15 December 2020, Carbon Brief, New climate models suggest faster melting of the Greenland Ice Sheet AYESHA TANDON
Greenland’s vast ice sheet could melt faster than previously thought over the 21st century, according to a new study.
The Greenland ice sheet is the second largest mass of ice on Earth, holding enough water to raise global sea levels by 7.2 metres. Even if warming in the coming decades is kept to low levels, melting from the Greenland ice sheet is expected to reach unprecedented rates in the coming decades, contributing significantly to global sea level rise.
The study, published in Nature Communications, compares estimates of future sea level rise from the Greenland ice sheet in new (CMIP6) models to the previous generation (CMIP5). The study finds that the 21st century sea-level contribution from the Greenland ice sheet is always higher in the CMIP6 models than in the corresponding CMIP5 models running the same emissions scenario. (See Carbon Brief’s detailed CMIP6 explainer.)
This is mainly because CMIP6 models project a greater temperature increase over the 21st century, the study finds. For example, the researchers find that in a very high emissions scenario, the new models predict a 22-day longer melt season than the old models by the end of the 21st century.
Overall, the researchers estimate that 21st century sea level rise from Greenland would be 2.6cm higher under a low-emission future scenario in the CMIP6 simulations, 2.8cm higher under a medium-emission scenario and 5cm higher under a high-emission scenario.
Modelling the Greenland ice sheet
To capture the full range of possibilities for the Greenland ice sheet over the coming century, the study explores a range of warming scenarios known as representative concentration pathways (RCPs).
Each RCP scenario details a different climate “forcing” – the extra amount of energy in the Earth’s system as a result of human activity. A higher forcing results in a greater rise in global temperature.
The study focuses on three of these scenarios: RCP8.5, RCP4.5 and RCP2.6. They describe, respectively, a baseline scenario of very high emissions, an intermediate scenario that broadly matches the trajectory the world is likely to follow in the coming decades, and a stringent mitigation scenario that limits warming to 2C.
The researchers use two sets of climate models from the Coupled Model Intercomparison Project (CMIP), a global modelling effort. These are taken from the latest CMIP6 suite of models and the older CMIP5 suite.
One key difference in CMIP6 is that it uses five “shared socioeconomic pathways” (SSPs), which are socioeconomic narratives that go alongside the RCPs.
CMIP6 models use combined RCP/SSP scenarios to give a more complete account of future changes to the planet. For example, the CMIP6 SSP58.5 scenario tells the story of fossil-fuelled economic and development, and details the high level of warming that will accompany it.
“I think it’s important to remember that SSP585 and RCP8.5 aren’t the same scenario so this work also shows that how we get to the end of the century is as or maybe more important than what the final emissions are.”
Mottram also tells Carbon Brief that “we knew already from the observed melt and runoff of Greenland over the past couple of decades that the CMIP5 models seemed to be on the low side”. She adds the CMIP6 models have “updated physics” and seem to be doing “a better job”.
Dr Stefan Hofer, the study’s lead author and a postdoctoral fellow at the University of Oslo, tells Carbon Brief that “on paper the CMIP6 models have a higher resolution and more sophisticated physics…However, we need to be careful, because at this stage we cannot say whether a CMIP6 world is more likely than a CMIP5 world”.
CMIP6 models run hotter
One of the most notable changes between results from the CMIP5 and CMIP6 models is that the latter predicts a greater temperature rise over the 21st century, the study finds. This is particularly notable in the Arctic, where “Arctic amplification” causes high-latitude temperatures to rise significantly above the global average.
The new study compares a range of different RCP and SSP scenarios in the CMIP5 and CMIP6 models, initially focusing on the highest emissions scenarios – RCP8.5 in CMIP5 and SSP58.5 in CMIP6. The figure below compares simulations of global and Arctic warming between the two.Projected warming in the 21st century in CMIP5 RCP8.5 (light and dark blue) and CMIP6 SSP58.5 (orange and red) models, for Arctic (top) and global (bottom) averages. Chart by Carbon Brief using Highcharts.
This figure shows that, in a high emissions scenario, CMIP6 models predict a global average temperature increase of 0.6C more than CMIP5 models by the year 2100. The difference is more pronounced in the Arctic, where CMIP6 models predict a 1.3C greater temperature rise than CMIP5 models.
Hofer explains that the warmer temperatures in CMIP6 are due to increased sensitivity to greenhouse gases. He tells Carbon Brief:
“The CMIP6 models seem to be more sensitive to greenhouse gas emissions and, therefore, global temperatures, but especially the Arctic warms more than in CMIP5. Therefore, despite similar anthropogenic forcing levels, Greenland would lose more mass in a CMIP6 world than in CMIP5.”
Mottram adds that higher temperatures may be a result of “feedbacks” that reinforce the human-caused warming:
“CMIP6 models show even higher temperatures in the Arctic, which affects sea ice and clouds and this seems to be an important feedback on Greenland ice sheet melt.”
The study authors note that the CMIP5 and CMIP6 models predict roughly the same global temperatures for the next few decades. The CMIP6 models only begin to predict significantly higher temperatures than CMIP5 after 2050. However, this divergence between the two sets of models occurs roughly 20 years earlier in the Arctic – around 2030.
Annual cycle of the Greenland ice sheet
The higher rates of warming in the CMIP6 simulations have a knock-on impact for the Greenland ice sheet.
This can be seen in the chart below, which shows the ice sheet’s “surface mass balance” (SMB) in different model simulations. The SMB compares how much snow the ice sheet accumulates through the year with how much it loses through melting – known as “ablation” – at its surface. (The ice sheet also loses ice via ocean melting at its edge and from icebergs breaking off.)
Greenland’s SMB is typically positive for much of the year, before plunging below zero as seasonal melting takes hold each summer.
This cycle can be seen in the figure below, which shows CMIP5 and CMIP6 simulations for the current climate (light blue and orange lines) and for the end of the century under the highest emissions scenario (red and dark blue).
The chart highlights that, while the differences between CMIP5 and CMIP6 models are “negligible” during 1981-2010, there are some notable differences by the end of the 21st century. Under the high emission scenario, in 2071-2100, the peak daily melt in the CMIP5 simulations is 15bn tonnes per day (Gt/day), whereas in CMIP6 it is 50% higher at 23Gt/day.
Furthermore, the melt season – where the ice sheet loses mass through the summer – starts seven days earlier and extends 15 days longer in CMIP6 than in CMIP5, the paper says.
Most of this change is due to a greater melting intensity, the authors note, with little difference in accumulation noticed between CMIP5 and CMIP6. This suggests that the increased temperatures and Arctic amplification are the main factors driving the increased melting from the Greenland ice sheet, they conclude.
Hofer outlines the meaning of this higher temperature for the Greenland ice sheet:
“Our results for the extreme high-emission scenarios (RCP8.5 and SSP585) show that in CMIP5 RCP8.5 the Greenland contribution to sea level rise would be roughly 9.9cm, but in CMIP6 SSP585 it would be 17.8cm. That’s roughly a 80% higher contribution, despite the same or similar radiative forcing from anthropogenic emissions in the extreme high-emission scenario.”
However, the study also finds that warmer temperatures can turn falling snow, which would normally contribute to accumulation, into rain. This can affect the surface albedo of the ice sheet – how well it reflects incoming sunlight – and lead to further melting. The high emissions CMIP6 scenario saw 47% more rainfall than the corresponding CMIP5 scenario.
When will thresholds be crossed in the Greenland ice sheet?
As global temperatures rise, the Greenland ice sheet is melting more quickly – 60% of the recent increase in Greenland’s sea-level contribution is due to enhanced surface runoff, the paper says.
When averaged over the entire year, the surface mass balance of the Greenland ice sheet is currently positive, meaning that accumulation outweighs melting. (Although the total mass balance – which includes melting at the ice sheet edges and the loss of icebergs – is increasingly negative, averaging at an annual 250bn-tonne loss over the past decade.)
However, as the climate continues to warm, the ice sheet may reach a point where annual surface melt outweighs accumulation and the surface mass balance becomes negative, on average.
The Intergovernmental Panel on Climate Change’s (IPCC) 2018 special report on 1.5C, notes that the “threshold at which annual mass loss from the ice sheet by surface melt exceeds mass gain by snowfall” would be a “useful indicator” for the fate of the Greenland ice sheet as it ties into other feedbacks and tipping points.
The figure below shows when this threshold could be crossed in different RCP scenarios, and using different models.
Surface mass balance from the Greenland ice sheet in CMIP5 (blue) and CMIP6 (red) under a high emissions (top), medium emissions (middle) and low emissions (bottom) scenario. Source: Hofer et al (2020).
In the high emissions scenario explored previously in this study, this threshold is crossed by 2046 in CMIP6, compared to 2058 in CMIP5. After the threshold is crossed, surface mass balance continues to decline.
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In the intermediate emissions scenario (RCP4.5), this threshold would be crossed in 2066 in CMIP6, but it would not be crossed at any point in the 21st century in CMIP5. Finally, in the low emissions scenario (RCP2.6), the threshold would never be crossed in either CMIP6 or CMIP5.
The study notes that the 21st century sea-level contribution from the Greenland ice sheet is always higher in the CMIP6 models than in the corresponding CMIP5 models, despite a similar level of global radiative forcing. Sea level rise is predicted to be 2.6cm higher in a low-emission future scenario, 2.8cm higher in a medium-emission scenario, and 5cm higher in the high-emission scenario in CMIP6 compared to CMIP5.
Mottram says that the inclusion of multiple different scenarios is an important feature of this paper:
“What is also very good about this analysis is that although they focus on the high end scenarios, they also give estimates for lower end scenarios, where they show the same effect – that the CMIP6 estimates of ice loss are higher than CMIP5.”
Hofer S. et al (2020) Greater Greenland Ice Sheet contribution to global sea level rise in CMIP6, Nature Communications, doi.org/10.1038/s41467-020-20011-8