I’ll do the reading so you don’t have to.

This is one of those papers that makes you sit up and evaluate what you’re doing, or at least it did for me. It’s short but seminal, and it made me terrified for the future, but also more resolutely committed to learning more about the climate system, and taking action to mitigate against the kind of nightmare future that this predicts.

Here’s a link to the paper I’m talking about. I’d highly recommend reading it. Full citation here: Trusel, L. D., Frey, K. E., Das, S. B., Karnauskas, K. B., Munneke, P. K., Van Meijgaard, E., & Van Den Broeke, M. R. (2015). Divergent trajectories of Antarctic surface melt under two twenty-first-century climate scenarios. Nature Geoscience.

This paper suggests that most Antarctic ice sheets are at risk of collapse by the end of the century, even assuming relatively aggressive emissions reductions consistent with RCP4.5 (the IPCC scenario which results in approximately 2°C of warming, the threshold that is considered ‘safe’, although above the target that has been agreed in the Paris Agreement, and still well below the trajectory we are currently on – RCP8.5, or around 4°C of warming). This includes those that are typically considered relatively stable because the accumulation of snow and ice roughly matches the amount of loss due to melt and runoff, such as the Shackleton and other East Antarctic ice sheets.

The total amount of water contained in Antarctic ice sheets would contribute more than 58 m of sea level rise globally. That’s a catastrophic amount of water – not only would it inundate coastal and island areas which are disproportionately located in developing countries, and which are relatively densely populated, but it would also increase the intensity of storm surges and flooding in low-lying areas across the world, causing further damage to homes, livelihoods and communities. Again, the brunt of the impacts will be borne by those who are least able to adapt or mitigate against the effects of climate change because they live in developing countries with fewer resources or political clout on the international stage to go up against major players like the US and EU. If we think the migrant crisis is bad now, we have a lot to learn.

Trusel et al. present a lot of information-dense figures, but Figure 3 is the one that stands out for me. The study uses a method called ensemble modelling, whereby you run numerous different models with varying strengths and weaknesses and representations of the Earth system, and combine the output to get a ‘best guess’ estimate that minimises the bias introduced by any one of the ensemble ‘members’. The ensemble projects a steady increase in the amount of melting across all Antarctic ice shelves as the century goes on, though the amount of melt under a higher emissions scenario is much more considerable. Y’know; RCP8.5, the scenario we are currently on track to achieve.

My own research focuses on the Antarctic Peninsula, which is the most rapidly warming region on the planet, having warmed by around 3°C since the ‘50s. I’m looking at a specific ice sheet called Larsen C, which is the largest ice sheet remaining on the peninsula, its neighbours Larsen A and B having succumbed to melting and collapsed into the sea in 1995 and 2003, respectively.

Trusel et al. use the conditions observed at the time of their collapse as a threshold against which to compare the stability of other ice shelves. We know from observations that depletion of the amount of air in the top layers of snow and ice (‘firn’) is a precursor of ice sheet collapse. That’s because this top firn layer is very porous and therefore acts as a buffer, absorbing excess meltwater in warm years, and allowing it to refreeze, rather than run off and be lost into the ocean. However, if this layer becomes saturated, this buffer is lost, and ponds of water begin to form on the surface and fill crevasses that extend under pressure, eventually causing sections of the ice sheet to calve off into icebergs.

Now, as we’re talking about ice sheets breaking up, it seems pertinent to mention this: during the last two summers in Antarctica, a huge crack has begun to extend across Larsen C, sparking fears that its stability may be in question. Further research suggests that the ice sheet as a whole is relatively stable, but that the volume of ice that this crack will cause to break off will represent just over 10% of the total surface area of the ice sheet. What this indicates is that Larsen C is reaching the same kinds of thresholds that were observed before Larsen A and B collapsed, so small changes like a few warm summers in succession could have considerable consequences.

The results of Trusel et al. indicate that Larsen C, where meltwater is currently produced at a rate of around 275 mm water equivalent (w.e.) per year, is on track to hit the threshold of 725 mm w.e. per year by the end of the 21st century under a relatively modest emissions scenario, RCP4.5. What is more shocking though, is by how much the threshold is projected to be overshot by if we keep emitting fossil fuels like we currently are. By 2100, under the RCP8.5 scenario, the meltwater production rates soar to around 2400 mm w.e. per year: that’s nearly nine times the current rates on the Larsen C, and more than three times the threshold level of melt considered a precondition of collapse.

Under the RCP8.5 scenario the outcomes are pretty terrifying: virtually all ice sheets will be lost from the Antarctic Peninsula. While the loss of floating ice shelves in itself will not contribute to sea level rise (they already displace their own weight in water – just like Archimedes in the bath, Eureka!), they in effect act as a plug, preventing all the ice that flows down from the mountains from spilling into the sea. Once these floating ice shelves are gone, there’s nothing to stop all that ice flowing rapidly downhill and into the ocean, as was observed after the loss of other ice sheets like Larsen A and B.

The loss of the Larsen C alone would contribute an estimate 27 cm of sea level rise. Add this to the other ice sheets that would potentially collapse under the level of warming that seems likely right now and we look set to lose a lot of land worldwide, not to mention the increased intensity and damage caused by natural events like flooding and storms.

It might seem like I’m spouting doom and gloom and terror – and I am in some ways, because this is scary stuff – but it’s very important to note the difference between the emissions scenarios: under a relatively ‘modest’ increase (consistent with 2°C by 2100), the only ice sheet that reaches the threshold of melt that suggests imminent collapse is the Larsen C. While this would certainly be detrimental worldwide, it’s a much less horrifying scenario than the hellish future that would materialise under 4°C of warming, where almost all ice sheets are lost around the Peninsula and many of the ice sheets around East and West Antarctica are also at risk.

What this means is that it is critical that people act to change our path from a trajectory set on achieving 4°C of warming to one where warming is much more minimal. It demonstrates that we can make changes now that will affect the outcomes for our future selves and for future generations.

This means taking action – preventing governments from making regressive decisions to expand destructive infrastructure and industries like airports and aviation, like oilrigs and oil exploration, like palm oil plantations and deforestation. It means making collective changes to our lifestyles, particularly in the developed world, to redress the inequality and within and between nations and to reduce the overall footprint of humanity on the climate.

It’s not impossible, but it is imperative. The feeling I got from this paper was one of cold dread. But eventually I realised that there’s no point feeling hopeless, because that doesn’t inspire change. We need to turn the fear into passion and commitment to prevent the outcomes we are afraid of – otherwise it could get a whole lot worse.

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About shakkka

Londoner, climate scientist, extremist. All views are my own.

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