much higher.6

The increase in CO2 from 2015 to 2016 was larger than the increase observed from 2014 to 2015 and the average over the last decade.
Direct measurements of atmospheric CO2 over the past 800,000 years provide proof that over the past eight swings between ice ages (glacials) and warm periods similar to today (interglacials) atmospheric CO2 varied between 180 and 280 parts per million (ppm), demonstrating that today’s CO2 concentration of 400ppm exceeds the natural variability seen over hundreds of thousands of years. Over the past decade, new high-resolution ice core records have been used to investigate how fast atmospheric CO2 changed in the past. After the last ice age, some 23,000 years ago, COconcentrations and temperature began to rise. During the period recorded in the West Antarctica ice core, fastest CO2 increases (16,000, 15,000 and 12,000 years ago) ranged between 10 and 15ppm over 100–200 years. In comparison, CO2 has increased by 120ppm in the last 150 years due to combustion of fossil fuel. 
Periods of the past with a CO2 concentration similar to the current one can provide estimates for the associated “equilibrium” climate. In the mid-Pliocene, 3–5 million years ago, the last time that the Earth’s atmosphere contained 400ppm of CO2, global mean surface temperature was 2–3℃ warmer than today, the Greenland and West Antarctic ice sheets melted and even some of the East Antarctic ice was lost, leading to sea levels that were 10–20m higher than they are today. During the mid-Miocene (15–17 million years ago), atmospheric CO2 reached 400–650ppm and global mean surface temperature was 3–4℃ warmer than today.

WMO, 2017, State of the Global Climate in 2017.6

Sea Level Rise

Plausibly the most severe climate change impact would be rapid and large sea level rise from the collapse of Earth’s ice sheets – the Greenland ice sheet, the West Antarctic ice sheet (WAIS) and the East Antarctic ice sheet (EAIS). This impact alone is a threat to the existence of global civilisation not only because of greater coastal flooding and storm surges, but also the financial and economic ramifications from the flooding of wealthy cities and ports, and the interruptions to international trade.

Shown below is land area in 2050 projected to be below annual flood level, due to 26cm sea level rise relative to year 2000.

World’s largest cities.12

Ice sheets, as opposed to ice shelfs, sit on bedrock that may be above or below sea level, and are dynamic – continuously gaining and losing mass. They gain mass from snow and rain that falls on them, which accumulates and compacts to form ice. This ice gradually drains to the ocean via rivers of ice known as glaciers which may terminate at the ocean, making them marine terminated glaciers. The point where an ice-sheet leaves the bedrock it rests upon, and becomes afloat is known as the grounding line. Here, the flow of ice leaves land and floats on the ocean surface, forming a tongue of ice known as an ice shelf.

Global warming has caused the flow rate of the glaciers that drain Greenland and WAIS to increase, putting the ice sheets out of mass balance (more ice going out, than forming from falling snow and rain), and their grounding lines have rapidly retreated.13

There are characteristics unique to the Antarctic ice sheet (AIS) that may cause rapid and large loss of ice, due to the amplifying feedback processes known as Marine Ice-Sheet Instability (MISI) and Marine Ice-Cliff Instability (MICI).

(a) Map of Antarctica showing the delineation between West and East Antarctica.14(b) West Antarctica showing ice shelves, ice grounded below sea level and ice covering land above sea level. Reprinted from Fig. 2. of Mercer, 1978, West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster.15

In contrast to the land-based East Antarctic ice sheet, the marine ice sheet in West Antarctica can exist only so long as its grounded position is buttressed by fringing ice shelves; in particular by the Ross and Filchner-Ronne ice shelves. Thus any environmental change that diminished or destroyed these ice shelves would also diminish or destroy the ice grounded below sea level; as the ice shelf fronts receded southward, their grounding lines would also recede and the grounded ice sheet would shrink and thin. Eventually after all the ice shelves had disappeared, ice cover would be confined to areas above sea level, and glaciers would terminate in ice cliffs between high and low water levels.

Mercer, J. (1978), West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster.16
Marine ice sheet and ice cliff instabilities

As far as anyone knows, no human had ever set foot on the West Antarctica glaciers until the International Geophysical Year, in 1957, a Cold War collaboration of the U.S. and the Soviet Union and other nations to expand the boundaries of scientific exploration. A team of scientists had trekked across the glaciers of West Antarctica, including Thwaites; by drilling ice cores and taking other measurements, they discovered that the ground beneath the ice was on a reverse slope and had been depressed further by the weight of the glaciers over millions of years. “Think of it as a giant soup bowl filled with ice,” says Sridhar Anandakrishnan, an expert in polar glaciology at Penn State University.

RollingStone, 2017, The Doomsday Glacier.17

Marine ice sheets rest on bedrock that is submerged below sea level (often by 2 to 3 km). The most well-researched marine ice sheet is the West Antarctic ice sheet (WAIS) where approximately 75% of the ice sheet’s area currently rests on bedrock below sea level. The East Antarctic ice sheet (EAIS), however, also has appreciable areas grounded below sea level (~35%), in particular around the Totten and Cook Glaciers.

Information on the ice and bed topography of WAIS suggests that it has about 3.3 m of equivalent global sea level grounded on areas with downward sloping bedrock.

These ice sheets are fringed by floating ice shelves, which are fed by flow from grounded ice across a grounding line (GL). The GL is free to migrate both seawards and landwards as a consequence of the local balance between the weight of ice and displaced ocean water. Depending on a number of factors, which include ice-shelf extent and geometry, ice outflow to the ocean generally (but not always) increases with ice thickness at the GL. Accordingly, when the ice sheet rests on a bed that deepens towards the ice-sheet interior, the ice outflow to the ocean will generally increase as the GL retreats. It is this feature that gives rise to the Marine Ice-Sheet Instability (MISI), which states that a GL cannot remain stable on a landward-deepening slope.

IPCC (2013), Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.18

Much of the WAIS sits on bedrock hundreds to thousands of metres below sea level. Today, extensive floating ice shelves in the Ross and Weddell Seas, and smaller ice shelves and ice tongues in the Amundsen and Bellingshausen seas provide buttressing that impedes the seaward flow of ice and stabilizes marine grounding zones. Despite their thickness (typically about 1 km near the grounding line to a few hundred metres at the calving front), a warming ocean has the potential to quickly erode ice shelves from below, at rates exceeding 10 m/yr/°C. Ice-shelf thinning and reduced backstress enhance seaward ice flow, grounding-zone thinning, and retreat. Because the flux of ice across the grounding line increases strongly as a function of its thickness, initial retreat onto a reverse-sloping bed (where the bed deepens and the ice thickens upstream) can trigger a runaway Marine Ice Sheet Instability (MISI).

DeConto, R., Pollard, D. (2016), Contribution of Antarctica to past and future sea-level rise.19
MISI and MICI processes. DeConto, R., Pollard, D. (2016), Contribution of Antarctica to past and future sea-level rise.19

The following quote refers to the image above.

Another physical mechanism previously under appreciated at the ice-sheet scale involves the mechanical collapse of ice cliffs in places where marine-terminating ice margins approach 1 km in thickness, with >90 m of vertical exposure above sea level. Today, most Antarctic outlet glaciers with deep beds approaching a water depth of 1 km are protected by buttressing ice shelves, with gently sloping surfaces at the grounding line (Fig. 2d). However, given enough atmospheric warming above or ocean warming below (Fig. 2e), ice-shelf retreat can outpace its dynamically accelerated seaward flow as buttressing is lost and retreating grounding lines thicken. In places where marine-terminating grounding lines are thicker than 800 m or so, this would produce >90 m subaerial cliff faces that would collapse (Fig. 2f ) simply because longitudinal stresses at the cliff face would exceed the yield strength (about 1 MPa) of the ice. More heavily crevassed and damaged ice would reduce the maximum supported cliff heights. If a thick, marine-terminating grounding line began to undergo such mechanical failure, its retreat would continue unabated until temperatures cooled enough to reform a buttressing ice shelf, or the ice margin retreated onto bed elevations too shallow to support the tall, unstable cliffs. If protective ice shelves were suddenly lost in the vast areas around the Antarctic margin where reverse-sloping bedrock is more than 1,000 m deep, exposed grounding-line ice cliffs would quickly succumb to structural failure, as is happening in the few places where such conditions exist today (the Helheim and Jakobsavn glaciers on Greenland and the Crane Glacier on the Antarctic Peninsula), hinting that a Marine Ice Cliff Instability (MICI) in addition to MISI could be an important contributor to past and future ice-sheet retreat.

DeConto, R., Pollard, D. (2016), Contribution of Antarctica to past and future sea-level rise.19

Major ice loss from the AIS stems from an increased discharge of grounded ice into the ocean, with ice shelves (the floating extensions of the grounded ice sheet) playing a crucial role. The buttressing provided by ice shelves can affect inland ice hundreds of kilometres away, and hence controls grounding-line retreat and associated ice flow acceleration. Ice shelves are directly affected by oceanic and atmospheric conditions, and any change in these conditions may alter their buttressing effect and impact the glaciers feeding them. For instance, increased sub-shelf melting causes ice shelves to thin, increasing their sensitivity to mechanical weakening and fracturing. This causes changes in ice shelf rheology and reduces buttressing of the inland ice, leading to increased ice discharge. Warming of the atmosphere promotes rainfall and surface melt on the ice shelves and causes hydrofracturing as water present at the ice-sheet surface propagates into crevasses or by tensile stresses induced by lake drainage.

Reduction of buttressing of ice shelves via the processes described above may eventually lead to the so-called marine ice sheet instability. For the WAIS, where the bedrock lies below sea level and slopes down towards the interior of the ice sheet, MISI may lead to a (partial) collapse of this marine ice sheet. This process, first hypothesized in the 1970s, was recently theoretically confirmed and demonstrated in numerical models. It arises from thinning and eventually flotation of the ice near the grounding line, which moves the latter into deeper water where the ice is thicker. Thicker ice results in increased ice flux, which further thins (and eventually floats) the ice, resulting in further retreat into deeper water (and thicker ice) and so on

Pattyn, F., Ritz, C., Hanna, E. et al. (2018), The Greenland and Antarctic ice sheets under 1.5 °C global warming.20
Pattyn, F., Ritz, C., Hanna, E. et al. (2018), The Greenland and Antarctic ice sheets under 1.5 °C global warming.20
Meltwater stratification

Glacier meltwater at a glacier’s outlet acts a lid trapping warm ocean water at the base of the glacier. This meltwater freshens the ocean water, making it less dense, reducing ocean heat circulation to the surface, preventing this heat venting to the atmosphere and space.21 22

As the ocean warms it begins to melt ice shelves, the tongues of ice that extend from the ice sheets into the ocean. These ice shelves buttress the land-based ice sheets. Thus as the ice shelves melt, the ice sheets expel ice into the ocean at a faster rate. This process is self-amplifying, because the melting icebergs freshen the ocean surface waters. Fresh water reduces the density of the ocean surface layer, thus reducing the ocean’s vertical overturning, which in turn reduces the release of ocean heat into the atmosphere and space. Instead, this ocean heat stays at depth, where it accelerates the rate of ice shelf melting.

The danger is that the ice discharge will pass a tipping point such that the amplifying feedbacks cause rapid acceleration of the melting process. It is even possible that, for a vulnerable portion of the Antarctic ice sheet sitting on bedrock well below sea level, the melting process could become self-sustaining. In that case, we say that a ‘point of no return’ is reached and it is too late to prevent discharge of massive amounts of ice, sufficient to raise sea level several meters. My colleagues and I estimate that this process could lead to multi-meter sea level rise in a period as short as 50 – 150 years, if greenhouse gases continue to increase rapidly.

If global warming continues unabated, portions of the ice sheets will become unstable, ice sheet disintegration will accelerate, and sea level will rise continuously. A majority of large U.S. and global cities are coastal. Continued high fossil fuel emissions will lead to eventual sea level rise that makes these cities dysfunctional, with consequences that are incalculable.

Hansen, 2018, Climate Change in a Nutshell: The Gathering Storm.23

Below Dr Hansen explains the amplifying feedbacks between the ocean and ice-sheets, in which he states: “These feedbacks raise questions about how soon we will pass points of no return and in which we lock in consequences that cannot be reversed on any time scale that people care about, consequences include sea level rise of several meters”:

Hansen, 2016. Ice Melt, Sea Level Rise and Superstorms: The Threat of Irreparable Harm.24
current and future trends

Earth’s climate has alternated between ice ages and warm periods. Civilisation developed during the most recent warm period, known as the Holocene, that lasted 11,700 years. The prior warm period is known as the Eemian, between 130,000 to 115,000 years ago.25 The best estimate of the maximum temperature of the Eemian, relative to preindustrial time, is between +0.5℃ and +1.0℃.25 The global surface temperature averaged over 2014–2018 wrt 1850-1900 was +1.04℃ according to the WMO,26 and in 2019 wrt 1880-1920 was +1.2˚C according to NASA GISS (Goddard Institute for Space Studies) global temperature analysis (GISTEMP).27 Temperature data of the Holocene (smoothed over centennial time periods) does not exceed +0.5℃.28

We conclude that the modern trend line of global temperature crossed the early Holocene (smoothed) temperature maximum (+0.5℃) in about 1985.

Hansen et al. (2017), Young people’s burden: requirement of negative CO2 emissions.29

Therefore we have warmed our climate beyond the temperature range of the Holocene, and are about to warm it beyond that of the Eemian.

The Holocene, over 11,700 years in duration, had relatively stable climate, prior to the remarkable warming in the past half century. The Eemian, which lasted from about 130,000 to 115,000 years ago, was moderately warmer than the Holocene and experienced sea level rise to heights 6–9 m (20–30 ft) greater than today.

Hansen et al. (2017), Young people’s burden: requirement of negative CO2 emissions.28

Professor Eric Rignot explains below the mapping of glaciers on Greenland and Antarctica, how these glaciers are changing and the consequences of rapid multi-meter sea level rise that may soon occur as a result of 1.5℃ to 2℃ warming:

Rignot, 2017. Watching the planet’s ice sheets disappear.30

Pine Island, Thwaites, Haynes, Smith, Pope, and Kohler Glaciers are among the fastest-flowing glaciers in continental Antarctica. Combined together, they drain one third of the West Antarctic Ice Sheet into the Amundsen Sea Embayment (ASE). Their mass flux into the southern Pacific Ocean is comparable to that of the entire Greenland Ice Sheet into the Arctic Ocean. Since first revealed with satellite radar interferometry in the 1990s, this sector has been significantly out of balance due to glacier speedup. Concurrent with the acceleration in ice flow, the glaciers have been thinning and their grounding-line positions—where ice goes afloat—have been retreating at a rate of 1 km/yr, one of the fastest retreat rates in the world. Together, these glaciers and their catchment basins combined contain 1.2 m global sea level rise.

Observations of the ASE of West Antarctica since 1973 indicate speedup of all the glaciers and a steady increase in ice discharge into the ocean from the collective ensemble of these large, major glaciers. During retreat across their respective ice plains, Pine Island and Smith Glaciers underwent a rapid increase in ice discharge. However, since 2009, the ice discharge of Pine Island has remained steady. Thwaites Glacier, which had experienced a steady flow since 1992, started to speed up in 2006 and has increased its ice discharge considerably since.

Mouginot et al. (2014), Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013.31

Map of Antarctic ice velocity in 2011, place names and drainage basins. Rignot et al. (2011), Ice flow of the Antarctic ice sheet.32

The Antarctic lost 40 billion tons of melting ice to the ocean each year from 1979 to 1989. That figure rose to 252 billion tons lost per year beginning in 2009, according to a study published Monday in the Proceedings of the National Academy of Sciences. That means the region is losing six times as much ice as it was four decades ago, an unprecedented pace in the era of modern measurements. (It takes about 360 billion tons of ice to produce one millimeter of global sea-level rise.)

“I don’t want to be alarmist,” said Eric Rignot, an Earth-systems scientist for the University of California at Irvine and NASA who led the work. But he said the weaknesses that researchers have detected in East Antarctica — home to the largest ice sheet on the planet — deserve deeper study.

“The places undergoing changes in Antarctica are not limited to just a couple places,” Rignot said. “They seem to be more extensive than what we thought. That, to me, seems to be reason for concern.”

“The traditional view from many decades ago is that nothing much is happening in East Antarctica,” Rignot said, adding, “It’s a little bit like wishful thinking.”

The Washington Post, 2019, ‘Ice loss from Antarctica has sextupled since the 1970s, new research finds'33

East Antarctica has the potential to reshape coastlines around the world through sea level rise, but scientists have long considered it more stable than its neighbor, West Antarctica. Now, new detailed NASA maps of ice velocity and elevation show that a group of glaciers spanning one-eighth of East Antarctica’s coast have begun to lose ice over the past decade, hinting at widespread changes in the ocean.

“Totten is the biggest glacier in East Antarctica, so it attracts most of the research focus,” said Catherine Walker, a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who presented her findings at a press conference on Monday at the American Geophysical Union meeting in Washington. “But once you start asking what else is happening in this region, it turns out that other nearby glaciers are responding in a similar way to Totten.”

Walker found that four glaciers west of Totten, in an area called Vincennes Bay, have lowered their surface height by about 9 feet (almost 3 meters) since 2008—before that year, there had been no measured change in elevation for these glaciers. Farther east, a collection of glaciers along the Wilkes Land coast have approximately doubled their rate of lowering since around 2009, and their surface is now going down by about 0.8 feet (0.24 meters) every year.

“The change doesn’t seem random; it looks systematic,” said Alex Gardner, a glaciologist with NASA’s Jet Propulsion Laboratory in Pasadena, California.

“Those two groups of glaciers drain the two largest subglacial basins in East Antarctica, and both basins are grounded below sea level,” Walker said. “If warm water can get far enough back, it can progressively reach deeper and deeper ice. This would likely speed up glacier melt and acceleration, but we don’t know yet how fast that would happen. Still, that’s why people are looking at these glaciers, because if you start to see them picking up speed, that suggests that things are destabilizing.”

There is a lot of uncertainty about how a warming ocean might affect these glaciers, due to how little explored that remote area of East Antarctica is.

For example, if it turned out that the terrain beneath the glaciers sloped upward inland of the grounding line—the point where glaciers reach the ocean and begin floating over sea water, forming an ice shelf—and featured ridges that provided friction, this configuration would slow down the flow and loss of ice. This type of landscape would also limit the access of warm circumpolar deep ocean waters to the ice front.

A much worse scenario for ice loss would be if the bedrock under the glaciers sloped downward inland of the grounding line. In that case, the ice base would get deeper and deeper as the glacier retreated and, as ice calved off, the height of the ice face exposed to the ocean would increase. That would allow for more melt at the front of the glacier and also make the ice cliff more unstable, increasing the rate of iceberg release. This kind of terrain would make it easier for warm circumpolar deep water to reach the ice front, sustaining high melt rates near the grounding line.

NASA, 2018, More glaciers in East Antarctica are waking up.34
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  20. Pattyn, F., Ritz, C., Hanna, E. et al. The Greenland and Antarctic ice sheets under 1.5 °C global warming. Nature Clim Change 8, 1053–1061 (2018). https://doi.org/10.1038/s41558-018-0305-8. https://iasc.info/images/media/s41558-018-0305-8.pdf()()
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