Impacts of Overshoot
“In pathways with no or limited overshoot of 1.5°C, CO2 emissions must decline 40-60% [median 50%] by 2030.” — IPCC, 2018: Summary for Policymakers
The IPCC Special Report on 1.5°C of Warming (2018) makes it clear that 40-60% emissions reductions (median 50%) are needed by 2030 to reach carbon neutrality by 2050 and remain close to 1.5°C of warming. Unfortunately, the UNFCCC Synthesis Report, released on February 26, 2021 reveals that almost no countries are taking their 2050 neutrality goals seriously by taking the needed steps to get there through 50% reductions by 2030. Instead, current national measures will only reduce global emissions by about 1% by 2030.
The result will be “overshoot” – exceeding not only the 1.5°C limit, but also the 2°C Paris Agreement. The decision by today’s governments to pursue overshoot carbon policies will have permanent and essentially irreversible impacts in the world’s cryosphere – snow and ice regions – as well as polar and near-polar oceans, with cold waters that acidify more rapidly. Cryosphere not only helps cool and regulate the climate system, but also holds much of the world’s fresh water resources. Once this ice melts however, or polar oceans acidify beyond the ability to support life, a return to lower temperatures and CO2 levels will not reverse many of these changes.
New research shows that our polar ice sheets, especially Antarctica will continue to lose ice and raise sea levels for many thousands of years, even after a return to lower temperatures. Glaciers, once lost, can re-grow; but only over many hundreds of years. Permafrost, once thawed continues to emit carbon for 1-2 centuries – meaning that overshoot will commit five-six generations of humans to some sort of carbon dioxide removal even once human carbon neutrality is reached, just to compensate for permafrost carbon emissions. Even Arctic sea ice will take at least several decades or a century to return, because the Arctic Ocean will remain warmer for centuries even after air temperatures decline.
The more rapid acidification of polar oceans and seas, forming the world’s richest fisheries is perhaps the most irreversible: taking 50-70,000 years to return to today’s levels. The shell-building animals at the base of the ocean food chain will go extinct; and with them, fish and animals on which they depend. Already today, at 1.2°C of warming and about 415ppm CO2, extensive damage to shellbuilding organisms has been documented by researchers in both polar regions.
The consequences from overshoot are dangerously underestimated by the policy world. While there are other near-irreversible impacts from exceeding 1.5° or 2°C, especially loss of coral reefs and a potential loss of the Amazon ecosystem, those from cryosphere stand out in their semi-permanent nature and global impacts. Massive unstoppable sea-level rise of 5-20 meters in just a few centuries. Loss of water resources reliant on glaciers and snowpack. Decline and extinction of cold-water species, including commercially important stocks of cod, salmon, lobster and shellfish. With overshoot, we also face greater reliance on untried carbon dioxide removal (CDR) methods; and potential trade-offs with food, biodiversity and other Sustainable Development Goals.
Why 1.5°C is better than 2°C
Sea Level Rise from Ice Sheets
“Marine ice sheet instability in Antarctica and/or irreversible loss of the Greenland ice sheet could result in multi-metre rise in sea level over hundreds to thousands of years. These instabilities could be triggered at around 1.5°C to 2°C of global warming.”
Glacier and Snow Loss
“Mid-latitude glaciers and snow in the Alps, southern Andes/Patagonia, Iceland, Scandinavia, New Zealand and North American Rockies can survive at 1.5°, but these glaciers will disappear almost entirely at 2°C, and snow cover decrease.”
Polar Ocean Acidification
“The level of ocean acidification due to increasing CO2 concentrations associated with global warming of 1.5°C is projected to amplify the adverse effects of warming, and even further at 2°C, impacting the growth, development, calcification, survival, and thus abundance of a broad range of species, for example, from algae to fish.”
IPCC, 2018: Summary for Policymakers Learn moreDisappearance of Summer Arctic Sea Ice
“The probability of a sea ice-free Arctic Ocean during summer is substantially lower at global warming of 1.5°C when compared to 2°C.”
Carbon Emissions from Permafrost
“High-latitude tundra and boreal forests are particularly at risk…Limiting global warming to 1.5°C rather than 2°C is projected to prevent the thawing over centuries of 1.5-2.5 million km2 of permafrost.”
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