On the heels of its August special report on climate and land, the Intergovernmental Panel on Climate Change (IPCC) in late September published another special report, this one focused on oceans and ice.

This latest report was authored by 104 climate scientists from 36 countries and reflecting findings in 6,981 studies. It contains a wealth of information – its Summary for Policymakers alone runs 45 pages – but some of the key points can be summarized as follows:

  • Sea-level rise is accelerating because the Greenland and Antarctic ice sheets are melting at an increasing rate;
  • The oceans, while still basic, are becoming more acidic, which will have serious consequences for coral reefs and shell-building marine species;
  • The oceans are warming rapidly, leading to more frequent marine heatwaves, which also damage coral reefs;
  • Researchers are beginning to see more of the most intense hurricanes, those in Category 4 and 5, which in combination with higher sea levels and increased precipitation pose increasing risks and damages to coastlines;
  • Melting of permafrost could become a major feedback, adding carbon emissions and global warming in energy scenarios with significant continued fossil fuel burning;
  • Ocean oxygen levels are declining, which in combination with rising temperatures drives marine species away from the equator toward the poles, and, in concert with algal blooms, could lead to reduced seafood catches at fisheries.

Sea-level rise is accelerating

The claim that sea-level rise is not accelerating has long been a popular myth among those challenging climate science. Measurement data indeed are noisy, making it difficult to detect a change in the rate at which the oceans are rising, but some recent studies have been able to detect an acceleration. The IPCC report confirms this conclusion, noting with high confidence that “Areas of concern in earlier IPCC reports, such as the expected acceleration of sea-level rise, are now observed.”

That acceleration is primarily the result of faster melting of the Greenland and Antarctic ice sheets. Over the past decade, the amount of ice loss from the Greenland ice sheet has doubled as compared with the rate during the previous decade, and ice loss from Antarctica has tripled. The IPCC report concluded that if the world meets the Paris Climate Agreement target of no more than a 2 degrees C (3.6 degrees F) warming, sea-level rise could increase by 1-2 feet by 2100 and by a total of 3 feet by 2300; in a worst case high-emissions scenario, sea level would increase by 3 feet by 2100 and by 12 feet by 2300.

Antarctic ice in particular poses substantial uncertainties in terms of future sea-level rise. The West Antarctic ice sheet alone has enough ice to raise sea levels by more than 10 feet, and recent studies have concluded that a large section of that ice sheet has gone into a state of irreversible retreat. Another recent study found that delaying by just five years reaching the threshold of  peak carbon emissions will add about 8 inches to sea-level rise, mostly as a result of Antarctic melting.

Extreme sea level events caused by long-term sea-level rise, in combination with factors like wind-driven storm surges or tides, currently tend to be once-per-century events, but research indicates they will become more commonplace. In many low-lying megacities and small islands, such events are projected to occur annually by 2050. Risks in some areas can be lessened by slowing global warming and using adaptation technologies, but the IPCC report notes that these can cost tens to hundreds of billions of dollars every year.

Ocean acidification – global warming’s evil twin

While human emissions of carbon cause global warming by increasing the atmospheric greenhouse effect, about 25-30% of global carbon emissions are absorbed by the oceans, resulting in a chemical reaction that creates carbonic acid and decreases ocean water pH.

The IPCC report notes that carbon pollution so far has decreased the average ocean pH from 8.2 to 8.1, which, because of its logarithmic scale, means the oceans are 25% more acidic than before the Industrial Revolution. By meeting the Paris Climate Agreement target of 2 degrees C, the world could limit the ocean acidification to 40%; but in a worst case high-emissions scenario, the oceans could become 150% more acidic than they were in 1850.

The IPCC report notes that even if the Paris target could be reached, “Almost all warm-water coral reefs are projected to suffer significant losses of area and local extinctions … These declines in coral reef health will greatly diminish the services they provide to society, such as food provision, coastal protection, and tourism.” High-emissions scenarios also pose a significant risk to other shell-forming species as a result of rising ocean acidification. But particularly vulnerable as a result of a one-two punch of ocean acidification and higher water temperature are coral reefs, among the most diverse ecosystems in the world and a critical source of food and habitat for about 25% of the ocean’s fish. Fishery maximum catch potential has declined 4% so far, and research indicates it will fall by another 6% even if the 2 degree C target is met … and could see a roughly 20% drop in a worst case high-emissions scenario.

Rapid ocean heating and stronger hurricanes

More than 90% of overall global warming goes into the oceans. In the mid-to-late 20th Century, the world’s oceans every second were absorbing heat at a rate equivalent to the energy of two Hiroshima atomic bombs. Since 1993, the rate is comparable to more than five atomic bombs per second. This increase is leading to more marine heatwaves, which are already twice as frequent as they were in the early 1980s and are “negatively impact[ing] marine organisms and ecosystems in all ocean basins over the last two decades.”

If the Paris agreement 2 degrees C target is somehow achieved, the IPCC report anticipates that by the end of this century there will be 20 times more marine heatwaves than in the late-19th Century. But in a worst case high-emissions scenario, they could become 50 times more frequent. These high ocean temperature events stress many marine species and, again, especially coral reefs, leading to coral bleaching.

High ocean temperatures will also fuel stronger hurricanes, which the IPCC report notes have been getting more intense. While it’s difficult to predict how the overall frequency of hurricanes will change in a warmer world, the research reviewed by the IPCC authors projects that the number of Category 4 and 5 hurricanes, the strongest categories, will increase. With increasing coastal populations, higher sea levels, bigger storm surges, and more intense precipitation, the forecast is for more damage from coastal flooding associated with hurricanes.

Melting permafrost carbon feedback and risk mitigation

The IPCC report notes that worldwide permafrost contains about twice as much carbon as is currently in Earth’s atmosphere, and it’s beginning to thaw in some places. Meeting the Paris 2 degrees C target, which many experts consider unlikely, would still result in about a 25% loss of permafrost, and to a loss closer to 70% in a worst-case scenario. In that high-emissions scenario, thawing permafrost could lead to a “release of tens to hundreds of billions of tons of permafrost carbon as CO2 and methane to the atmosphere by 2100 with the potential to exacerbate climate change.” For comparison, humans currently release about 10 billion tons of carbon into the atmosphere every year, so this feedback could be equivalent to adding decades’ worth of human carbon emissions into the atmosphere if fossil fuel use continues unabated.

There is substantial uncertainty about the amount of permafrost thaw and carbon release that will happen in these scenarios, but that’s a key take-home point for the public and policymakers from this report: Even if the world somehow meets the Paris targets, the outlook is for significant adverse impacts in the form of coral reef decline and damages to associated marine ecosystems. Expert research reviewed by the IPCC authors cautions of a reduction in fishery catches that will “elevate the risk to nutritional health in some communities highly dependent on seafood.” Hurricanes and coastal flooding will become worse as the oceans keep warming and rising, and there’s an increasing risk of very large sea-level rise associated with potential ice sheet collapse.

The more the world warms beyond the Paris Climate Agreement threshold of 2 degrees C, the greater these risks become. The IPCC report is best viewed as a risk assessment document: The slower the transition away from fossil fuels, the greater the risks of triggering negative, and in some cases potentially catastrophic, outcomes. The scientists look to policy makers to determine how best to manage and reduce those risks by reducing carbon emissions and curbing their damages through adaptation measures.