2.2.2.1 Ice sheets

The most widespread physical impact of changes in ice sheets is rising sea levels. Sea level rise contributions from ice sheets in the present day represent around 1.45 mm of sea level rise per year (IMBIE Team, 2018). Under future climate change, the proportion of contribution coming from ice sheets will increase. However, typical modelling approaches struggle to accurately represent ice sheet dynamics, leading many studies to underestimate projections of sea level rise (Siegert et al., 2020).

Reconstructing past ice-sheet change and sea level rise can provide an analogue for sea level rise under tipping points. Around 125,000 years ago, when it was around 1°C warmer than today, it is estimated that global sea levels were around 6-9m higher than present (Dutton et al., 2015). Periods of very rapid sea level change have previously occurred, potentially up to 4m per century. These ‘melt-water pulses’ are thought to have occurred during periods of ice sheet collapse (International Cryosphere Climate Initiative, 2023). 

Today, modelling estimates that total loss of the Greenland Ice Sheet (GrIS) could lead to a total of 7.5m additional sea level rise (Morlighem et al., 2017). While the Antarctic Ice Sheet is much larger, and has a greater sea level potential, the East Antarctic Ice Sheet is more stable and less susceptible to tipping elements. However, much of the West Antarctic Ice (WAIS) Sheet is grounded below sea level, making it more susceptible to processes associated with large-scale ice loss. The estimated possible contributions from the WAIS are around 5m of sea level rise (Pan et al., 2021). While complete loss of all ice sheets is highly unlikely, significant losses from the WAIS and GrIS could be triggered at relatively low levels of warming (1.5-3°C). Ice sheets respond relatively slowly to change, meaning substantial mass loss would likely occur over thousands of years, if triggered (Armstrong McKay et al., 2022). 

Using ‘structured expert judgement’ (SEJ) in the IPCC 6th Assessment Report, Fox-Kemper et al., (2021) explore a ‘high-end storyline’ of ice sheet loss under a high-emissions scenario to complement standard modelling approaches. The storyline explores substantial contributions from Greenland and the Antarctic to sea level rise (including MICI and MISI, although it does not require both). This is a qualitative approach, and describes how projections of high sea level rise should not be ruled out. Fox-Kemper’s projections show up to 2.3m rise by 2100 (95^th percentile, SSP5-8.5) and,  while they are low-confidence, this storylines approach shows they cannot be discounted, based on process-based understanding of possible tipping points within the cryosphere. Passing ice-sheet tipping points accelerates the rate of sea level rise and dramatically increases the magnitude of impacts (Armstrong McKay et al., 2022). Acceleration of melting ice sheets cannot be reversed or stopped on the timescales of millennia. Exploring such high-end scenarios is important for adaptation approaches where there are low risk tolerances, such as the construction of nuclear power sites at coastal locations. 

Rising sea levels have the most immediate and significant impact upon coastal communities, with numerous detrimental consequences (Figure 2.2.2). Around 10 per cent of the global population live within 10m of sea level worldwide, with most of the world’s megacities located within coastal areas (Neumann et al., 2015). This low-elevation coastal zone (LECZ) also generates around 14 per cent of the world GDP (Kummu et al., 2016). The inundation of coastal regions would lead to flooding of cities, damage to costly infrastructure, and even the complete loss of low-lying nations such as the Marshall Islands. Inundation of coastal regions would also impact natural systems, in turn resulting in negative impacts for fishing, agriculture, tourism and other ecosystem-based services. Such changes might force migration and would result in severe economic damages.

Under 1.5°C global warming by 2100, (Rockström et al., 2023)project that as many as 170 million people could be exposed to sea level rise. Population exposure increases significantly to 500 million over the long term (multi-century sea level rise), based on no adaptation and static population dynamics (Figure 2.2.3). One study estimates that, in a case of Antarctic instability (where sea level rise reaches over 2m by the end of the century, in line with the ‘high-end storyline’ presented in the IPCC 6th Assessment Report (Fox-Kemper et al., 2021) a total of 480 million people (based on current population dynamics) would be vulnerable to an annual coastal flood event by 2100 (Kulp and Strauss, 2019). 

Bangladesh, India, Indonesia and the Philippines are projected to experience the highest increases in populations living within the flood risk zone, increasing vulnerability in the nations. Such changes threaten populations and may result in displacement, with migration putting pressures on inland areas and cascading impacts across systems (Hauer et al., 2017).

The loss of atoll island nations is one of the most well-known examples of impacts from ongoing sea level rise (Oppenheimer et al., 2019) and begs many legal questions, including whether the loss of a nation results in ‘statelessness’, as well as having implications for access to resources such as maritime fishing zones, upon which communities depend (Hauer et al., 2020; Vidas et al., 2015). 

The effectiveness of adaptation options under rising sea level, and the ability to adapt, remains a knowledge gap (Magnan et al., 2022). Limits to adaptation action will be reached in many different types of coastal environments within this century, even before tipping points are considered. It is suggested that 1m of global sea level rise would present challenges to adaptation approaches (O’Neill et al., 2017), leading to significant questions about our ability to adapt to high-end scenarios of sea level rise triggered by passing tipping points (see further discussion in Chapter 3.3).