Tandon reported in late 2020 that:
- 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.
- The new models predict a 22-day longer melt season than the old models by the end of the 21st century.
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.
Greenland's ice sheet could melt at a rate not seen in the last 130,000 years. 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-fueled economic and development, and details the high level of warming that will accompany it. Climate scientist Dr Ruth Mottram of the Danish Meteorological Institute, who was not involved in the study, explains the importance of these scenarios: “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.
Read on ... very interesting.
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