- Once people have cumulatively emitted approximately 1,000 gigatons of carbon in total, then the southern part of the Greenland Ice Sheet will melt eventually causing the sea level to rise by almost six feet.
- Once humans have cumulatively emitted approximately 2,500 gigatons of carbon in total, the whole Greenland Ice Sheet will eventually melt and the sea level rise would rise by 6.9 meters, or 22.6 feet.
- And right now, now we are at approximately 500 gigatons of carbon emissions released.
New research published this week in the journal Geophysical Research Letters found that carbon emissions are halfway to a tipping point after which 6 feet of sea level rise from the melting of the Greenland Ice Sheet would be unstoppable.
"Once we have emitted more than ~1,000 gigatons carbon in total, we won't be able to stop the southern part of the Greenland Ice Sheet to melt entirely in the long term, even if we would entirely stop emitting carbon then. This melting would cause a sea level rise by ~1.8m," Dennis Höning, a climate scientist at the Potsdam Institute for Climate Impact Research who is the lead author of the study, told CNBC. (1.8 meters is 5.9 feet.)
"Although this melting would take hundreds of years, future generations won't be able to stop it," Höning said.
The further the Earth overshoots the first tipping point of 1,000 gigatons of carbon emissions, the faster the Greenland Ice Sheet will melt.
And right now, now we are at approximately 500 gigatons of carbon emissions released.
"Certainly, the coastal regions would be most affected by this, especially in poorly developed countries without modern coastal management," Höning told CNBC.
Höning said scientists have found in previous studies that the Greenland Ice Sheet could melt completely with global warming somewhere between 1 and 3 degrees Celsius (1.8 degrees to 5.4 degrees Fahrenheit.)
But the methodologies of previous studies have been less accurate because the assumptions made in those models have been overly simplistic and therefore unrealistic, Höning told CNBC.
"While exploring tipping points connected to temperatures is useful to understand the stability of the system, in the real world it's the cumulative carbon emission that detemines whether or not a tipping point will actually be crossed," Höning told CNBC. "This is why we studied the connections between cumulative carbon emissions and tipping of the Greenland Ice Sheet with a fully coupled Earth system model, that includes all relevant feedback processes, for the first time."
Höning used the CLIMBER-X computer system that models the evolution of the Earth over long time periods and measures everything in his paper, titled: Multistability and Transient Response of the Greenland Ice Sheet to Anthropogenic CO2 Emissions.
Measuring the melting of the Greenland Ice Sheet is challenging because it takes a really long time for it to melt, and it doesn't happen at a consistent pace.
"Once a critical threshold is crossed, the system's behavior qualitatively changes and approaches a completely new equilibrium. This is due to self-reinforcing feedback mechanisms: When the ice sheet melts, its surface gets exposed to warmer air temperatures at lower altitudes, and melting inevitably continues," Höning told CNBC.
Höning said it's more accurate to measure total, cumulative carbon emissions, which are those released since 1850.
If total emissions of carbon stay below the 1,000 gigatons of carbon emissions threshold, then the melting Greenland Ice Sheet would "only" contribute tens of centimeters to total sea level rise, he added.
The second tipping point noted in the research would occur once 2,500 gigatons of carbon emissions have been released into the atmosphere, at which point the whole Greenland Ice Sheet will melt and sea level rise would rise by 6.9 meters, or 22.6 feet.
"A complete melting will take time, hundreds or even thousands of years, especially if we cross the threshold only by a bit," Höning said. "Even though the atmospheric CO2 concentration will decline on these long timescales, it won't decline at a pace fast enough to stop melting the ice sheet."
In theory, artificial carbon dioxide removal technology could pull enough carbon out of the atmosphere fast enough after one of these tipping points has been reached to claw back the inevitable melting. But it's not a case study worth pursuing because the technology to remove that volume of emissions from the atmosphere doesn't exist right now.
"We are not even close to a point where carbon removal is efficient. Avoiding carbon emissions is in any case much cheaper than the energy required to capture this carbon again," Höning told CNBC.