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Jonathan Bamber is Professor of Physical Geography at the University of Bristol (United Kingdom); Michael Oppenheimer is Professor of Geosciences and International Affairs at Princeton University.
Antarctica is farther from civilization than any other place on Earth. Greenland's ice cap is closer to home, but about a tenth the size of its southern brother. Together, these two ice masses contain enough frozen water to raise the world average. sea level 65 meters if they were to melt suddenly. But what is the probability that this will happen?
The icecap of Antarctica is about one and a half times larger than Australia. What happens in one part of Antarctica may not be identical to what happens in another – just as the east and west coasts of the United States can experience very different reactions, for example, in case of El Niño climate change. There are periodic climatic events resulting in wetter conditions in the southern United States, warmer conditions in the north and drier conditions on the northeast coast.
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The ice in Antarctica is about 5 km thick in some places and we have no idea of the conditions at the base, although these conditions play a key role in determining the speed at which the ice can react to climate change, including how fast it can flow to and into the ocean. A warm, moist base lubricates the bedrock under the ice and allows it to slide on it.
These problems have made it particularly difficult to produce model simulations of how ice sheets will respond to climate change in the future. Models must take into account all the processes and uncertainties we know and ignore – "known unknowns" and "unknown unknowns," as Donald Rumsfeld put it. As a result, several recent studies suggest that previous reports of the Intergovernmental Panel on Climate Change may have underestimated the importance of the melting of ice sheets that would contribute to future sea level.
What the experts say
Fortunately, models are not the only tools for predicting the future. The structured expert judgment is a method taken from a study published by one of us in 2013. The experts give their judgment on a problem difficult to model. Their judgments are combined to take into account their ability to assess their own uncertainty. This provides a rational consensus.
The approach was used when the consequences of an event are potentially catastrophic, but our ability to model the system is weak. These include volcanic eruptions, earthquakes, the spread of vector-borne diseases such as malaria and even aircraft accidents.
Since the study conducted in 2013, scientists who model ice caps have improved their models by attempting to integrate processes generating positive and negative reactions. Impurities on the surface of the Greenland Ice Cap have a positive effect because they enhance melting by absorbing more heat from the sun. The stabilizing effect of bedrock formation when the underlying ice thins, thus reducing the weight on the bed, is an example of negative feedback, as it slows down the ice melting speed.
The record of observations of ice change, mainly from satellite data, has also increased in length and quality, helping to improve knowledge of recent ice sheet behavior.
With colleagues from the United Kingdom and the United States, we embarked on a new structured expert judgment exercise. With all the new research, data and knowledge, you can expect that uncertainties around the amount of ice cap melting contributing to sea-level rise have been reduced. Unfortunately, this is not what we found. What we have seen is a range of future results that are getting worse.
Increasing uncertainty
We brought together 22 experts in the United States and the United Kingdom in 2018 and combined their judgments. The results are sobering. Rather than reducing uncertainty about the future behavior of the ice cap over the past six years, it has increased.
If the increase in global temperature remains below 2 ° C, the experts' best estimate of the average contribution of sea-level ice sheets was 26 centimeters. They concluded however that there was a 5% chance that the contribution would reach 80 centimeters.
If this is combined with the other two main factors that affect the sea level – the melting of glaciers around the world and the dilation of the ocean's water as it warms up – the l? Average global sea-level rise could exceed one meter by 2100. Small island states are experiencing every two days the current flood, which only occurs once a year. once every hundred years, and become uninhabitable.
For a climate change scenario closer to the status quo – where our current trajectory of economic growth continues and global temperatures rise by 5 – the outlook is even bleaker. The best estimate of the experts in this case is 51 cm sea level rise caused by melting ice sheets from here 2100, but with a 5% probability that the sea level will increase by more than two meters by 2100, which could move some 200 million people.
Let's try to put this in context. It is estimated that the Syrian refugee crisis has caused the emigration of about one million people in Europe. This has happened over several years rather than a century, which has left much less time for countries to adjust. Nevertheless, sea-level rise caused by migration of this size could threaten the existence of nation-states and create unimaginable stress on resources and space. It's time to change course, but not much, and the longer we wait, the harder it becomes, the bigger the mountain must be.
This article is republished from The conversation under Creative Commons license. Click here to subscribe to the Climate Action newsletter. Climate change is inevitable. Our answer to that is not.
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