Melting Continental Glaciers Remain a World-Flooding Enigma

"The potential contributions of ice sheets remain the largest source of uncertainty."


Research released this week uncovers a situation that’s far worse than we thought. Scientists have seriously underestimated just how much water will melt off continent-sized glaciers, aka ice sheets, that are shrinking because of Earth’s increasingly hot atmosphere. The news opens a set of bigger questions facing scientists: How severe will sea level rise become by the end of the 21st century?

In a paper published on Monday in the journal Proceedings of the National Academy of Science, an international team of scientists explain that global sea levels could rise more than six feet by 2100. That is is twice the high end of a 2013 range predicted by the United Nations Intergovernmental Panel on Climate Change. Just six years ago, that UN group predicted that if humans continue emitting greenhouse gases at current rates — a worst-case scenario — sea levels would rise between 52 and 98 centimeters (between 20.5 and 38.6 inches) by the end of the century. The new study, on the other hand, predicts that the number will be more than 200 centimeters — over 6.5 feet., which puts a lot more cities underwater.

“Miami? Underwater. Norfolk? Swamped. New York City? Fuggedaboutit,” was the opening sentence in a USA Today story on the study.

The huge difference all comes down to just how difficult it is to predict how ice sheets will behave over the coming decades.

“Severe limitations remain in the predictive capability of ice sheet models,” write the study’s authors, led by first author Jonathan Bamber, Ph.D., a professor of physical geography at the University of Bristol in the United Kingdom. “As a consequence, the potential contributions of ice sheets remain the largest source of uncertainty in projecting future [sea level rise].”

One of the main sources of this unpredictability actually has very little to do with the ice sheets themselves, though.

Perito Moreno Glacier Opal Iceberg
The Perito Moreno Glacier in Los Glaciares National Park in the south west of Santa Cruz province, Argentina. 

Predicting the future of the Earth depends in great part on how effectively governments respond to the threat of global heating in the short term. Most climate change predictions use models that represent what the global climate will be like based on what humans choose to do today.

For instance, one scenario in the new paper reflects what could happen if world communities shift away from fossil fuels toward renewable energy — basically what is laid out in the Paris Agreement. As such, the best-case scenario in the paper predicts how ice sheets might melt if the global average temperature only rises by 2 degrees celsius. But the worst-case scenario predicts what might happen if our current CO2 and other greenhouse gas emissions continue at the current pace. Under that model, the researchers behind this week’s new paper say global average temperatures could rise by 5 degrees celsius.

When a panel of experts used these two scenarios to estimate how ice sheets will contribute to sea level rise by 2100, they demonstrated that the best-case scenario would result in up to 32 inches of sea level rise, while the worst-case scenario would lead to about 5 feet of sea level rise in the same time period. Once they include the contribution of glaciers and factor in how water expands as it warms, that worst-case scenario ends up resulting in 6.5 feet of sea level rise.

These numbers, though, are only as good as the data we have today.

Fortunately, satellite data, ice core samples, and fossil records have all contributed to building an unprecedented library of data that can help inform climate predictions. The researchers say that while the data increases prediction accuracy, there’s still a big gap.

Nonetheless, long-term [sea level rise] projections remain acutely uncertain, in large part because of inadequate understanding of the potential future behaviors of the Greenland and Antarctic ice sheets and their responses to future global climate change. This limitation is especially troubling, given that the ice sheet influence on [sea level rise] has been increasing since the 1990s and has overtaken mountain glaciers to become the largest barystatic (mass) contribution to [sea level rise]. In addition, for any given future climate scenario, the ice sheets constitute the component with the largest uncertainties by a substantial margin, especially beyond 2050.

In short, ice sheets are an enigma. But they’re crucial to accurately estimating the effects of climate change, so scientists are trying to figure them out. For instance, it’s not totally clear whether changes in ice sheet melting observed in recent years are due to natural weather variations or global climate change. These questions will be crucial to our understanding of future sea level rise scenarios, and scientists are just beginning to fill in these gaps in understanding.

For now, though, we understand climate change well enough to know this for sure: Even if things change for the better, we’re facing some serious challenges in the near future.

Abstract: Despite considerable advances in process understanding, numerical modeling, and the observational record of ice sheet contributions to global mean sea-level rise (SLR) since the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change, severe limitations remain in the predictive capability of ice sheet models. As a consequence, the potential contributions of ice sheets remain the largest source of uncertainty in projecting future SLR. Here, we report the findings of a structured expert judgement study, using unique techniques for modeling correlations between inter- and intra-ice sheet processes and their tail dependences. We find that since the AR5, expert uncertainty has grown, in particular because of uncertain ice dynamic effects. For a +2 °C temperature scenario consistent with the Paris Agreement, we obtain a median estimate of a 26 cm SLR contribution by 2100, with a 95th percentile value of 81 cm. For a +5 °C temperature scenario more consistent with unchecked emissions growth, the corresponding values are 51 and 178 cm, respectively. Inclusion of thermal expansion and glacier contributions results in a global total SLR estimate that exceeds 2 m at the 95th percentile. Our findings support the use of scenarios of 21st century global total SLR exceeding 2 m for planning purposes. Beyond 2100, uncertainty and projected SLR increase rapidly. The 95th percentile ice sheet contribution by 2200, for the +5 °C scenario, is 7.5 m as a result of instabilities coming into play in both West and East Antarctica. Introducing process correlations and tail dependences increases estimates by roughly 15%.