Key Cryosphere Messages from the IPCC AR6 Synthesis Report: Climate Change 2023
Link to the Summary for Policy Makers: https://www.ipcc.ch/report/ar6/syr/
The IPCC on March 20, 2023 approved the final installment of the Sixth Assessment Report (AR6), the “Synthesis Report,” bringing together the world’s leading scientists to create a comprehensive assessment of current climate change. The Summary for Policy Makers (SPM) is the result of over five days of negotiations between governments together with these scientists; it is linked above. The full Report on which the SPM is based will be available in coming weeks.
The main cryosphere conclusions from the Synthesis Report can be found below. From a cryosphere perspective, it is important to note that much of the Synthesis Report is based on the SROCC released in 2019 (referencing literature only up to 2018), as well as the AR6 WG1 Physical Science Report released in 2021 (literature up to January 2021). The Synthesis Report does not include cryosphere papers published after those dates, although cryosphere research and observations continue to evolve exceedingly quickly, as evidenced by the weekly Cryosphere Capsules.
The Synthesis Report makes clear that the world is approaching a major temperature threshold: the 1.5°C/2°C guardrails set by the Paris Agreement. Only “deep, rapid and, in most cases, immediate” emissions reductions this decade will keep global temperatures below 1.5°C with no to limited overshoot (B.6.1). Emissions must decline almost 50% by 2030 to uphold the 1.5°C guardrail, and avoid irreversible and cascading damage from the impact of greenhouse gas emissions on the cryosphere (B.6.2).
Most notably, the global and intergenerational implications of cryosphere loss are cited throughout the Report. Communities living near mountain glaciers, permafrost, and Arctic sea ice experience increasing hazards from cryosphere loss almost immediately as the ice melts, but larger frozen regions such as the Greenland and Antarctic ice sheets respond over centuries, with future sea levels exponentially increasing as global temperatures rise.
Since the next IPCC report will be released around 2030, this Report is situated at a critical moment. Earth’s cryosphere responds to each fraction of warming above 1.5°C with devastating consequences, even if temperatures are lowered again. The choices made within the next few years will determine the amount of cryosphere remaining on our planet and therefore, the future of generations to come.
Figure SPM.1 (c): Observed (1900–2020) and projected (2021–2100) changes in global surface temperature (relative to 1850–1900), which are linked to changes in climate conditions and impacts, illustrate how the climate has already changed and will change along the lifespan of three representative generations (born in 1950, 1980 and 2020) […] Credit: Figure from the IPCC AR6 Synthesis Report: Climate Change 2023, accessible here: https://www.ipcc.ch/report/ar6/syr/
Every increment of warming will multiply and intensify future hazards from cryosphere regions. Floods, landslides, and freshwater shortages from glacier retreat and snow loss pose a serious threat to mountain regions across the world (B.2.1). These climate hazards have the potential to cause irrevocable damage to communities and ecosystems, and they will only become more widespread and pronounced if temperatures exceed 1.5°C.
Nearly half of the world’s population lives in regions that are “highly vulnerable” to climate change, with more intense heatwaves and weather extremes increasing risk to human health if temperatures continue to rise (A.2.2). In the last decade, deaths from floods, droughts and storms were 15 times higher in highly vulnerable regions compared to those in in most resilient areas (A.2.2).
If warming goes beyond 1.5°C, limited freshwater resources will pose hard adaptation limits for small islands, high mountain regions, and downstream communities dependent on glacier and snow melt (B.4.2). The growing impact of climate change on cryosphere regions is already approaching irreversibility, such as with glacier retreat limiting freshwater availability in high mountain areas and permafrost thaw destabilizing land in the Arctic (A.2.3). Approximately 3.5 billion people currently experience severe water scarcity for at least part of the year (A.2.4), which will increase as retreating glaciers decrease seasonal freshwater supplies, and as droughts occur more frequently.
With further warming, every region will experience stronger and more prolonged climate and weather extremes. Heatwaves and droughts become more frequent, tropical cyclones and storms more intense, and dry landscapes more vulnerable to wildfires (B.1.4). Heavier precipitation will intensify local flooding (B.2.1). In some coastal regions, extreme sea level events that currently occur only once per century will happen annually by 2100 (B.1.4). Rising temperatures pose an increasing risk to local transportation, water, sanitation and energy systems; thereby jeopardizing human health, with the most harmful impacts concentrated amongst economically and socially marginalized communities (A.2.7).
Climate change has already caused substantial damages and irreversible losses in freshwater, cryosphere, and ocean ecosystems (A.2.3). Over recent decades, heat extremes have also led to mass mortality events that jeopardize local biodiversity, with hundreds of species unable to survive in their current environments (A.2.3). With every increase in warming, oceans become more acidic and deoxygenated, which dissolves the shells of marine species and prevents larger organisms from drawing enough oxygen into their tissues (B.1.3). These increasingly hostile marine environments will cause reduced catches for fisheries and collapse of regional stocks, especially in polar and near-polar ecosystems.
Figure SPM.3 (a): Risks of species losses as indicated by the percentage of assessed species exposed to potentially dangerous temperature conditions, as defined by conditions beyond the estimated historical (1850-2005) maximum mean annual temperature experienced by each species, at GWLs of 1.5°C, 2°C,3°C and 4°C. Credit: Figure from the IPCC AR6 Synthesis Report: Climate Change 2023, accessible here: https://www.ipcc.ch/report/ar6/syr/
Earth’s cryosphere directly reacts to peaks in temperature and CO2 emissions. Even temporarily passing 1.5°C will result in “irreversible” impacts on polar, mountain and coastal communities by increasing glacier melt and sea level rise from ice sheets (B.7.2). The higher temperatures rise, and the longer they remain above 1.5°C, the more societies will be exposed to greater risks; including freshwater scarcity and sudden flooding, heatwaves, and droughts (B.7.2). Surpassing this temperature threshold will increase damage to infrastructure, threaten livelihoods, diminish agricultural productivity, and force low-lying coastal settlements to either adapt or move inland.
Any period of overshoot will increase warming through positive feedbacks, such as increased wildfires, drying of peatlands, permafrost thawing with related emissions, and weakening natural land carbon sinks (B.7.1). Already today, permafrost releases large amounts of greenhouse gases into the atmosphere on the same level as some top-20 national emitters. These feedbacks will make the return below 1.5°C even more challenging. The larger the overshoot, the more net negative CO2 emissions would be needed to return to 1.5°C.
Rising global temperatures increase the likelihood of crossing major cryosphere thresholds, which can trigger abrupt and irreversible changes in Earth’s climate system. These thresholds elevate the risks of species extinction or permanent biodiversity loss, especially in oceans and Arctic regions (B.3.2).
Even at today’s temperatures, the world is committed to unavoidable sea level rise for several centuries to millennia due to deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years (B.3.1). Risks for coastal ecosystems, people and infrastructure will continue to increase beyond 2100 (B.2.2). However, low and very-low emissions pathways would slow the acceleration of further ice loss, and limit our commitment to future sea level rise.
Over the next 2000 years, global mean sea levels will rise by 2–3m even if warming is limited to 1.5°C (B.3.1). If warming levels fall between a sustained 2°C and 3°C, the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly (B.3.2). These two ice sheets alone hold 9-11 meters of sea-level rise. The speed of ice loss, and related sea-level rise will increase with higher temperatures. With very high emissions and taking into account instability processes, 2m may occur already by 2100, and 15m by 2300, continuing to rise further over subsequent millennia (B.3.3). The probability of ice sheet instability processes triggering very large future sea level rise increases with higher global warming levels (B.3.3).
The AR6 Synthesis Report provides a message of both urgency and hope. Although the window for action is closing, the world still has the necessary knowledge, tools, and financial resources to keep global temperatures within 1.5°C. These findings will help inform decisions at COP28, which will be hosted by the United Arab Emirates in Dubai this December. The science speaks for itself: there is no negotiating with the melting point of ice.
Published Mar. 24, 2023 Updated Mar. 24, 2023 6:03 pm