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.

Future Sea-Level Rise and the Paris Agreement

The signing of the Paris Agreement in December 2015 signaled the world’s renewed focus on limiting global temperature rise to below 2 degrees Celsius, with a goal to lessen the adverse impacts of climate change. However, one of these impacts, sea-level rise, is already occurring and will continue long after emissions and temperatures stabilize. In other words, policies and decisions made now will set sea-level rise on a course to higher or lower levels. To better assess these effects, a recent paper published in Nature Communications examined the implications of the Paris Agreement’s goals on global sea levels up until the year 2300.

Photo of the Cop 21 logo
Logo for the UNFCC’s COP 21 where the Paris Agreement was signed (Roberto Della Seta/Twitter).

If we are to achieve the 2 degree Celsius goal of the Paris agreement, global greenhouse gas (GHG) emissions must peak and subsequently decline in the near future. This decline would coincide with the removal of emissions already in the atmosphere, through natural sinks, carbon capture and storage technologies, or both; ultimately leading to global net-zero GHG emissions sometime between 2050 and 2100. Most previous studies examining sea-level rise under different climate change scenarios only looked forward to 2100, and though a few extended farther into the future, none had yet to consider the implications of meeting the aims of the Paris Agreement.

The goal of this study was to fill this gap and assess the legacy of the Paris Agreement on sea level rise beyond the 21st century, author Alexander Nauels told GlacierHub. Another important motivation for the study was to investigate the effect of delayed climate mitigation action on future sea-level rise, he added.

Sea-level rise due to climate change is driven by several elements, including the thermal expansion of the oceans as they warm, the retreat of mountain glaciers, and the mass loss of ice sheets in Antarctica and Greenland. These elements react on different timescales to increasing temperatures ranging from hundreds (shallow water thermal expansion and glaciers) to thousands (major ice sheets) of years. Thus, emissions today will lock in future sea-level rise well into the future.

Photo of the Drang-Drung Glacier
Drang-Drung Glacier in Northern India. Mountain glaciers like it are one of the elements responsible for sea-level rise analyzed in this study (Source:sandeepachetan/Creative Commons).

To explore the relationship between the provisions of the Paris Agreement and sea-level rise, the study utilized a carbon cycle and climate model composite, together with a sea-level model. These models were driven by fossil fuel and industry emission scenarios that meet the Paris Agreement’s goal of limiting temperature rise to 2° C. These scenarios resemble the IPCC’s Representative Concentration Pathways (RCP) 2.6 scenario where emissions peak by 2020 and then decline thereafter. The emissions in these scenarios were limited to fossil fuels and industry because as Nauels states they are, “…by far the most important emission share when it comes to global decarbonistion.”

The scenarios chosen met either the net-zero GHG emissions goal of the Paris Agreement, seeing a gradual temperature decline over time due to GHG removal by carbon sinks, or a net-zero CO2 goal that would only limit temperature rise to 2° C. Why the two different scenario groups? Joeri Rogelj, another author of the study, told GlacierHub that they wanted to be able to distinguish between scenarios that only stabilize warming, partially meeting the Paris Agreement’s targets (net-zero CO2) and ones that fully comply with the Paris Agreement’s targets (net-zero GHG). This distinction enabled the authors to analyze the effect that delayed or insufficient mitigation action would have on sea-level rise.

Aerial Photo of Antartica
Aerial view of Antartica. The Antartic ice sheet is one of the elements responsible for sea-level rise analyzed in this study (Source: Pylyp Koszorús/Twitter).

There was a stark difference between the more stringent requirements of the Paris Agreement, slowly decreasing temperature through carbon sinks and action that would only stop temperature rise at 2° C. Under net-zero GHG scenarios, median sea-level rise was 73-123 cm, while under net-zero CO2 scenarios the median rise was a much higher level at 116-164 cm. Sea-level rise also continues through 2300 in all scenarios, emphasizing the need for immediate mitigation action, although, the rate begins to slow soon after emissions peak at 0.06-0.7 cm and 0.33-0.49 cm per year for the net-zero GHG and net-zero CO2 scenarios, respectively. Ominously, under net-zero CO2 scenarios, results showed that the possibility of sea-level rise of up to 5 m by 2300 was within the 90% confidence interval.

Figure of the sea-level rise response for partially meeting the Paris Agreement
Sea level rise response from the four contributors analyzed when the Paris Agreement’s goals are partially met (net-zero CO2) (Source: Mengel et al. 2018).

What happens if humanity only stabilizes temperatures instead of meeting the goals of the Paris Agreement?  When the authors compared the net-zero GHG and net-zero CO2 scenario groups, they found that median sea-level rise was 40 cm higher for the net-zero CO2 scenario. Another relevant factor for 2300 sea-level rise is the timing of the emissions peak. If the peak in global emissions is delayed by five years, an additional 20 cm of rise was found to occur in 2300 and when based on the 95th percentile the rise is an additional 1 m.

There is a good chance that global temperatures will increase by more than 1.5° C at least temporarily, with a 2017 study putting the chances of staying below a higher threshold of 2° C at 5%. The authors assessed this possible ‘temperature overshoot’ and found for every 10-year period where temperature rise is greater than 1.5° C a 4 cm increase in median sea-levels is expected. Overall, if global temperatures top 1.5° C no scenario showed median sea-level rise less than 1.2 m by 2300.

Figure of the sea-level rise response to fully meeting the Paris Agreement
Sea level rise response from the four contributors analyzed when the Paris Agreement’s goals are met in full (net-zero GHG) (Source: Mengel et al. 2018).

Lastly, the authors examined the connections between sea-level rise and the Paris Agreement’s Nationally Determined Contributions (NDC), the emission reduction goals of individual countries. If implemented in full, the NDCs would lead to a median sea-level rise between 1.45 and 1.64 meters under the net-zero CO2 scenarios and a median sea-level between 1.05 and 1.23 meters under the net-zero GHG scenarios. 95th percentile estimates for the NDCs were even more dramatic, with net-zero CO2 and net-zero GHG sea-level rises between 4.1 to 4.8 m and 2.3 to 3 m respectively.

Further research is needed to develop more precise estimates of sea-level rise into the future, according to Rogelj. He proposes several concrete steps inculding better continuous observations and improved model development for Antarctic ice sheet instabilities and Greenland ice discharge, both of which contributed the most to this study’s uncertainty ranges.

The findings of this study point to continued sea-level rise up until 2300, even if global GHG emissions reach net-zero levels. However, the authors note that high-end scenarios “can be halved through early and stringent emission reductions,” highlighting the urgent need for fast action on climate change from individuals all the way up to the world’s biggest countries.

Below the Ice: Subglacial Topography in West Antartica

When traversing the broad white expanses of West Antarctica’s Pine Island Glacier (PIG) by snowmobile, you might think the main attraction would be the surface of the rapidly receding river of ice. However, for the authors of a recently published study in Nature Communications, the real draw was not the surface but the rock beneath—the subglacial topography of Antarctica’s most rapidly melting glacier.

Photo of snowmobile pulled radar
Snowmobile pulling survey radar on Pine Island Glacier (Source: Damon Davies/British Antarctic Survey).

Utilizing ice penetrating radar towed by snowmobile, the study’s authors were able to compile the first high-resolution maps of PIG’s underlying bed topography. What sets these maps apart from previous surveys is the detail and diversity of the rugged underlying landscape according to Ted Scambos of the National Snow & Ice Data Center who was not one of the authors of the study. Where previously conducted airborne studies found relatively level topography, this work showed more varied, and often rugged topography—findings that earlier studies had missed, because of the inability of planes to conduct very close parallel surveys.

Why is improved glacier bed delineation crucial for analyzing and predicting glacial retreat rates? It has to do with basal traction or, simply, bottom ice flow. Although we might imagine a glacier sliding as smoothly as an ice cube on a table on a hot summer day, in fact glacial movement is often slowed by two factors, friction and drag. The first of these components is the friction where ice meets the bed below. This factor is highly dynamic, changing as ice melts, flows, and refreezes; friction is also affected by subglacial till, sediment in the glacier bed which was eroded by the glacier, as it moves and freezes.

The second factor, drag, is the more static component of glacial movement. It reflects the size and orientation of undulations in the bedrock below. The net result is the sum of the first, more variable component and the second, more constant component. But earlier work had measured only the sum of the two—making it difficult to predict how the sum might vary. This study marks a major step in removing this limitation. Researchers were able to estimate the two components separately and come up with more precise predictions.

Ariel view of Pine Island Glacier meeting the sea (Source: NASA Ice/Creative Commons).

The glaciers of remote Pine Island Bay (PIB) have received a good deal of attention lately. In May, Rolling Stone published an article examining the Thwaites glacier, West Antarctica’s other rapidly shrinking glacier, and its contribution to rapid sea level rise. Then, in November, Grist published a piece titled “Ice Apocalypse” on the possibility of a rapid glacier collapse in PIB.

Pine Island Glacier, one of the most rapidly retreating glaciers in Antarctica, is estimated to have contributed up to 10 percent of observed global sea level rise, according to the study’s authors. Because of already problematic sea level rise and the societal threats posed by the rapid collapse of these glaciers, many studies have attempted to project the PIG’s future retreat. However, despite all of the focus on the PIG’s retreat, one condition has remained uncertain: the slowing of the glacier’s seaward movement, due to the forces deep below the surface where ice meets terrain.

Photo of figure showing Pine Island subglacial topography
Pine Island subglacial topography derived from study observations (Source: Bingham et al.).

Previous survey methods were unable to separately resolve glacial friction and drag. They could only measure the sum of the two, leading to inaccuracies in ice sheet models that predict retreat rates. These inaccuracies contributed to high variability in bottom ice flow predictions. Given the improved clarity of bed topography observed in this study, the authors were able to conclude that previous inconsistencies must be associated with an incomplete picture of topography beneath glaciers.

The study’s observations of the PIG painted a detailed picture of the landscape beneath the ice. Utilizing these observations, the authors were able to compare them to satellite data outlining the glacier’s recent movements and shrinkage. The comparisons revealed an interesting relationship, the movement of the glacier differed in its tributaries. What was the reason for this variation? It turns out that the slower advancing tributaries corresponded to rougher bottom terrains, with the coarse tributaries, for example, advancing toward the coast where melting occurs, two to three times slower than their smoother counterparts. These findings indicate that bedrock undulations under the PIG impact the glacier’s flow considerably more than changes in friction, a result not previously observed.This discovery allows researchers to make more precise predictions, by summing each of the different tributaries of PIG.

Photo of Thwaites glacier.
Thwaites Glacier, the other rapidly shrinking glacier in Pine Island Bay (Source: NASA’s Marshall Space Flight Center/Creative Commons).

The study shows the large influence of subglacial topography on the retreat of PIG, a topic of great importance to society for its potential disastrous impacts. While the results reveal the importance of these landscapes for glacial recession, the authors note that more research is needed to better measure terrain beneath glaciers. Specifically, they underscore the significance of these needed improvements for PIG’s counterpart, the Thwaites glacier. Like PIG, Thwaites appears to have similar diversity in underlying topography. Nonetheless, the glacier has exhibited a rapid recent retreat, faster than that of even the smoothest PIG tributaries. This is a disturbing fact given that the study’s authors state that the glacier has the “potential for rapid and irreversible retreat, and a considerable contribution to sea-level rise.”

How fast the glaciers of PIB and West Antarctica retreat in the future is still difficult to predict. Nevertheless, as exemplified by this study, scientists continue to develop better methods and models in the face of extreme conditions in one of the most remote and inhospitable places on Earth. But while remote, what happens in the coming years to the ice in PIB has the potential to change the world.

Photo Friday: The Dry Valleys of Antarctica

The National Snow and Ice Data Center (NSIDC) houses an excellent Glacier Photograph Collection, including a special collection of photos of the McMurdo Dry Valleys, a row of snow-free valleys in Antartica. However, that doesn’t stop from glaciers from entering into the picture.

About dry valleys and the MucMurdo Dry Valley photo collection, the NSIDC comments:

“While the valleys themselves are notably ice-free, a number of glaciers terminate in the valleys, some acting as outlets to the East Antarctic Ice Sheet. Studies show that the majority of the glaciers in this area are receding. Glaciers were photographed in the course of geologic studies and help document the conditions of the glaciers and how they may have changed.”

Enjoy some of the photos below.

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To view the rest of the collection, click here.

Many thanks to NSIDC and its Glacier Photograph Collection for the use of these photos. These photos are held by the Data Conservancy at Johns Hopkins University.  Please contact Keith Kaneda for further questions about the collection.

Photo Friday: Seals taking it easy on icebergs

Seals are some of the cutest animals found in the Arctic and the Antarctic. This week’s photo friday features seals carrying out their daily activities on icebergs, which are important environmental features in their chilly habitats. The photos include leopard seals and crabeater seals among other species.

Photo Friday highlights photo essays and collections from areas with glaciers. If you have photos you’d like to share, let us know in the comments, by Twitter @glacierhub or email us at glacierhub@gmail.com.