#Greenland; #MassiveIceSheetsheds; #SeaLevelRise
Greenland, Aug 23 (Canadian-Media): Greenland's massive ice sheet saw a record net loss of 532 billion tonnes last year, raising red flags about accelerating sea level rise, according to new findings, https://phys.org/news/2020 news reports said.
Until 2000, Greenland's ice sheet accumulated as much mass as it shed
That is equivalent to an additional three million tonnes of water streaming into global oceans every day, or six Olympic pools every second.
Crumbling glaciers and torrents of melt-water slicing through Greenland's ice block—as thick as ten Eiffel Towers end-to-end—were the single biggest source of global sea level rise in 2019 and accounted for 40 percent of the total, researchers reported in the journal Communications Earth & Environment.
Last year's loss of mass was at least 15 percent above the previous record in 2012, but even more alarming are the long-term trends, they said.
"2019 and the four other record-loss years have all occurred in the last decade," lead author Ingo Sasgen, a glaciologist at the Helmholtze Centre for Polar and Marine Research in Germany, told AFP.
The ice sheet is now tracking the worst-case global warming scenario of the UN's climate science advisory panel, the IPCC, noted Andrew Shepherd, director of the Centre for Polar Observation and Modelling at the University of Leeds.
"This means we need to prepare for an extra ten centimetres or so of global sea level rise by 2100 from Greenland alone," said Shepherd, who was not involved in the study.
Redraw world's coastlines
If all of Greenland's ice sheet were to melt, it would lift global oceans by seven metres (23 feet).
Even a more modest rise of a couple of metres would redraw the world's coastlines and render land occupied today by hundreds of millions of people uninhabitable.
Until 2000, Greenland's ice sheet—covering an area three times the size of France—generally accumulated as much mass as it shed.
Runoff, in other words, was compensated by fresh snowfall.
But over the last two decades ago, the gathering pace of global warming has upended this balance.
The gap is widening at both ends, according to the study, which draws from nearly 20 years of satellite data.
Changing weather patterns—also a consequence of climate change—has resulted in less cloud cover, and thus less snow. These high pressure systems have also resulted in more, and warmer, sunny days, accelerating the loss of mass.
In 2019, the ice sheet lost a total of 1.13 trillion tonnes, about 45 percent from glaciers sliding into the sea, and 55 percent from melted ice, said Sasgen. It gained about 600 billion tonnes through precipitation.
A study in the same journal last week concluded that the Greenland's ice sheet has passed a "tipping point", and is now doomed to disintegrate, though on what time scale is unknown.
'Alarm bells ringing'
Sasgen says it is too soon to know if we have reached a point of no return, but agrees that the ice sheet is likely to continue losing mass, even in colder years.
"But that doesn't mean that trying to limit warming doesn't matter," he added.
"Every decimal degree you save in terms of warming will save a certain amount of sea level rise, both in magnitude and speed."
At the other end of the world, the West Antarctic Ice Sheet—which holds another six metres worth of sea level rise—is similarly thought to be teetering on a tipping point, with many experts convinced it has already passed it.
Scientists not involved in the research were not surprised by the findings, but expressed concern.
"The ice sheet has lost ice every year for the last 20 years," said Twila Moon, a research scientists at the University of Colorado.
"If everyone's alarm bells were not already ringing, they must be now."
Stuart Cunningham, an oceanographer from the Scottish Association for Marine Science, warned about the potential impact on the North Atlantic circulation, a current that keeps northwestern Europe five to ten degrees Celsius warmer that similar latitudes elsewhere on the globe.
"Climate models show this circulation can be switched off by adding fresh water to the North Atlantic," he said, noting this happened during the end of the last ice age.
"This tipping point in the climate system is one of the potential disasters facing us."
From 1992 to 2018, Greenland lost about four trillion tonnes of mass, causing the mean sea level to rise by 11 millimetres, according to a December 2019 study in Nature.
#Genomes; #Rhinos; #Extinction; #climateChange
The extinction of prehistoric megafauna like the woolly mammoth, cave lion, and woolly rhinoceros at the end of the last ice age has often been attributed to the spread of early humans across the globe, https://phys.org/news reports said.
This image shows the preserved, reconstructed remains of a baby woolly rhinoceros named Sasha that was discovered in Siberia. Image Credit: Albert Protopopov
Although overhunting led to the demise of some species, a study appearing August 13 in the journal Current Biology found that the extinction of the woolly rhinoceros may have had a different cause: climate change. By sequencing ancient DNA from 14 of these megaherbivores, researchers found that the woolly rhinoceros population remained stable and diverse until only a few thousand years before it disappeared from Siberia, when temperatures likely rose too high for the cold-adapted species.
"It was initially thought that humans appeared in northeastern Siberia fourteen or fifteen thousand years ago, around when the woolly rhinoceros went extinct. But recently, there have been several discoveries of much older human occupation sites, the most famous of which is around thirty thousand years old," says senior author Love Dalén, a professor of evolutionary genetics at the Centre for Palaeogenetics, a joint venture between Stockholm University and the Swedish Museum of Natural History. "So, the decline towards extinction of the woolly rhinoceros doesn't coincide so much with the first appearance of humans in the region. If anything, we actually see something looking a bit like an increase in population size during this period."
To learn about the size and stability of the woolly rhinoceros population in Siberia, the researchers studied the DNA from tissue, bone, and hair samples of 14 individuals. "We sequenced a complete nuclear genome to look back in time and estimate population sizes, and we also sequenced fourteen mitochondrial genomes to estimate the female effective population sizes," says co-first author Edana Lord, a Ph.D. student at the Centre for Palaeogenetics.
By looking at the heterozygosity, or genetic diversity, of these genomes, the researchers were able to estimate the woolly rhino populations for tens of thousands of years before their extinction.
"We examined changes in population size and estimated inbreeding," says co-first author Nicolas Dussex, a postdoctoral researcher at the Centre for Palaeogenetics. "We found that after an increase in population size at the start of a cold period some 29,000 years ago, the woolly rhino population size remained constant and that at this time, inbreeding was low."
This stability lasted until well after humans began living in Siberia, contrasting the declines that would be expected if the woolly rhinos went extinct due to hunting. "That's the interesting thing," says Lord. "We actually don't see a decrease in population size after 29,000 years ago. The data we looked at only goes up to 18,500 years ago, which is approximately 4,500 years before their extinction, so it implies that they declined sometime in that gap."
The DNA data also revealed genetic mutations that helped the woolly rhinoceros adapt to colder weather. One of these mutations, a type of receptor in the skin for sensing warm and cold temperatures, has also been found in woolly mammoths. Adaptations like this suggest the woolly rhinoceros, which was particularly suited to the frigid northeast Siberian climate, may have declined due to the heat of a brief warming period, known as the Bølling-Allerød interstadial, that coincided with their extinction towards the end of the last ice age.
"We're coming away from the idea of humans taking over everything as soon as they come into an environment, and instead elucidating the role of climate in megafaunal extinctions," says Lord. "Although we can't rule out human involvement, we suggest that the woolly rhinoceros' extinction was more likely related to climate."
The researchers hope to study the DNA of additional woolly rhinoceroses that lived in that crucial 4,500-year gap between the last genome they sequenced and their extinction.
"What we want to do now is to try to get more genome sequences from rhinos that are between eighteen and fourteen thousand years old, because at some point, surely they must decline," says Dalén. The researchers are also looking at other cold-adapted megafauna to see what further effects the warming, unstable climate had. "We know the climate changed a lot, but the question is: how much were different animals affected, and what do they have in common?"