The fate of the Greenland Ice Sheet, Earth's second-largest ice mass, hangs in the balance as it melts at an unprecedented rate, threatening global sea levels and ocean circulation. However, recent studies co-authored by Professor Laurence Smith from Brown University challenge the accuracy of current climate models.
Professor Smith and his team have uncovered a critical oversight in these models: the failure to account for processes that retain meltwater on the Ice Sheet. This omission leads to overestimations of meltwater runoff, a key contributor to rising sea levels.
"Current climate models are largely accurate," Smith acknowledges, "but they overlook how meltwater can refreeze or pond, which can significantly impact our predictions."
The team's fieldwork focused on the ablation zone, a lower-elevation area where snow loss exceeds accumulation. Here, seasonal snowpack melts, exposing dark ice that absorbs more sunlight, leading to further melting.
By measuring the flow rate of a river draining meltwater on the Ice Sheet's surface and drilling holes to monitor refreezing, the researchers discovered that meltwater retention is a major source of inaccuracy.
"Meltwater produced during the day fills the pore spaces of the top ice layers, but at night, temperatures drop, and the water refreezes," Smith explains. This process, modeled computationally, shows that heat is redirected to remelting existing ice rather than generating new runoff.
Projecting these findings across the southwestern region, the study estimates that "roughly 11 to 17 gigatons of meltwater are retained and refrozen annually, equivalent to 9% to 15% of modeled annual runoff."
In flatter areas of the ablation zone, meltwater ponds, creating a positive feedback loop that darkens the Ice Sheet, increasing its absorption of solar radiation, and accelerating melting.
In a separate study, the team investigated the warming effect of these ponds. Ice sheets typically have high albedo, or reflectivity, due to their white color. However, as temperatures rise and ice melts, the Ice Sheet becomes darker, absorbing more solar radiation and melting faster.
To quantify this effect, the researchers used drones to capture images of the Ice Sheet at different GPS points, which were then compared to satellite maps of water coverage and albedo variations.
The study found that meltwater ponding accounted for approximately 1% of total heating from sunlight across the Greenland Ice Sheet in the summer of 2019. However, depending on factors like time of year, elevation, and area size, ponding can significantly impact albedo variability and heating.
"The fieldwork supporting this research is intense," Smith says, but for lead author Jonathan Ryan, it's rewarding. "We camp right in the melt zone, and it's a unique, high-pressure experience that fosters strong teamwork."
Looking ahead, Smith and his team aim to bridge the gap between climate models and surface processes, exploring what's happening on and beneath the Earth's surface in the Greenland Ice Sheet.
"We want the best physical understanding possible to get into these models," Smith emphasizes.
What are your thoughts on these findings? Do they challenge your understanding of climate change impacts? Share your thoughts in the comments below!
