Two of the lead authors of the Intergovernmental Panel on Climate Change's (IPCC) new report explain how global warming is affecting the ocean—and all of us.
The whole planet is observing drastic, unprecedented changes in the Earth’s atmosphere, oceans, and polar regions that are unequivocally a result of human activities. In fact, some of these changes that have already been set in motion—such as continued sea level rise—are irreversible. This is just a sampling from the main conclusions of the latest Intergovernmental Panel on Climate Change (IPCC) Report, released last August, 2021, in which 234 scientists from around the globe summarized the current climate research on how the Earth is changing as temperatures rise and what those changes will mean for the future.
Some of the most concerning conclusions of the report relate to the impacts of global warming on the oceans, which cover almost three-quarters of Earth’s surface, and the cryosphere (frozen water portion of the Earth covering another 10%). According to climate researchers, the global sea level has been rising at an accelerating rate since about 1970, and, over the last century, it has risen more than in any other century in at least 3000 years.
These changes are already affecting all people on Earth, but especially those in the Arctic, low-lying coastal zones and high mountain regions. As a result of impacts to the ocean and cryosphere, communities around the world are already seeing their water resources disappear, experiencing floods and landslides, facing changes in food supply, and witnessing the degradation of ecosystems, infrastructure, recreation, and culture. According to IPCC projections, the intensity of these and other impacts will depend on what actions the global community takes today to reduce emissions.
Jaqueline Sordi, on behalf of The Earth & I, interviewed Dr. Aimee Slangen** and Dr. Helene Hewitt***, two of the lead authors of the report’s chapter on Earth’s oceans, ice, and sea level rise, about the profound changes underway.
Could you describe the IPCC report’s latest assessment on the state of the ocean?
Dr. Helene Hewitt: The ocean has warmed, which has contributed to sea level rise since water expands as it becomes warmer. The ocean has become more acidic and the area of sea ice in the Arctic is reducing. We have also seen an increase in extreme events in the ocean including marine heat waves and coastal flooding. Ocean warming, ocean acidification, and sea level rise are all projected to continue over this century.
Dr. Aimee Slangen: The global heat content of the ocean has increased since at least 1970 and will continue to increase over the 21st century. The Greenland ice sheet, the Antarctic ice sheet, and the glaciers around the world have lost mass over the observed period, and this will continue throughout this century. The sea level will continue to rise through 2100 because all contributors (including ocean warming and the loss of ice mass on land) will continue throughout this century.
How and why are these assessments different from previous reports?
Dr. Hewitt: In this report the assessment shows that the changes we have seen in recent decades are unprecedented. The report has a greater focus on changes in extremes and assesses the regional changes that have been and will be experienced.
Dr. Slangen: There is again more evidence showing the changes, and we have better models to project future changes. As a result, this report is a refinement with more details than the previous report.
What's the report's most important overall message in terms of ocean changes?
Dr. Hewitt: The latest IPCC report confirms that the climate system, including the ocean, has experienced widespread, rapid, intensifying, and unprecedented changes. While deep and rapid reductions in emissions will limit climate change in the near surface, the deep ocean responds slowly, so some changes in the ocean that have already occurred will be irreversible for centuries to millennia.
Dr. Slangen: We know that the ocean is warming and that sea level is rising. The rate of sea level rise in the 20th century was faster than in any century in the past 3000 years, seeing as it has risen over 20 centimeters (almost 8 inches) since 1900. Sea level will continue to rise, but the speed is strongly determined by the amount of greenhouse gas emissions and how fast they can be reduced.
At the current rates, how much sea level rise is now considered unavoidable?
Dr. Slangen: Even if greenhouse gas emissions are reduced completely and quickly, we expect a sea level rise of about 40 centimeters (almost 16 inches) by the end of the century. This is because the processes that cause sea level rise, such as ocean warming or ice sheet melt, will not respond immediately. It will take time before they adjust and find a new equilibrium.
On the other hand, if there are no emission reductions, we expect a sea-level rise of about 80 centimeters (over 31 inches). It could even be more than a meter (over 39 inches) if accelerated ice mass loss on Antarctica takes place. So, what we do right now will impact the sea level rise in the long term and, specifically, the rate of sea-level rise. We have to ask the question: Will it be the current rate of 4 millimeters per year, or will it be much more?
What are you most concerned about occurring, avoiding, or preventing in regards to changes to the ocean or ice as a result of global warming?
Dr. Slangen: We are most concerned about Antarctica, because there is a gigantic amount of freshwater stored in the ice sheet. To give a sense of the scale, if the Antarctic ice sheet were melted completely, that would translate to 58 meters or 190 feet of sea-level rise. While a complete melt is not something that could happen on a human timescale, parts of the ice sheet may experience a ‘runaway’ effect due to ice sheet and ice cliff instabilities if we keep warming the climate. It is however a very difficult place to do research in, so we don’t yet have all the knowledge we would like about [the state of the ice in] Antarctica in order to exactly say how much, how fast, and when we would expect large contributions [towards sea level rise] from Antarctica.
Is there a “maximum” sea level rise scenario if global warming continues unabated?
Dr. Hewitt: In the assessment, we look at a worst-case scenario. This would only occur if the world followed a high emissions pathway, eventually leading to a large loss of ice from the Antarctic ice sheet over the next centuries. We can’t exclude the possibility of sea level rise approaching 2 meters (over 6 feet) by 2100.
Is it still possible to avoid a catastrophic scenario? If so, what could we do?
Dr. Hewitt: The science is clear: while some sea level rise is unavoidable, deep and rapid reductions in emissions will limit the warming of the ocean and the melting of ice sheets and glaciers—all of which contribute to sea level rise. This is our best chance of limiting sea level rise over this and future centuries.
Dr. Slangen: I agree with Dr. Hewitt. What the planet needs is rapid, strong, and sustained greenhouse gas emission reductions.
*Jaqueline Sordi is a Brazilian journalist and biologist, specializing in science and environmental journalism. She has a master’s degree in environmental journalism at UCLA and is currently a Ph.D. candidate in communications at Federal University of Rio Grande do Sul.
**Dr. Aimee Slangen is a researcher at NIOZ Royal Netherlands Institute for Sea Research, Department of Estuarine and Delta Systems, and Utrecht University Netherlands and one of the lead authors of Chapter 9 of the IPCC Sixth Assessment Report on “Ocean, cryosphere, and sea level change.”
***Dr. Helene Hewitt is a coordinating Lead Author of the Ocean, Cryosphere and Sea Level Change chapter and Science Fellow at the Met Office Hadley Centre in the United Kingdom.