Ice sheets under 1.5C warming – Expert Reaction

Under the Paris Agreement, nations agreed to limit global warming to less than 2C above pre-industrial levels, but even that might not be enough to stop the collapse of Greenland and Antarctic ice sheets.

A review of our current understanding of ice sheet processes, which includes a New Zealand author, suggests both ice sheets may have tipping points at or slightly above the 1.5-2C threshold. If the sheets collapse, it could lead to irreversible loss of mass and drainage basins, and urgent research is needed to better improve projections, the researchers say.

The SMC asked New Zealand experts to comment on the study, please feel free to use these comments in your reporting.

Associate Professor Rob McKay, Antarctic Research Centre, Victoria University of Wellington, comments:

“This paper highlights that ice sheets in Antarctica and Greenland have tipping points, after which melting rates will rapidly accelerate and the ice sheet retreat potentially becomes unstoppable.

“While the reason for this differs between hemispheres, with melting of Greenland controlled by atmospheric warming and Antarctica by oceanic warming, it demonstrates that the temperature threshold to reach these tipping points in both regions is between 1.5 to 2C, which suggests that even if we do meet the Paris climate agreement targets, we will be extremely close to the point of no return for accelerated retreat.

“While this melting will play out over hundreds to thousands of years, it is apparent from this work that the more we overshoot the 1.5C target, the more rapid this accelerated ice sheet melt will be.”

No conflict of interest, but I work in the same research centre as author Nicholas Golledge.

Professor Christina Hulbe, School of Surveying, University of Otago, comments:

“The new review article by Pattyn and others synthesises the work of several research groups into one clear message: we are very close to triggering irreversible change in Earth’s polar ice sheets.

“It’s been known for a long time that both the Greenland and Antarctic ice sheets are vulnerable to climate change and that the physical processes governing them have tipping points, thresholds beyond which the ice sheets are guaranteed to shrink no matter what we do next.

“Which processes matter most are different in the north and the south and the different research groups use somewhat different approaches to represent them in computer models. But all the computer models all point in the same direction: the threshold for irreversible ice loss in both Greenland and Antarctica is somewhere between 1.5 and 2C global mean warming. We’re already at a bit more than 1C warming.

“Where the models diverge is just how fast the ice sheets retreat once the threshold is crossed. The differences between models are helpful though because they show which processes and which regions need more study. New Zealand is an international leader in this kind of research. We conduct challenging fieldwork in Antarctica and the Southern Ocean and we’re actively engaged in building better models to predict what will happen next and how it will play out for us here at home.

“The mayor of Dunedin (where I live) calls sea level rise a slow-moving disaster. One of my predecessors here in the School of Surveying, Professor Emeritus John Hannah, says it’s also an opportunity to think differently about coastal development. Not all parts of our coastline have the same vulnerability or will respond in the same way as sea level rises.

“The new report makes another point that is sometimes lost in the conversation: even if we meet the Paris targets and keep the warming in check, we’re still committed to continued ice loss over the 21st century, and with it continued sea level rise. I would add to this caution that some of the tipping processes may have already been invoked, at least in some parts of Antarctica but understanding that in detail requires more work.”

No conflict of interest.

Professor Tim Naish, Antarctic Research Centre, Victoria University of Wellington, comments:

“This review in Nature Climate Change is extremely timely given the recent release of the IPCC Special Report on “Global Warming of 1.5C”, and especially given that we are close to 1.5C. Without some degree of direct carbon extraction from the atmosphere, we are unlikely to avoid it.

“This review summarises the progress made since the IPCC’s 5th Assessment Report in 2013 on understanding the poorly known, but potentially very significant, dynamic contribution of the polar ice sheets to future sea-level rise. The authorship team represent the very best of the world’s ice sheet modellers including Victoria University of Wellington (VUW), Antarctic Research Centre’s Dr Nick Golledge.

“Nick’s Nature paper in 2015 was one of the first to suggest that there may be a ‘threshold’ or ‘tipping point’ in the Antarctic ice sheet somewhere close to the Paris climate agreement target of 2C. Above this level of warming, parts of the ice sheets could be committed to irreversible loss over coming centuries to millennia.

“This review assesses the latest research and reinforces the potential of this threshold for both ice sheets, and by implication, the importance of achieving the Paris target of 2C. It also acknowledges that large uncertainties still exist and that there is an urgency for further research.”

No conflict of interest declared.

Dr Huw Horgan, Senior Lecturer, Antarctic Research Centre, Victoria University of Wellington, comments:

“This study really emphasises the vulnerability of these ice sheets to warming, and clearly show how their fate is linked to choices we make about emissions.

“Most striking are the tipping points, which once crossed potentially commit us to much greater rates of sea level rise for centuries to millennia. These model results show that the ice sheets have tipping points very close to the global temperature increases that business as usual scenarios would have us reach by the middle of this century.

“The possibility of non-linear responses, whereby steady changes in forcing result in a disproportionate increase in the loss of grounded ice to the ocean, means that the possible upper rates of sea level rise are significantly greater than we are currently experiencing.

“This study really shows us that there’s no room for complacency here. We understand the problem well enough to know what we need to do. And what we don’t understand is also motivation to act sooner rather than later. If I were looking for reasons to be optimistic I would point to the great work being done by the science community in this area, and the engagement of the public. But we do need a pathway to accomplish what we now know we need to do.”