Center for Climate and Life Fellow Explains IPCC Sea Level Warning
By Bill Chaisson
The Intergovernmental Panel on Climate Change (IPCC) released a “Special Report on Oceans and the Cryosphere in a Changing Climate” on Wednesday. It states that the 1.5°C increase in temperature will raise sea level enough to displace 280 million people, four times the current number of refugees in the world. Pierre Dutrieux, a 2019 Climate and Life Fellow and an assistant research professor at Lamont-Doherty Earth Observatory, is an oceanographer whose research is focused on the relationship between the world’s oceans and the polar ice sheets. Dutrieux’s work in part explains how, why, and when this dire projection is going to happen.
A June 14 press release from the IPCC announced the circulation of the draft report for final review. The report summarizes the effects of climate change on oceans and ice, but it “will also discuss how nature and society can respond to the risks this poses and achieve climate-resilient development.” Member governments were invited to submit comments on the draft ahead of the September 20-23 approval session in Monaco.
A draft of the report was leaked to the French news agency AFP in August. On August 29, New York magazine reported that the United Nations research group, known in the past for its conservative assessments of changing climate, is now adopting a more urgent tone.
Matt Stieb’s story warns that the “oceans were poised to unleash misery” if warming isn’t halted before a 1.5°C rise in global temperature. The relationship between rising temperature, ice, and oceans is straightforward: warming oceans melt polar ice and meltwater raises sea level, which leads to coastal flooding. Flooding damage is exacerbated by stronger storms produced by a warmer, more moisture-laden atmosphere. By 2100, the draft states, annual flood damages are expected to increase by 10 to 1,000 times current levels.
The October 2018 IPCC report took a comprehensive look at the consequences of a 1.5°C rise in global temperature. In response to this report, New York magazine’s David Wallace-Wells wrote, “At two degrees, the melting of ice sheets will pass a tipping point of collapse, flooding dozens of the world’s major cities this century. At that amount of warming, it is estimated, global GDP, per capita, will be cut by 13 percent.”
“Since the beginning of the satellite record in the early 1970s,” Dutrieux said, “scientists have identified one sector, the Amundsen Sea sector, that holds a very significant amount of ice and where the balance between snow precipitation upstream and flux of ice into the ocean downstream does not hold. In fact, this sector, part of the West Antarctic Ice Sheet, has been losing ice at a consistently accelerating rate, with present rates of loss from this sector alone being equivalent to 4.5 centimeters per century of global sea-level rise. As an example, terminus velocity of Pine Island and Thwaites Glaciers almost doubled since the late 1980s, now surpassing 4 kilometers per year or 10 meters per day.”
Ice shelves, the floating extension of the Antarctic polar ice sheets are an important factor and a focus of Dutrieux’s research. They have been thinning for some time and are in danger of breaking up.
Dutrieux takes the wider perspective before diving into his own research. “In terms of
net Antarctic and Greenland contributions to global sea level,” the
oceanographer said, “numerical model predictions over the next decades to
century vary widely depending on ice physics parameters and external (ocean/atmosphere) factors. They oscillate from 10 centimeters, approximately 4 inches, to 2 meters, approximately 6 feet, by 2100, with a median around 20-40 centimeters.”
Dutrieux’s research area is westward along the West Antarctic coast from the Antarctic Peninsula, directly south of the Pacific Ocean. “The current imbalance in the Amundsen Sea sector is driven by the presence next to the ice of ocean water that is much warmer—around 3 degrees Celsius warmer—than in other sectors. This water efficiently melts and thins the ice shelves, releasing friction and accelerating ice flow, all factors that have been measured using multiple techniques to date. Geometric factors, namely the fact that the ice sheet is resting on a base that is deepening inland, means that melt at the ice base and stretching arising from ice acceleration exposes ever thicker columns of ice, making the system susceptible to unstable, self-sustained retreat and acceleration. Ultimately, however, available oceanic heat very much seems to dictate the pace of the retreat.”
According to Dutrieux, oceanographers have identified the atmospheric changes that are driving oceanic ones. Wind belts are shifting, driving currents carrying heat poleward. Historically, the circumferential currents that surround Antarctica have been a barrier to poleward transfer of heat, but that is changing.
“The latest development in our understanding,” Dutrieux said, “arises from special numerical simulations of the atmosphere/ocean system, where the large atmospheric variability, mostly arising from the tropical Pacific, is captured. In these simulations, the chaotic variability is overlaid by a trend in the wind. This trend is small, from the beginning of the 20th century over the course of a century the wind has shifted—on decadal average—from mostly preventing the flow of warm water close to the ice to being neutral today. This long-term shift, driven by anthropogenic global warming, seems to have led to the present situation.”
The domino effect from atmospheric warming to ocean-current shifting to ice-sheet melting to sea-level rise is only one described in the new IPCC report. Other sections address the cascading consequences of rising global temperature on El Niño, Arctic permafrost, and ocean productivity.
In a somewhat unnerving conclusion, Dutrieux noted that our ability to predict the future behavior of ice sheets requires an understanding of “a combination of natural variability and chaotic behavior of the ice/ocean/atmosphere, and global warming.”