By Tianyi Sun
The Intergovernmental Panel on Climate Change has published yet another alarming report about the dangers we face from the climate crisis.
The Special Report on the Ocean and Cryosphere in a Changing Climate synthesizes the latest science on how the oceans and frozen parts of the world have changed, and will continue to change, because of global warming.
More than 100 scientists from 36 countries summarized findings from almost 7,000 peer reviewed research studies. The authors addressed over 30,000 comments from expert reviewers and governments in 80 countries.
A major focus of the report is sea level rise, a climate change impact that is especially serious to those who live in coastal regions – which is more than a quarter of the world’s population. Recent advances in science, such as higher quality data, improved physical understanding, and agreements across modeling studies have improved understanding of the threat of sea level rise.
Here are four of the report’s most important takeaways on sea level rise:
- Sea level rise is accelerating
The annual rate of today’s sea level rise is unprecedented over the past century.
Seas rose two-and-a-half times faster from 2006 to 2015 than between 1901 and 1990. Projections show that the rate of sea level rise will become even faster by the end of the century, even if we drastically curb our climate pollution.
The current acceleration of sea level rise is mainly attributed to ice melting in Greenland and Antarctica. The Antarctic ice sheet in particular lost triple the amount of ice in mass over the period from 2007 to 2016 than during the previous decade. If we take no steps to intervene, this could signal the onset of irreversible change that could cause several meters of sea level rise within a few centuries. It is also possible that the West Antarctic ice sheet could collapse due to its severe instability.
- Rare extreme flooding events may begin to occur annually
Rare hundred-year extreme flooding events are expected to become annual occurrences in many places by midcentury. That prediction holds true for all the scenarios considered in the report – from business-as-usual climate pollution levels to drastic cuts in emissions.
Some coastal areas have already seen as increase in the frequency of higher coastal sea levels during events like high tides and storm surges. Many more low-lying cities and islands are at risk. Category 4 and 5 hurricanes that often yield the biggest storm surges and coastal floods are projected to become more intense, last longer, and cause heavier rainfall.
Coastal migration and urbanization make flooding events even more damaging. The number of people who live at low elevations (considered around 30 feet above sea level or less) is projected to increase, from 85 million today to 239 million by 2100. Without serious adaptation measures, flood risks may increase by 2 to 3 orders of magnitude.
- We have enough information to implement adaptation plans
Global mean sea level has already risen by around half a foot since 1900 (with nearly four inches of that since 1970). Scientists are fairly confident that global mean sea level will rise by nearly another foot by midcentury regardless of our emissions trajectory, providing a reliable basis for adaptation planning.
However, local sea level rise depends on the location, and can be 30 percent lower or higher than the global mean due to factors such as uneven ocean heat uptake, ocean circulation and wind patterns, gravitational effect of land ice loss, and human activities. For example, sea level has already risen by around one foot since 1900 off the coast of New York City.
Adaptation measures are needed to address committed sea level rise. These include reduced population and urban infrastructure exposure in low-lying regions as well as coastal protections such as seawalls and dikes, which are cost efficient for urban and densely populated regions.
- We can achieve long-term stability of sea level with actions to curb emissions
While sea level will continue to rise for centuries beyond 2100, due to continuing deep ocean heat uptake and ice loss from the Greenland and Antarctic ice sheets, it can be stabilized eventually with aggressive actions to rapidly curb climate pollution.
If we are able to reduce emissions rapidly, substantial ice loss from the major ice sheets may be avoided. For example, it is estimated that the Greenland ice sheet will reach a point of no return and have perpetual ice loss between 1 degree Celsius and 4 degrees Celsius above preindustrial temperatures. If temperature is stabilized under the 2 degree Celsius international temperature target, we may avoid the Greenland ice sheet entering a state of decline with the potential loss of most or all of the ice sheet.
Emissions cuts can also prevent the huge economic losses that are expected to occur with anticipated sea level rise from a business-as-usual emissions scenario. The latest projections show up to four feet of sea level rise by the end of the century, which could cost up to 9 percent of the global Gross Domestic Product in the absence of adaptation measures.
The report’s concerns beyond sea level rise
Sea level rise is one of the many concerns that the report has raised. Loss of mountain glaciers, permafrost and snow are threatening the communities that depend on mountain ice for water resources, and may release tons of carbon into the atmosphere that had been trapped in frozen soils or permafrost.
Marine heatwaves were also assessed in the report for the first time. They were found to now be more frequent and intense, causing widespread bleaching of tropical coral reefs and forcing fish species to migrate hundreds and thousands of miles just to survive. The ocean is becoming warmer, more acidic and less oxygenated so quickly that many marine ecosystems will not be able to keep up.
Accelerating sea level rise and other extreme water events show the urgent need for us to deploy large-scale adaptation measures. But the fate of our oceans and ice regions ultimately depends on our actions to mitigate climate pollution this decade. If we can curb emissions at scale and stabilize temperatures soon, we may be able to avoid catastrophic damages to marine ecosystems, huge economic losses to coastal communities, and increasing loss of life along the coast, and keep the biggest ice sheets intact for generations to come.