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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    The Greenland Ice Sheet is close to a melting point of no return    
    Once we emit about 1000 gigatons of carbon, much of the massive ice sheet   
   will melt irreversibly: We've emitted 500 gigatons so far    
      
     Date:   
         March 27, 2023   
     Source:   
         American Geophysical Union   
     Summary:   
         A new study using simulations identified two tipping points for   
         the Greenland Ice Sheet: releasing 1000 gigatons of carbon into   
         the atmosphere will cause the southern portion of the ice sheet to   
         melt; about 2500 gigatons of carbon means permanent loss of nearly   
         the entire ice sheet. Having emitted about 500 gigatons of carbon,   
         we're about halfway to the first tipping point.   
      
      
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   FULL STORY   
   ==========================================================================   
   The Greenland Ice Sheet covers 1.7 million square kilometers (660,200   
   square miles) in the Arctic. If it melts entirely, global sea level   
   would rise about 7 meters (23 feet), but scientists aren't sure how   
   quickly the ice sheet could melt. Modeling tipping points, which are   
   critical thresholds where a system behavior irreversibly changes, helps   
   researchers find out when that melt might occur.   
      
      
   ==========================================================================   
   Based in part on carbon emissions, a new study using simulations   
   identified two tipping points for the Greenland Ice Sheet: releasing 1000   
   gigatons of carbon into the atmosphere will cause the southern portion   
   of the ice sheet to melt; about 2500 gigatons of carbon means permanent   
   loss of nearly the entire ice sheet.   
      
   Having emitted about 500 gigatons of carbon, we're about halfway to the   
   first tipping point.   
      
   "The first tipping point is not far from today's climate conditions,   
   so we're in danger of crossing it," said Dennis Ho"ning, a climate   
   scientist at the Potsdam Institute for Climate Impact Research who led   
   the study. "Once we start sliding, we will fall off this cliff and cannot   
   climb back up."  The study was published in AGU's journal Geophysical   
   Research Letters, which publishes short-format, high-impact research   
   spanning the Earth and space sciences.   
      
   The Greenland Ice Sheet is already melting; between 2003 and 2016, it   
   lost about 255 gigatons (billions of tons) of ice each year. Much of   
   the melt to date has been in the southern part of the ice sheet. Air   
   and water temperature, ocean currents, precipitation and other factors   
   all determine how quickly the ice sheet melts and where it loses ice.   
      
   The complexity of how those factors influence each other, along with the   
   long timescales scientists need to consider for melting an ice sheet of   
   this size, make it difficult to predict how the ice sheet will respond   
   to different climate and carbon emissions scenarios.   
      
   Previous research identified global warming of between 1 degree to 3   
   degrees Celsius (1.8 to 5.4 degrees Fahrenheit) as the threshold beyond   
   which the Greenland Ice Sheet will melt irreversibly.   
      
   To more comprehensively model how the ice sheet's response to   
   climate could evolve over time, Ho"ning's new study for the first   
   time used a complex model of the whole Earth system, which includes   
   all the key climate feedback processes, paired with a model of ice   
   sheet behavior. They first used simulations with constant temperatures   
   to find equilibrium states of the ice sheet, or points where ice loss   
   equaled ice gain. Then they ran a set of 20,000-year-long simulations   
   with carbon emissions ranging from 0 to 4000 gigatons of carbon.   
      
   From among those simulations, the researchers derived the 1000-gigaton   
   carbon tipping point for the melting of the southern portion of the ice   
   sheet and the even more perilous 2,500-gigaton carbon tipping point for   
   the disappearance of nearly the entire ice sheet.   
      
   As the ice sheet melts, its surface will be at ever-lower elevations,   
   exposed to warmer air temperatures. Warmer air temperatures accelerate   
   melt, making it drop and warm further. Global air temperatures have to   
   remain elevated for hundreds of years or even longer for this feedback   
   loop to become effective; a quick blip of 2 degrees Celsius (3.6 degrees   
   Fahrenheit) wouldn't trigger it, Ho"ning said. But once the ice crosses   
   the threshold, it would inevitably continue to melt. Even if atmospheric   
   carbon dioxide were reduced to pre- industrial levels, it wouldn't be   
   enough to allow the ice sheet to regrow substantially.   
      
   "We cannot continue carbon emissions at the same rate for much longer   
   without risking crossing the tipping points," Ho"ning said. "Most of   
   the ice sheet melting won't occur in the next decade, but it won't be   
   too long before we will not be able to work against it anymore."   
       * RELATED_TOPICS   
             o Earth_&_Climate   
                   # Global_Warming # Climate # Snow_and_Avalanches #   
                   Ice_Ages   
             o Fossils_&_Ruins   
                   # Early_Climate # Ancient_DNA # Evolution # Origin_of_Life   
       * RELATED_TERMS   
             o Ice_sheet o Greenland_ice_sheet o Antarctic_ice_sheet o   
             Ice_shelf o Carbon_dioxide o Ice_age o Carbon_dioxide_sink   
             o Carbon_cycle   
      
   ==========================================================================   
   Story Source: Materials provided by American_Geophysical_Union. Note:   
   Content may be edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Dennis Ho"ning, Matteo Willeit, Reinhard Calov, Volker Klemann,   
      Meike   
         Bagge, Andrey Ganopolski. Multistability and Transient Response of   
         the Greenland Ice Sheet to Anthropogenic CO 2 Emissions. Geophysical   
         Research Letters, 2023; 50 (6) DOI: 10.1029/2022GL101827   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230327163212.htm   
      
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