The loss of ice from glaciers and ice sheets is probably one of the clearest early indicators of climate change as it occurs before us. The receding of long-standing ice and glaciers has been happening since the 19th century but this has accelerated in recent decades.
Places such as Greenland are critical, as total ice loss there could spell a significant increase in global ocean levels and affect global ocean circulation that would affect our climate. Approximately 80% of Greenland is covered by an ice sheet, which can be as thick as 3 kilometers (2 miles). Now, we have improved methods for quantifying ice loss and have a better understanding what is happening to ice in Greenland.
Ice sheet calving and ice loss
Calving of ice sheets (when chunks of ice break off at the terminus, or end, of a glacier) is an important process in the loss of ice. The mass of the Greenland ice sheet has significantly decreased in recent years, a result of intensified surface melting and the breaking off of icebergs. As glaciers flow toward the sea and calve, they are usually replenished by new snowfall on the interior of the ice sheet that gets compacted into ice. Due to rising air and ocean temperatures, the ice sheet is losing mass at an accelerating rate, partially through an increased rate of calving, and additional meltwater is flowing into the sea.
In a recent study, using 36,328 manually-derived and artificial intelligence (AI)-derived observations based on different data sets of glacier terminus positions collected from 1985 to 2022, researchers were able to generate 120-meter-resolution ice-sheet projections for Greenland.
Greenland’s ice lost is 20% more than previously thought
Based on this, researchers estimate the Greenland Ice Sheet (GrIS) has lost 5,091 ± 72 km2 in area, which is 1,034 ± 120 gigatonnes of ice that has calved and melted away. Most estimates today on ice loss have been looking at the thinning of ice in glaciers, while not focusing as much on the historical calving and receding extent of ice. Combining the two, estimating the receding and thinning of ice, researchers now believe Greenland’s ice lost is about 20% more than current estimates.
To derive the results, researchers used cleaned data from different sources. This included 3,437 glacier terminus positions from the MEaSUREs Annual v2[2] from 1985 and 21,990 weekly to monthly terminus positions which derive fromSentinel-1 synthetic aperture radar since January 2015. The CALFIN dataset provides 19,665 data points from 1985 and 39,013 terminus positions were obtained from the TermPicks dataset.[3] There are 153,281 terminus positions also derived from the AutoTerm dataset, which includes Landsat imagery results.[4]
Other important results used for the total estimate in ice loss incorporated the BedMachine Greenland Version 5 for measuring ice thickness and using MEaSUREs ITS_LIVE velocity mosaics to determine ice velocity. This helps to estimate ice terminus advances.[5] Overall mass and change in ice can be calculated by differences between February 1985 and February 2022 using a 12-month moving mean calculated. Root sum square of uncertainties between 1985-2022 measurements are used to calculate error.
Greenland’s ice loss affects ocean circulation
This amount does not add much to global sea levels; however, it does affect ocean circulation and heat energy transferred by ocean circulation. In general, Greenland is now losing 193 ± 25 km2 ice per year, where glaciers expand to a maximum extent by May and recede to a minimal extent by September/October. However, the overall retreat indicates that ice in Greenland is highly sensitive to overall global climate. Using 95 Greenlandic glaciers, seasonality can be shown to be the most clear indicator of yearly change in ice loss, but overall loss is affected by increasing global temperatures.[1]
Overall, this technique has shown that oceanic circulation would be affected, where the Atlantic Meridional Overturning Circulation (AMOC) would be the most impacted. This would most directly affect precipitation and storms in Europe and the North Atlantic.
The implication of the results suggest not only have we underestimated the effects of ice melt on global ocean circulation, which has consequences on overall climate as circulation models may now need updating to account for the extra water derived from Greenland, but as the pace of melt increases, overall sea level change will need to be reconsidered. If we are now losing more ice than previously thought from Greenland, and possibly other regions, sea level change may affect cities and populations faster than previously thought. More accurate ice loss projections may require a reevaluation of major glaciers and ice sheets in Antarctica as well, given its even larger role in changing sea levels.
Using satellite imagery, flow models, and AI-derived observations, it is clear that ice has been in retreat and thinning in places like Greenland for decades and has been substantially underestimated in previous studies. Now, using this knowledge, we will need to update our understanding of how this excess water will affect our climate and future, with more accelerated ice loss likely to occur.
References
[1] For more on the recent work looking at ice loss since 1985 in Greenland, see: Greene, C.A., Gardner, A.S., Wood, M., Cuzzone, J.K., 2024. Ubiquitous acceleration in Greenland Ice Sheet calving from 1985 to 2022. Nature 625, 523–528. https://doi.org/10.1038/s41586-023-06863-2.
[2] For more on the MEaSUREs data, see: https://nsidc.org/data/measures/data datase.
[3] The CALFIN data can be found here: https://nsidc.org/data/nsidc-0764/versions/1. The publication for TermPicks can be found here: Goliber, S., Black, T., Catania, G., Lea, J.M., Olsen, H., Cheng, D., Bevan, S., Bjørk, A., Bunce, C., Brough, S., Carr, J.R., Cowton, T., Gardner, A., Fahrner, D., Hill, E., Joughin, I., Korsgaard, N.J., Luckman, A., Moon, T., Murray, T., Sole, A., Wood, M., Zhang, E., 2022. TermPicks: a century of Greenland glacier terminus data for use in scientific and machine learning applications. The Cryosphere 16, 3215–3233. https://doi.org/10.5194/tc-16-3215-2022.
[4] The AutoTerm data can be found here: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1095/.
[5] More on MEaSUREs ITS_LIVE can be found here: Gardner, A., Fahnestock, M., Scambos, T., 2023. MEaSUREs ITS_LIVE Antarctic Grounded Ice Sheet Elevation Change, Version 1. https://doi.org/10.5067/L3LSVDZS15ZV.
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Fonte : National Geographic
