We see far greater risk of massive irreversible sea level rise (SLR) at 2°C, on a scale of 12–20 meters or more in the long term. The climate record of the Earth over the pat few million years is quite clear:
Sea Level Rise from Ice Sheets
We see far greater risk of massive irreversible sea level rise (SLR) at 2°C, on a scale of 12–20 meters or more in the long term. The climate record of the Earth over the pat few million years is quite clear:
Sea Level Rise from Ice Sheets
Arctic Sea Ice
At 1.5°C global warming, it is unlikely that Arctic sea ice will melt completely in any given summer; and if it does melt completely, that ice-free period will be brief. In contrast, by 2°C the Arctic Ocean is expected to be ice free in summer for several months. This long ice-free period will warm the Arctic Ocean, feeding back to raise regional air temperatures and accelerating Greenland melt and associated sea level rise; increasing permafrost thaw and associated carbon emissions; and also leading to a decrease in snow cover. All of these will in turn make for faster rates and scale of overall global warming, making efforts to address the problem that much harder.
Many parts of the Arctic ecosystem depend on the existence of thicker, multi-year sea ice. These will likely collapse with the complete disappearance of multi-year ice cover by 2.0°C global warming. This impact is amplified by our observation already today of more frequent ocean “heat waves.” Human communities are of course also impacted, especially Arctic indigenous cultures reliant on the reliable presence of sea ice for many thousands of years.
Scientific Editors
Dirk Notz, Max Planck Institute for Meteorology/University of Hamburg, Germany Julienne Stroeve, University College London/University of Manitoba
Scientific Reviewers
Jennifer Francis, Woods Hole Research Center Walt Meier, National Snow and Ice Data Center Ronald Kwok, NASA Jet Propulsion Laboratory
Paul Wassmann, UiT - The Arctic University of Norway
Sea Ice's Role
Arctic sea ice serves as an important regulator of temperature in the northern hemisphere, acting as a “global refrigerator” because this large area of ice-covered ocean – the size of the U.S. and Russia combined – reflects most of the sun’s rays back into space during the entire 6-month polar summer “day”, cooling the planet. It has served this role in the climate system almost continuously for over 200,000 years. The extent of Arctic sea ice that survives the entire summer has however declined by at least 35% since 1972, when reliable satellite measurements became available. In contrast to reflective ice, the darker ocean water absorbs heat, amplifying Arctic and overall global warming. In addition, whereas until quite recently most of the sea ice in the Arctic was very thick multi-year ice, with an average lifetime of several years and wide-spread winter sea-ice thickness of 3 meters or more, today’s ice is mostly formed the previous winter, and thinner than 2 meters. The total volume of Arctic sea ice has therefore declined by nearly two-thirds, far more than its area.
Arctic sea ice break up. Image: Patrick Kelley / US Coast-Guard
Arctic sea ice has declined in close correlation to the rise in CO2 emissions.
ICCI, 2019: Cryosphere1.5º: Where Urgency and Ambition Meet – Why Cryosphere Dynamics Must Mean 1.5° Pathways for 2020 NDCs.
Sea Ice in the Arctic Ocean. Image: Armin Rose / Shutterstock
Arctic Amplification
This extreme recent loss of summer sea ice is one of the causes of “Arctic amplification”, which refers to the greater rise in temperature that has been observed in the high latitudes of the northern hemisphere compared to the rest of the globe. It also carries significant weather, ecological, and economic consequences. These include loss of livelihood for indigenous cultures dependent on stable sea ice for hunting and fishing. It also has been suggested to include influences on the jet stream, which has changed mid-latitude weather systems, as exemplified by the extreme cold or warm periods in recent years that can be related to a more “wobbly” jet stream and less stable polar front zones. Sea ice loss relates to ecosystem loss, especially for marine species that have evolved with an ice “ceiling” much of the year, and those that depend on these in the food chain.
What About Antarctic Sea Ice?
Sea ice around the continent of Antarctica has been comparatively stable over the past few decades of satellite observation, growing in some regions and decreasing in others. However, recent observations document very sharp declines beginning in 2014, equal to or exceeding those in the Arctic but occurring over the space of only a few years, rather than decades. If this trend holds, sea ice-dependent habitats along Antarctica’s coast and in the Southern Ocean would begin to show similar negative impacts as those in the Arctic.
Sea Ice in the Southern Ocean
Below the Arctic Sea Ice. Image: Peter Thor, SMHI
Loss of Multi-year Ice
Summer Arctic sea ice extent has often been seen as a bellwether of climate change, with great attention paid to the September minimum each year. In reality however, sea ice thickness and extent has declined for all months; and the consensus of sea ice scientists is that the nature of Arctic Ocean ice cover already has fundamentally changed and crossed a threshold to a new state. Thinner and younger ice has replaced much of the multi-year ice that circulated several years around the North Pole, before being discharged south along Greenland through the Fram Strait. This “ecosystem of ice” no longer exists. Instead, more than half of Arctic sea ice now consists of first-year ice that largely melts each summer, and with the “older” ice existing on average for only 2–3 years.
Multi-year Arctic Sea Ice Loss 2000–2019.
Thicker multi-year sea ice, which used to dominate the Arctic environment has nearly vanished today.
(Ron Kwok, JPL)
Abrupt Loss Events
Despite this fundamental change already at today’s temperatures, public focus remains on when the first ice-free summer will occur: something which the Arctic likely has not experienced since at least the Holocene spike in warming after the last Ice Age 8,000 years ago; and possibly not since the warm Eemian period 125,000 years ago, which today’s temperatures almost equal (and when sea-level was 4–6 meters (13–20 feet) higher than today).
Like many climate change impacts, Arctic sea ice loss over the past three decades has not occurred gradually, but sometimes in abrupt loss events when combinations of wind, as well as warmer temperatures pressed extent lower.
It is likely that near-complete loss of summer sea ice (defined as dipping below 15% of the Arctic Ocean, or 1 million square kilometers) will occur with such a sudden event, then perhaps not occur again for several years; until total-loss summers become more frequent and (if temperatures continue to rise) by the end of this century, become the norm for some portion of each summer.
Sea Ice in the Southern Ocean
Melt pools atop sea ice in the Arctic Ocean in August 2009.
Image: Pablo Clemente-Colon / National Ice Center
Ice Free Summers
The occurrence of the first ice-free summer is therefore very unpredictable, but most scientists are fairly confident one could occur before 2040 given current temperature pathways. In making their projections, the SROCC and SR1.5 relied on the more numerous global studies, since fewer focus on the Arctic region or take into account recent observations of sea ice decline. Global models however, especially in the past but even those currently used, underestimate the actual sea ice loss that has been observed since about 1990.
In contrast, studies that incorporate more regional models, together with observations (even if these are fewer in number) track current sea ice much better, though still underestimating current losses slightly. These predict ice-free summers starting somewhere around 1.7 degrees, with longer ice-free periods each summer by 2°C.
Both regional and global models however agree that ice-free summers will become the norm in the Arctic should temperatures rise much above 2°C, with ice-free summers far more rare at 1.5°C. This in turn will minimize the impacts noted above in terms of lower ice sheet and glacier loss; lower levels of permafrost thaw and carbon release; and less disruption to Arctic marine ecosystems.
Ice-free conditions by month and temperature. Latest research projects ice-free conditions ranging from briefer periods in September around 1.7°, to several months by 2°C.
Based on Notz and Stroeve (2018).
Feedbacks and Risks
The global impact of complete Arctic summer sea ice loss is likely to further accelerate global warming and its impacts.
Given the greater absorption of solar heat from open water, it will lead to higher fall and winter temperatures in the Arctic, as well as potentially affecting the weather patterns of the middle latitudes of the northern hemisphere with unusual weather patterns that remain difficult to predict, but likely involve incidences of persistent weather (drought or rainy periods) such as the extreme drought seen in Scandinavia in the summer of 2018, which led to crop and livestock losses as well as extensive wildfires.
Additional permafrost loss and especially melting on the margins of Greenland and from Arctic land glaciers would lead to greater release of greenhouse gases from permafrost, and higher sea-level rise. The scale of such impacts is highly unpredictable, as the Arctic has never been ice-free in modern human existence.
Arctic Sea Ice. Image: Wikimedia Commons
Freight ship in Arctic waters.Image: a_medvedkov / Adobe Stock
Global Impacts
Finally, while some Arctic governments declare that an ice-free summer Arctic will bring economic opportunity, it is important to balance such statements with the global impacts elsewhere. In other words, the 2°C above pre-industrial that creates the summer ice-free conditions that will allow exploitation of Arctic resources, will also lead to the risks and societal disruptions noted elsewhere in this report, such as 4–10 (or more) meters committed long-term sea-level rise, and potential fisheries and ecosystem damage from acidification. Such adverse impacts almost certainly will eclipse temporary economic benefits brought by an ice-free summer Arctic, even among Arctic states and those moving to invest there.
Learn more:
IPCC Special Report: Global Warming of 1.5°C
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
ICCI The Cryosphere1.5° Report
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