Global environmental consequences of twenty-first-century ice-sheet melt
Nicholas R. Golledge, Elizabeth D. Keller, Natalya Gomez, Kaitlin A. Naughten, Jorge Bernales, Luke D. Trusel, and Tamsin L. Edwards
The problem: Global policymakers rely heavily on scientific studies to inform their decisions about climate-related policies. However, many climate change scenarios outlined in these studies’ models fail to address the ice-ocean-atmosphere feedbacks that may be triggered by ice sheet melting.
What data were used? To incorporate ice-ocean-atmosphere feedbacks in their estimation of climate consequences, the authors use climate models and data from 23 empirical studies. These data include measurements of the changes in total ice mass, surface mass balance, ice shelf melt, and iceberg calving of the Antarctic and Greenland ice sheets.
Total ice mass is simply the volume of ice that makes up the ice sheet. Measuring the change in mass over time tells us whether the ice sheet is shrinking or growing, and at what rate that mass is changing. Spoiler alert: the ice sheets are most definitely shrinking.
A component of the changes to total ice mass is surface mass balance. This concept describes the balance between net accumulation and net ablation occurring on the surface of the ice sheet. The key process in accumulation is snowfall, while ablation is the process of melting. Thus, we can determine surface mass balance by subtracting the amount of melting from the amount of snowfall.
Two other components of determining changes to total ice mass are ice shelf melt and iceberg calving (Figure 1). Ice shelves are areas of the ice sheet that extend off the continent and over ocean water. When they melt, they directly feed the ocean with freshwater that had previously been trapped in frozen form. Similarly, the process of icebergs calving (i.e. when ice chunks separate from a marine-terminating glacier) removes mass from the ice sheet and adds it to the ocean.
Methods: The authors input a series of different climate scenarios into a model that predicts the effects of climate warming and ice sheet melting on ice-ocean-atmosphere feedback loops (Figure 2). Their model varies the monthly and yearly climate conditions over a period from 1860 to 2100 to assess the effects of thinning ice sheets. Starting the model in the past means that they can compare the predictions of the model with actual data from the 23 supplementary studies that collectively span 1900 to 2017.
Results: The model scenarios paired with empirical climate studies come to three main conclusions concerning the future thinning of ice sheets. First, as the Greenland Ice Sheet loses mass, the increasing amount of fresh meltwater will slow circulation of the Atlantic Ocean. The Atlantic meridional overturning ocean circulation (AMOC) is driven by temperature and salt gradients. Thus, a large contribution of cold freshwater would alter the speed of AMOC. Second, as Antarctica continues to deliver meltwater to the ocean, warm water will be trapped below the ocean surface. This creates a positive feedback loop in which Antarctic ice loss will increase from meltwater input to the ocean. Finally, the model predicts that any future ice sheet melt with elevate global temperature variability and contribute an additional 25 centimeters (10 inches) to sea level by the year 2100.
Why is this study important? Our climate is warming, the ice sheets are melting, and the consequences of those changes are substantial. We study past climates to better understand our modern and future climates, and we heavily rely on model predictions to look toward the future. This paper address a key component missing from many climate models (the ice-ocean-atmosphere feedbacks that may result from future ice sheet melting) and shows that some models have underestimated the severity of ice sheet melting consequences. Model predictions are critical tools in global policymaking, so ensuring that those models are comprehensive is essential. Moreover, this paper calls for continued observations of the effects of climate change on ice sheets, oceans, and the atmosphere. Further incorporation of data into models will only help improve their predictions of a future climate that demands new environmental policies.
Citation: Golledge, N.R., Keller, E.D., Gomez, N., Naughten, K.A., Bernales, J., Trusel, L.D., and Edwards, T.L., 2019, Global environmental consequences of twenty-first-century ice-sheet melt: Nature, p. 65-72, https://doi.org/10.1038/s41586-019-0889-9.