An Existential Threat Part 1

This page explains that the myriad of changes we are causing Earth to undergo now pose a threat to the existence of civilisation.

Despite repeated warnings about climate change, civilisation has chosen not to be precautionary. Instead civilisation 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 the global warming that has directly resulted from our emissions, but because of the ways Earth’s climate system has responded, and will continue to respond. These responses are known as feedbacks. Feedbacks act on different timescales, so some are fast and others are slow. They are also either negative feedbacks that reduce, 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. For example, no amount of solar panels and wind turbines will arrest the collapse of an ice sheet.

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 

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. Although rates of change in the Anthropocene are necessarily assessed over much shorter periods than those used to calculate long-term baseline rates of change, and therefore present challenges for direct comparison, they are nevertheless striking. 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

As explained in Part 1, 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.

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.5

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 2017.5

The WMO states:6

  • Global mean temperatures in 2018 were 1℃ above pre-industrial levels
  • Average global temperature for 2014 to 2018 was 1.04℃ above that for the pre-industrial period 1850 to 1900
  • 2018 was the fourth warmest on record and the past four years – 2015 to 2018 – were the top four warmest years in the global temperature record
Global mean temperature anomalies, with respect to the 1850 to 1900 baseline, for five global datasets. Reprinted from figure 1 of WMO, 2018, State of the Global Climate in 2018.6

Most of the remaining headings in this section are adapted from the paper ‘Assessing ‘‘Dangerous Climate Change’’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature’ by Dr Hansen et al.7

Sea Level Rise

Plausibly, the climate change impact with most severe consequences for civilisation will be rapid multi-metre 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.

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 (see figure below). Here, the flow of ice leaves land and floats on the ocean surface, forming a tongue of ice known as an ice shelf.

MISI (Marine Ice-Sheet Instability) due to a marine based ice sheet on a bed with retrograde slope. Reprinted from Fig. 1a of Pattyn, 2018, The paradigm shift in Antarctic ice sheet modelling.8

Global warming has caused the flow rate of the glaciers that drain the 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.9

Unfortunately for civilisation, there are characteristics unique to the ice sheets that will cause them to rapidly melt and fracture due to amplifying feedbacks: (i) three of the glaciers that drain the Greenland ice sheet, some of those that drain EAIS and all of those that drain WAIS have submerged grounding lines that lie on bedrock with a reverse, or retrograde slope as shown in the diagram above, and (ii) almost the entirety of WAIS is grounded on bedrock below sea level. The amplifying feedbacks are: (i) as the glaciers retreat, their terminating face becomes taller and exposed to deeper and deeper warm ocean water due to the retrograde slope of the bedrock, ever increasing the rate of melt and fracture, (ii) the ice cliff at the face of glacier above sea level becomes taller, less stable and more likely to collapse, (iii) friction of the ice flowing in the glacier reduces, causing the glacier’s velocity to increase, (iv) the end of the glacier becomes lighter, lifting off the bedrock allowing warm ocean water to melt it from underneath, and (v) the glacier meltwater at the glacier’s outlet acts a lid trapping warm ocean water at the base of the glacier. The 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.

All these amplifying feedbacks will continuously accelerate glacier melt and fracture. These are explained further below, along with evidence from Earth’s climatic past (paleoclimate data) that demonstrates rapid multi-metre sea level rise is expected.

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.10

This is shown in the diagram below as cross-hatched areas in (b).

(a) Map of Antarctica showing the delineation between West and East Antarctica.11(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.12

Then, in 1974, Hans Weertman, a materials scientist at Northwestern University, figured out that these glaciers in West Antarctica were more vulnerable to rapid melting than anyone had previously understood. He coined a term for it: “marine ice-sheet instability.” Weertman pointed out that warm ocean water could penetrate the grounding line, melting the ice from below. If the melting continued at a rate that was faster than the glacier grew – which is currently the case – the glacier would slip off the grounding line and begin retreating backward down the slope, like “a ball rolling downhill,” says Howat, the Ohio State glaciologist. As the glacier becomes grounded in deeper and deeper water, more of the ice is exposed to warming ocean water, which in turn increases the rate of melt. At the same time, parts of the glacier want to float, which places additional stress on the ice, causing it to fracture. As the face of the glacier collapses, or “calves,” more and more ice falls into the sea. The farther the glacier retreats down the slope, the faster the collapse unfolds. Without quite meaning to, Weertman had discovered a mechanism for catastrophic sea-level rise.

Mercer saw that Weertman’s breakthrough had big implications. In a 1978 paper called “West Antarctic Ice Sheet and the CO2Greenhouse Effect: A Threat of Disaster,”13 Mercer focused on the floating ice shelves that buttress the West Antarctica glaciers. Because they are thinner and floating in the ocean, as the water warms they will be the first to go. And when they do, they will not only reduce friction that slows the glaciers’ slide into the sea, they will change the balance of the glaciers, causing them to float off the grounding line. And that, in turn, will advance their retreat down the slope. Mercer argued that this whole system was more unstable than even Weertman had realized. “I contend that a major disaster – a rapid (16-foot) rise in sea level, caused by the deglaciation of West Antarctica – may be imminent,” he wrote

RollingStone, 2017, The Doomsday Glacier.10
RollingStone, 2017, The Doomsday Glacier.10

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 p.10-11.14

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.15 The best estimate of the maximum global surface temperature of the Eemian, relative to preindustrial time, is between +1℃ and +1.5℃ (+1.8℉ to +2.7℉).15 The global surface temperature averaged over 2009–2018 was +0.93℃ relative to preindustrial time, and averaged over 2014–2018 was +1.04℃.16 17 Temperature data of the Holocene (smoothed over centennial time periods) does not exceed +0.5℃.

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

‘Young people’s burden: requirement of negative CO2 emissions, Hansen et al, 2017’.18

Therefore our climate has heated beyond the temperature range of the Holocene, and entered 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.

‘Young people’s burden: requirement of negative CO2 emissions, Hansen et al, 2017′19

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 will soon occur as a result of 1.5℃ to 2℃ warming:

Rignot, 2017. Watching the planet’s ice sheets disappear.20

Below Dr James 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.21

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 2013.9

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'22
Map of Antarctic ice velocity in 2011, place names and drainage basins. Reprinted from Fig. 1. of Rignot et al., 2011, Ice flow of the Antarctic ice sheet.23

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.24
Hansen, 2018, Aerosol Effects on Climate & Human Health: Urgent Research Needs.25
Hansen, 2006, The Case for Action by the State of California to Mitigate Climate Change.26
Hansen, 2006, The Case for Action by the State of California to Mitigate Climate Change.26
  1. UN Secretary-General’s press encounter on climate change()
  2. Professor James Hansen’s warning of multi-metre sea level rise in The Guardian()
  4. p. 54, box 1.1, M. R. Allen, O. P. Dube, W. Solecki, F. Aragón–Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M. Wairiu, K. Zickfeld, 2018, Framing and Context. In: 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, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)]. In Press.()
  7. p.6, Hansen J, Kharecha P, Sato M, Masson-Delmotte V, Ackerman F, et al. (2013) Assessing ‘‘Dangerous Climate Change’’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature. PLoS ONE 8(12): e81648. doi:10.1371/journal.pone.0081648()
  8. Pattyn, F., 2018. The paradigm shift in Antarctic ice sheet modelling. Nature communications9(1), p.2728()
  9. Mouginot, J., E. Rignot, and B. Scheuchl (2014), Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013, Geophys. Res. Lett.41, 1576–1584, doi:10.1002/2013GL059069()()
  11. Abrahamsen, E.P., 2012. Oceanographic conditions beneath Fimbul Ice Shelf, Antarctica (Doctoral dissertation, University of Southampton()
  12. Mercer, J.H., 1978. West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster. Nature271(5643), p.321()
  14. Climate Change in a Nutshell: The Gathering Storm()
  15. Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616,, 2017()()
  16. p6,
  17. The previous two references use slightly different definitions for preindustrial. Hansen uses 1880 – 1920 and the WMO uses 1850 – 1900. The difference in temperature of these two periods is negligible in the context here.()
  18. p 581, Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616,, 2017()
  19. p. 580-581, Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616,, 2017()
  23. Rignot, E., Mouginot, J. and Scheuchl, B., 2011. Ice flow of the Antarctic ice sheet. Science333(6048), pp.1427-1430()