What does COP26 mean for global glacier and ice sheet change?

This article is based on an invited lecture given by Bethan Davies at the Cryosphere Pavilion at COP26. You can watch it on YouTube here.

Earth’s glaciers and ice caps (here, we differentiate from the two ice sheets, the Antarctic and Greenland ice sheets) are shrinking at an alarming rate. Each year, global glaciers are losing more mass than is being replenished in each accumulation season. 28 trillion tonnes of ice were lost from 1994 to 2017[1], and rates of ice loss have risen by 57% since the 1990s[1].

How much ice is there in the world?

Globally, Earth has 153.8 x103 gigatonnes (Gt) of ice. A gigatonne is a billion tonnes; a gigatonne of water forms a towering block of water 1 km high, 1 km wide, and 1 km deep. That is the height of Mount Snowdon in Wales! The Earth’s glaciers have enough ice to raise global sea levels by 32 cm.

Globally shrinking glaciers

At present, global glacier ice volume is shrinking at a rate of 267±16 Gt per year, and is accelerating by 48±16 Gt per decade[2]. This is like putting 267 Mt Snowdons into the ocean, each year! Ice lost from melting contributes significantly to sea level rise, and is estimated to contribute 0.75 mm per year.

Global ice loss. From Slater et al., 2021

Globally shrinking ice sheets

Between 1992 and 2017, the Antarctic ice sheet lost approximately 2603±563 Gt of ice [1]. Ice sheet loss in Antarctica has tripled since 2012, when compared with ice loss from the previous two decades [3]. The resulting ice shelf thinning and collapse in the Antarctic Peninsula has caused the speed up of glaciers further up stream[4], which causes reduced ice shelf buttressing.

Ice sheets in the Arctic are experiencing similar losses to Antarctica. Between 1992 and 2018, the Greenland ice sheet lost a staggering 3902±342 Gt of ice. Roughly half of the ice lost in this period was a result of increased meltwater runoff, which had been enhanced by atmospheric warming following several unusually warm summers[5,6]. The remaining ice loss is likely to be a result of increased glacier discharge[1].

Between 1992 and 2017, ice loss from the Greenland and Antarctic ice sheets combined has caused global sea level to rise by 17.8±1.8 mm[7].

What was decided at COP26?

COP26 deliberations and decided policies will likely lead to an estimated warming of 2.6-2.7oC by 2100[8]. However, if countries met all conditional and unconditional NDCs by 2030, warming could be reduced to 2.4oC. Current NDCs sit just below SSP 2-4.5, and well below the worst case scenarios (SSP 3-7.0 and SSP 5-8.5).

Comparison of global emissions under IPCC, with total global emissions according
to nationally determined contributions. Winning et al., 2019.

After the Paris agreement agreed to limit global warming below 1.5oC, it was predicted that almost half of global ice volume will be lost by the year 2100[8]. Since COP26, the target for warming has increased to a maximum of 2.7oC, therefore, it is likely that over half of global ice volume will be lost by 2100.

What does this mean in terms of sea level rise?

This degree of ice melt, alongside thermal expansion, is predicted to cause sea level rise of ~56 cm by 2100[1]. At present, sea level is rising at a rate of 3.1 mm per year[1]. Over 680 million people live in low-lying coastal areas, which are especially susceptible to coastal flooding as a direct result of global sea level rise.

In the UK, coastal flooding is often caused by storm surges. Rising sea levels from ice melt will increase the height of sea level extremes, which will result in more frequent coastal flood events in the UK. Projected increases in coastal flooding has been strongly linked to sea level rise, and less so to changes frequency of extreme weather events and storm surges[9]. Roughly £150 billion of assets are at risk of coastal flooding in the UK[10]. In London alone, the number of residents at risk of tidal flooding has increased from 1.25 to 1.3 million over the past 5 years, which equates to an increase from £200 billion to £275 billion in property value[11]. As ice loss continues to contribute to sea level rise, the risks of coastal flooding in the UK will only continue to rise.

What does this mean for water resources?

Mountain Glaciers store and provide vital water to downstream communities. Glacial retreat is threatening water security in these communities and is already causing severe water shortages in many countries that rely on mountain glaciers for a dependable water supply. Communities surrounding the Himalayas are particularly vulnerable to water shortages directly related to glacial retreat. With current COP26 NDCs, only 40% of glaciers in Central Asia will remain by 2100, threatening the lives of hundreds of millions of people.

Was COP26 a success?

Despite its praise in the media, the outcomes of COP26 are not sufficient to adequately reduce the impact of climate change on global ice loss. Even if all commitments to reduce global GHG emissions in line with SSP 2-4.5 are kept, global temperature change will exceed the threshold required to retain our life-giving glaciers.

However, COP26 was successful in one respect; there was a universal commitment to reduce global greenhouse gas emissions, and a global acknowledgment of the climate crisis. This is an important milestone.

Further reading

References

1 Slater, T., Lawrence, I.R., Otosaka, I.N., Shepherd, A., Gourmelen, N., Jakob, L., Tepes, P., Gilbert, L. and Nienow, P., 2021. Earth’s ice imbalance. The Cryosphere15(1), pp.233-246.

2 Hugonnet, R., McNabb, R., Berthier, E., Menounos, B., Nuth, C., Girod, L., Farinotti, D., Huss, M., Dussaillant, I., Brun, F. and Kääb, A., 2021. Accelerated global glacier mass loss in the early twenty-first century. Nature592(7856), pp.726-731.

3 Mouginot, J., Rignot, E., and Scheuchl, B.: Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013, Geophys. Res. Lett., 41, 1576–1584, https://doi.org/10.1002/2013GL059069, 2014. 

4 Hogg, A. E., Shepherd, A., Cornford, S. L., Briggs, K. H., Gourmelen, N., Graham, J. A., Joughin, I., Mouginot, J., Nagler, T., Payne, A. J., Rignot, E., and Wuite, J.: Increased ice flow in Western Palmer Land linked to ocean melting, Geophys. Res. Lett., 44, 4159–4167, https://doi.org/10.1002/2016GL072110, 2017. 

5 Enderlin, E. M., Howat, I. M., Jeong, S., Noh, M.-J., van Angelen, J. H., and van den Broeke, M. R.: An improved mass budget for the Greenland ice sheet, Geophys. Res. Lett., 41, 866–872, https://doi.org/10.1002/2013GL059010, 2014. 

6 Bevis, M., Harig, C., Khan, S. A., Brown, A., Simons, F. J., Willis, M., Fettweis, X., Broeke, M. R. van den, Madsen, F. B., Kendrick, E., Caccamise, D. J., Dam, T. van, Knudsen, P., and Nylen, T.: Accelerating changes in ice mass within Greenland, and the ice sheet’s sensitivity to atmospheric forcing, P. Natl. Acad. Sci., 116, 1934–1939, https://doi.org/10.1073/pnas.1806562116, 2019. 

7  The IMBIE Team: Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature, 579, 233–239, https://doi.org/10.1038/s41586-019-1855-2, 2020 (data available at: http://imbie.org/data-downloads/). 

8 Edwards, T.L., Nowicki, S., Marzeion, B., Hock, R., Goelzer, H., Seroussi, H., Jourdain, N.C., Slater, D.A., Turner, F.E., Smith, C.J. and McKenna, C.M., 2021. Projected land ice contributions to twenty-first-century sea level rise. Nature593(7857), pp.74-82.

9  Edwards, T., 2017. Current and future impacts of sea level rise on the UK. Foresight–future of the sea evidence review. Government office for science, UK.

10 Howard, T., Lowe, J. and Horsburgh, K., 2010. Interpreting century-scale changes in southern North Sea storm surge climate derived from coupled model simulations. Journal of Climate23(23), pp.6234-6247.

11 Pace, S., 2021. Urban infrastructure inundation risk from permanent sea-level rise scenarios in London (UK), Bangkok (Thailand) and Mumbai (India): A comparative analysis. Master Thesis in Geographical Information Science.

What is the ice volume of Thwaites Glacier?

Thwaites Glacier in West Antarctica is currently the focus of a major scientific campaign. Why is Thwaites Glacier of so much interest, however? How much ice is there, and how much would sea levels rise if it all melted?

Thwaites Glacier is roughly the size of UK (176 x103 km2). The glacier terminus is nearly 120 km wide, and the bed of the glacier reaches to >1000 m below sea level. Pine Island Glacier and Thwaites Glacier together account for 3% of grounded ice-sheet area, but they receive 7% of Antarctica’s snowfall1.

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Choosing the future of Antarctica

In a new article in the journal Nature, Stephen Rintoul and colleagues present two very different visions of Antarctica’s future, from the perspective of an observer looking back from 2070. In one vision, humanity continues to exploit Earth’s natural resources (such as fossils fuels) and does little to protect the environment, and in the other, there is a global movement towards conservation. The article shows how Antarctica will change over the next 50 years, should either of these two situations occur.

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Mass balance of the Antarctic ice sheet from 1992 to 2017

A new paper with a whole host of authors has just been published in Nature (IMBIE Team, 2018). It provides a new estimate of mass balance of the entire Antarctic Ice Sheet over the last 25 years, the longest and most thorough estimate of this to date.

This article argues that the Antarctic Peninsula, the smallest ice sheet in Antarctica, has lost an average of 20 Gigatonnes (Gt) of ice per year over the 25 year study. This increased during the study and especially since the year 2000.  The West Antarctic Ice Sheet lost 53±29 Gt yr-1 from 1992-1997, but this accelerated to 159±26 Gt yr-1 from 2012-2017. The East Antarctic Ice Sheet is more stable, with small gains (with large errors) over the study period. Continue reading

The Larsen C Ice Shelf growing rift

Antarctic Peninsula ice shelves | Ice shelf collapse on the Antarctic Peninsula | Rifting on Larsen C | Impact of calving the large iceberg | Sea level rise following ice-shelf collapse | References | Comments |

Antarctic Peninsula ice shelves

The Antarctic Peninsula is fringed by floating ice shelves. They are floating extensions of the glaciers on land, and receive mass by snowfall and marine freeze-on. They lose mass by melting at their base and by calving icebergs. Larsen C Ice Shelf, the largest ice shelf on the Antarctic Peninsula, is currently being closely watched. Following a series of high-profile ice-shelf collapse events on the Antarctic Peninsula over the last few decades, all eyes are watching Larsen C and wondering when, and if, it will collapse.

A growing rift on Larsen C Ice Shelf

Those concerns are growing more acute as a large rift on Larsen C Ice Shelf is growing rapidly, threatening to soon calve a huge iceberg, equivalent to losing 10% of the area of the ice shelf. This could destabilise the ice shelf, making it more susceptible to a total collapse.

Larsen C rift animation uses #Sentinel1 InSAR to illustrate recent jumps in rift progression. From Prof. Adrian Luckman.

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The Global Last Glacial Maximum

Around 27,000 years ago, ice sheets reached their maximum across the world, after a period of global cooling caused by variations in the Earth’s orbit around the sun. There was a massive ice sheet in North America (the Laurentide Ice Sheet)[1, 2], a large Eurasian Ice Sheet covering Britain, Ireland and Scandinavia as well as northern Europe[3], an ice sheet in Antarctica[4], the Himalaya and Patagonia[5, 6]. Land near the ice sheets that escaped glaciation was cold, with tundra vegetation. Northern Europe was frequented by ice-age animals such as mammoth, reindeer and arctic hare. There was a landbridge betweeb Britain and Europe, and animals could walk freely across it. Numerous human artefacts from this time are scattered across the landscape.

Ice sheets at the Last Glacial Maximum worldwide, around 27,000 to 21,000 years ago. From data in Ehlers et al., 2011.

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Will we enter another ice age?

There are a number of web and news articles around surrounding the question of whether or not we will enter another ice age. Many of these questions arise from the idea that a collapse or significant melting of the Greenland Ice Sheet will produce enough fresh water to shut down the global thermohaline circulation, dropping us into a new ice age in the next 10,000 years.

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Is the West Antarctic Ice Sheet collapsing?

Marine ice sheet instability

The West Antarctic Ice Sheet (WAIS) is the world’s most vulnerable ice sheet. This is because it is grounded below sea level, and marine ice sheets such as these are susceptible to rapid melting at their base. Fast-flowing ice streams draining the WAIS (Pine Island Glacier and Thwaites Glacier in particular) into the Amundsen Sea have a grounding line on a reverse bed slope, becoming deeper inland. Recession of the grounding line means that the ice stream is grounded in deeper water, with a greater ice thickness. Stable grounding lines cannot be established on these reverse bed slopes1, because ice thickness is a key factor in controlling ice flux across the grounding line. Thicker ice in deeper water drives increased calving, increased ice discharge, and further grounding-line recession in a positive feedback loop2, 3.  This process is called Marine Ice Sheet Instability4.

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Sea level rise over the next 2000 years

A new paper by Levermann et al. in PNAS uses the record of past rates of sea level rise from palaeo archives and numerical computer models to understand how much sea level rise we can expect per degree of warming in the future. These data suggest that we can expect a global sea level rise of 2.3 m per 1°C of warming within the next 2000 years: well within societal timeframes. A 2°C of warming would result in a global sea level rise of 4.8 m within 2000 years. This would inundate many coastal cities in Europe alone, and cause untold economic and societal damage.

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