Solar Geoengineering Could Limit Sea-Level Rise from Cryosphere

Of the many impacts caused by climate change, sea-level rise threatens to be one of the most devastating due to the thermal expansion of the oceans and the melting of ice and glaciers on land. These impacts, along with numerous others related to rising global temperatures, may in the future motivate a country, a group of countries, or even a very rich individual to pursue solar geoengineering, a controversial proposal for limiting the amount of solar radiation that reaches the Earth’s surface. A recent study in The Cryosphere assessed the efficacy of such a solar geoengineering attempt at limiting global sea-level rise.

Geoengineering, as it relates to climate change, falls into two categories. The first, atmospheric carbon removal, entails physically removing carbon dioxide from the air to reduce greenhouse gas concentrations and limit temperature rise. The second, solar geoengineering, involves injecting sulfur dioxide or another aerosol into the stratosphere to reflect a portion of incoming solar radiation, again limiting temperature rise.

Because of the temperature-reducing effect of solar geoengineering, research suggests that such a proposal would also reduce sea-level rise. However, just how effective solar geoengineering could be in limiting sea-level rise had not yet received sufficient research, according to Peter Irvine, the lead author of the study, who spoke with GlacierHub. Irvine and his team of scientists hoped to “shed some light on the complexities of the sea-level rise response to solar geoengineering, make an initial evaluation of its efficacy, and to bring this issue to the attention of the cryosphere research community,” Irvine told GlacierHub.

Photo of the sun. Solar geoengineering could limit the amount of sunlight that reaches the Earth’s surface (Source: climatemediat/Twitter).

Initially, the researchers conducted a literature review on the small number of studies that explored the cryosphere’s potential response to solar geoengineering. A 2009 study that examined the Greenland Ice Sheet found that under a scenario where atmospheric concentrations were quadrupled, solar geoengineering could slow or even prevent the collapse of ice sheets. Conversely, a 2015 study determined that while solar geoengineering could slow melting, glaciers and ice sheets would not recover to past states. Lastly, a 2017 study focusing on high-mountain Asia, found that solar geoengineering would stop temperature increases; however, 30 percent of glaciated area would still be lost.

Following the literature review, the researchers evaluated the potential effect of solar geoengineering on three aspects of sea-level rise. The first aspect was thermosteric sea-level rise, or more simply, the thermal expansion of ocean waters. Because temperature is the dominant influence on thermosteric sea-level rise (warmer water is less dense than cooler water), the decrease in solar radiation reaching the Earth’s surface due to solar geoengineering would limit sea-level rise.

Secondly, the researchers examined solar geoengineering’s effect on the surface mass balances of glaciers and ice sheets. Surface mass balance is primarily affected by the surface melt rate, which is the result of the availability of energy at the surface of the ice. Thus, a change in solar radiation reaching the Earth’s surface would likely reduce surface melt. The analysis of large volcanic eruptions offers an analogous example to what might happen to surface melt if solar geoengineering were pursued because the dust and ash released during an eruption blocks some incoming solar radiation.

Photo of Pinatubo eruption Solar geoengineering could have a similar effect to the 1991 eruption of Pinatubo (Source: USGS/Twitter).

One study examined by the researchers showed that surface mass balances in Greenland were at their maxima in the year after the El Chicón and Pinatubo eruptions in 1982 and 1991, respectively. Similarly, another study in Greenland found that in the years following the El Chicón and Pinatubo eruptions, surface runoff was the third lowest and lowest, respectively, between the years 1958 and 2006, further reinforcing the expectation that solar geoengineering would limit surface mass balance reductions.

In addition to its effect on temperature, solar geoengineering would also affect the global hydrologic cycle and subsequently sea-level rise. Warming temperatures due to climate change will likely lead to more precipitation worldwide; however, if solar geoengineering is pursued, this increase could be offset. This precipitation change would affect both Greenland and Antarctica, according to Irvine.

In Greenland, surface melt changes, not precipitation accumulation, are the primary influence on surface mass balances; therefore, solar geoengineering would likely have a positive effect by reducing temperatures, with the decrease in precipitation unlikely to lead to mass balance decline. In Antarctica, on the other hand, increased precipitation due to climate change has had a positive effect on surface mass balance, thus a decrease in precipitation due to solar geoengineering would negatively impact mass balances.

The third and final sea-level rise aspect examined by the researchers to evaluate the efficacy of solar geoengineering was ice lost through calving and eventual ice-sheet collapse. Calving, the scientific name for icebergs breaking off a glacier at its terminus, depends on the speed at which ice flows, which itself is driven by climatic changes.

Photo of a calving glacier in Greenland A calving glacier in western Greenland (Source: NASA_ICE/Twitter).

In Antarctica, warming water known as circumpolar deep water (CDW) is the primary driver of calving. CDW is pushed below and then up into glacial cavities by surface winds, where the warm water melts the ice. This melting drives calving and leads to the thinning of glaciers.

While solar geoengineering would likely lower air temperatures, it is unlikely to reduce the temperature of CDW and limit melting and subsequent calving from below. In addition, a 2015 study found that solar geoengineering is unlikely to limit the upwelling of CDW and could even increase upwelling. However, this finding has yet to be replicated, according to Irvine, and it is not clear whether the results are exclusive to the model used.

There is also no guarantee that solar geoengineering would be able to prevent glacial collapse due to marine ice sheet instability. This collapse occurs when a glacier retreats past its grounding line (where ice meets underlying bedrock) and continues to retreat inland until it reaches another stabilizing ridge. The process might already be occurring at West Antarctica’s Thwaites and Pine Island glaciers, which are extremely vulnerable due to the sloping topography upon which they rest.

Nevertheless, retreat and possible collapse might be preventable, says a 2016 study. It showed that returning water to cooler conditions reversed glacial retreat. This finding indicates solar geoengineering may be useful to prevent marine glaciers from destabilizing. While encouraging, it remains likely that certain glaciers, especially those in West Antarctica, will continue to experience significant ice loss regardless of whether solar geoengineering is pursued or greenhouse gas emissions are dramatically reduced.

Based on their study, the researchers lay out four areas in need of future research. First is the need to evaluate the sea-level rise response to solar geoengineering scenarios in conjunction with climate change scenarios so that the efficacy of solar geoengineering and greenhouse gas emissions reductions can be compared. Second is the need to employ regional models of surface mass balance in order to assess the effectiveness of solar geoengineering to limit mass balance losses. Third, the researchers recommend additional evaluation of the effect of solar geoengineering on the CDW upwelling and stability of glaciers and ice-shelves. Finally, the researchers recommend the evaluation of sea-level rise risk, alongside the numerous other risks and challenges associated with solar geoengineering.

Diagram of glacier melting from below Diagram depicting a glacier melting from below due to warming ocean currents (Source: EuroGeosciences/Twitter).

The potential for solar geoengineering to limit sea-level rise from the cryosphere is still up for debate, but as this study shows, it may have the potential to reduce temperature and curb some aspects of sea-level rise, including surface mass balance losses and ocean thermal expansion. However, for other aspects, mainly the melting of glaciers from below by warm waters, it may be unlikely that solar geoengineering can limit sea-level rise contributions. Nonetheless, when it comes to the society-altering impact of sea-level rise, solar geoengineering could be a part of humanity’s response.

Project Aims to Better Understand “Doomsday” Glacier

On April 30, 2018, the largest joint United States-United Kingdom Antarctic project since the 1940s was announced at the British Antarctic Survey in Cambridge. The International Thwaites Glacier Collaboration (ITGC) will focus on the Thwaites glacier of West Antarctica, one of the world’s largest and fastest melting glaciers.

The Thwaites has already contributed to 4 percent of observed global sea-level rise, but the rapid melting of the Thwaites is not scientists’ only concern. The glacier acts as a sort of plug, protecting the rest of the West Antarctic ice sheet from melting. But if the Thwaites were to collapse, most of the West Antarctic ice sheet would be destabilized, likely leading to its impending collapse. This so called “‘Doomsday”’ scenario would cause sea levels to rise 10 feet on average across the Earth.

Photo of the Thwaites Glacier from above
Aerial view of the Thwaites Glacier (Source: Earth Insitute/Twitter).

The ITGC, a $25 million, five-year collaboration between the U.S. National Science Foundation and UK Natural Environment Research Council, will include six scientific field studies with over 100 scientists using a number of different technologies and techniques to analyze the changes to the Thwaites and surrounding ocean.

Getting to the white expanses of the Thwaites is no easy task due to its remote location in Antarctica. In fact, only a handful of people have actually stood on the glacier. For this reason, most scientific research takes place at the more accessible national research stations, according to Jessica O’Reilly, an anthropologist at Indiana University who studies the Antarctic. “Deep field” projects like the ITGC are much rarer because of logistical challenges like coordinating multiple flights to remote areas in erratic weather and the sheer cost of such endeavors, she added.    

This threshold system is further unique to West Antarctica, according to O’Reilly, because it is a marine ice sheet, meaning its grounding line (where ice meets the underlying bed) is under water instead of on land. “Therefore, not only does air surface temperature interact with the ice sheet, but the warming ocean underneath it can also destabilize it,” she said. This distinction makes the Thwaites and rest of the West Antarctic extremely vulnerable to melting, a reality that has inspired a geoengineering proposal to build underwater walls at the grounding line of glaciers like the Thwaites to slow down melting and possibly prevent collapse.

NERC figure depicting the project's different missions
NERC figure depicting the project’s different missions (Source: NERC).

The U.S.-UK research endeavor and its six-field missions will assess just how at risk the Thwaites is to a catastrophic collapse. One of these missions will be led by Penn State Glaciologist Sridhar Anandakrishnan, who spoke to GlacierHub about the project. In Antarctica, Anandakrishnan and his team will be conducting geophysical surveys to characterize the base of the glacier. The surface where ice meets bedrock affects the way the Thwaites flows and is important for projecting how the glacier will retreat. Presently, according to Anandakrishnan, there is a need for improved information on this surface at the Thwaites. “Is it soft or hard? Is it smooth or rough? And so on,” he said.

To obtain this information, Anandakrishnan’s team will use both seismic and radar techniques. From a seismic approach, small explosives will be set off right below the glacier’s surface. The team will then listen for the explosion’s echo from the bottom of the glacier. Based on the time it takes for the echo to return and its strength, the team will be able to better understand the surface where glacial ice meets bedrock.

The radar approach involves a similar process where the team will emit a radar pulse, detect the reflected pulse, and interpret the time and amplitude of the returned pulse for information such as ice thickness, according to Anandakrishnan.

Thwaites Glacier from above
The seemingly endless expanses of the Thwaites Glacier (Source: Hannah Hickey/Twitter).

In addition to this field mission, the five others will examine the Thwaites from different focuses. These include measuring the glacier’s melting at its grounding line; measuring ocean circulation and glacial thinning underneath the floating portion of the glacier using autonomous submersibles; sampling bedrock beneath the glacier to better understand its past retreats and recoveries; analyzing the glacier’s margins to study what controls its width and speed; and examining sediments deposited in the ocean near the glacier to reconstruct past environmental changes and the Thwaites’s response to these changes.

Two other non-field missions will utilize computer models and simulations to assess processes that could cause the rapid retreat and collapse of the glacier and to improve projections on its future behavior and contribution to sea level rise.  

Photo of Boaty McBoatface
The autonomous submersible Boaty McBoatface which will participate in the project (Gizmodo/Twitter).

According to Anandakrishnan, these missions fall in line with one of the main goals of the ITGC: to better understand the Thwaites by improving modeling and projections of the future state of the glacier. Even without a collapse, the Thwaites could contribute up to one meter of sea-level rise over the next century, a change that would have devastating effects on the world’s coastal communities.

If the goals of the project can be accomplished, “we can better estimate what this glacier would do under various future climate scenarios,” Anandakrishnan said. Overall, the U.S.-UK Antarctic project will be a big step forward for humanity’s understanding of a glacier that could have a profound impact on society if it were to collapse.