Source: Carbon Brief
The scale of loss from the Greenland ice sheet will be higher this century than in any other over the past 12,000 years, a new study concludes.
However, the rate of mass loss still depends heavily on future greenhouse gas emissions, the study shows – the ice sheet could lose ice up to four times faster if global emissions are very high than if global climate goals are met.
The research, published in Nature, focuses specifically on southwestern Greenland in order to draw conclusions for the fate of the ice sheet as a whole.
Before the start of the human industrial era, the highest rate of mass loss occurred around 12,000 years ago during the early Holocene. At this time, southwestern Greenland was losing ice at a rate of 6,000bn tonnes per century.
Over the past two decades, it has been losing ice at a rate of around 6,100bn tonnes per century, the study says.
If the global target of limiting global warming to 2C is achieved, this level will rise to 8,800bn tonnes per century.
However, if future emissions are very high, it could rise to up to 35,900bn tonnes per century – adding almost 10cm to global sea levels, the lead author tells Carbon Brief.
Island of ice
The Greenland ice sheet is a mass of frozen freshwater extending about 1.7m square kilometres, covering most of the island of Greenland. It is three times the size of Texas. At its “northern dome” – its tallest point – it stands around 3,000m above sea level.
As a result of climbing air and ocean temperatures, the ice sheet is losing mass each year.
The rapid melting of the ice sheet has consequences for sea level rise. From 1993-2010, ice loss from the Greenland ice sheet caused sea levels to rise at a rate of 0.33mm a year. (The total rate of sea level rise over the same period was around 3.2mm a year.)
The new research takes a rigorous look back at how the ice sheet has changed over the past 12,000 years. To do this, it uses a combination of improved ice core data and high-resolution climate models.
It focuses on southwestern Greenland – a region responsible for around 25-50% of the ice sheet’s mass loss, explains study lead author Prof Jason Briner, a glaciologist and climate change researcher from the University at Buffalo. He tells Carbon Brief:
“If you compare the rate of ice change in our study area to rates of the entire ice sheet, there is a very close, almost one-to-one, relationship. So…we are very confident that the trends of ice changes in our study area and the entire ice sheet are similar.”
The chart below shows how the rate of ice mass loss and gain has changed from 12,000 years ago to present day, according to the study.
It also shows a range of projections for the speed of future ice mass loss by 2100.
On the chart, yellow circles show the projected rate of ice mass loss from model simulations using a scenario where the world successfully meets its goal of limiting global warming to below 2C above pre-industrial levels (“RCP2.6”).
Meanwhile, red circles show the projected rate of ice mass loss from model simulations using a scenario where future greenhouse gas emissions are very high (“RCP8.5”).
Rate of Greenland ice sheet mass change from 12,000 years ago to present and projected mass change by 2100 under RCP2.6 and RCP8.5. Data source: Briner et al. (2020). Chart by Joe Goodman using Highcharts.
The chart shows that the rate of ice loss at present is similar to that during the early Holocene – a time period from around 7,000 to 12,000 years ago when the ice sheet was receiving large amounts of heat from the sun, Briner explains:
“During the early Holocene, summers were pretty warm, maybe 3 to 5C warmer than they were in the 20th century. This was because the northern hemisphere was receiving more energy from the sun then.
“If you look at the rate of ice loss on Greenland – say, this past decade – that’s about what it was during this early Holocene warm period. Despite the fact that Greenland is receiving less energy from the sun that it was in the early Holocene, the ice is melting about just as much because of global warming.”
The chart also shows how the rate of mass loss from the ice sheet is likely to change by the end of the century.
It shows that, by 2100, the ice sheet will be losing ice faster than at any other century in the past 12,000 years.
However, if the world successfully limits global warming to below 2C, the rate of ice mass loss is likely to be around 8,800-10,600bn tonnes per century by 2100, the study says.
But if future emissions are very high, the rate of ice mass loss could reach 14,000bn-35,900bn tonnes per century by 2100, it adds.
The projected range is larger under the high emissions scenario because scientists are less sure of how the ice sheet could react to such high levels of warming, explains study author Dr Josh Cuzzone, a sea level and ice researcher from the NASA Jet Propulsion Laboratory. He tells Carbon Brief:
“As the climate warms, feedbacks and climate sensitivity – how much our planet warms for a doubling of CO2 – really matter for how hot our planet will get and, ultimately, how much ice mass loss we get.
“Because each individual model [used in the study] has different climate sensitivity and parametrisations of processes that lead to different feedback responses, the spread really starts to increase the more carbon we put into the system and warm.”
(Carbon Brief has previously published an explainer on “tipping points”, which covers the potential for a threshold of Greenland ice melt, beyond which its eventual collapse is irreversible.)
The loss of land ice from Greenland will have consequences for sea level rise, the authors say.
They calculate that, under the 2C scenario, southwestern Greenland could add 2.4cm to global sea levels per century by 2100. Under the high emissions scenario, this could rise to 9.9cm.
This level of sea level rise from Greenland would be unprecedented, says Dr Peter Kuipers Munneke, a polar glaciologist from Utrecht University in the Netherlands, who was not involved in the research. He tells Carbon Brief:
“The paper does show that mass losses from Greenland will become unprecedented. It will lead to a larger-than-ever contribution to sea level rise from Greenland.
“But, in early humanity, the rate of rising sea levels has probably been much greater due to the melting of ice sheets that no longer exist today: in Eurasia and North America. So, although the mass loss from Greenland may start to exceed historical rates, this doesn’t necessarily mean that rates of sea level change will become unprecedented this century.”
The study puts to rest the “climate trope” that warming in Greenland during the early Holocene dwarfed that of human-caused climate change, says Dr Ruth Mottram, a climate scientist at the Danish Meteorological Institute (DMI) in Copenhagen, who also was not involved in the research. She tells Carbon Brief:
“[What] I like about this study is they really attempt to set present observed changes into a longer term context to show that at present we are much closer in terms of ice sheet melt to the Holocene thermal optimum than at any other period in the last 12,000 years.”
(In 2019, Carbon Brief published a factcheck explaining why ice core records do not show that past warming in Greenland exceeds that of human-caused climate change.)
The researchers have done a “really nice job” of reconstructing Greenland’s past using modelling and ice core data, says says Prof Jonathan Bamber, a glaciologist from the University of Bristol. He tells Carbon Brief:
“The only caveat to that is that they are tuning the model on a region that [with] land-terminating [glaciers] so excluding the effects of ocean processes. Nonetheless, the results show that the Greenland ice sheet will be entering a phase of mass loss that it hasn’t experienced in the last 12,000 years and potentially since the last interglacial some 115,000 years ago.”
(Carbon Brief published an in-depth interview with Bamber in 2019.)
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