PREVIEW

Scientists say efforts to limit global temperature increase to 1.5°C under the Paris Climate Agreement may fail to save the world’s ice sheets, resulting in potentially catastrophic sea level rises.

Research led by Durham University, UK, suggests the target should instead be closer to 1°C to avoid significant losses from the polar ice sheets and an accompanying acceleration in sea level rises.

Currently, around 230 million people live within one metre of sea level and melting ice represents an existential threat to those communities, including several low-lying nations.

The climate scientists’ work suggests that while we should continue to work to reduce temperature rises, even optimistic estimates mean we should prepare to adapt to worst case scenarios.

Lead author Professor Chris Stokes, in the Department of Geography, Durham University, UK, said: “There is a growing body of evidence that 1.5 °C is too high for the ice sheets in Greenland and Antarctica. We’ve known for a long time that some sea level rise is inevitable over the next few decades to centuries, but recent observations of ice sheet loss are alarming, even under current climate conditions.

“Limiting warming to 1.5°C would be a major achievement and this should absolutely be our focus. However, even if this target is met or only temporarily exceeded, people need to be aware that sea level rise is likely to accelerate to rates that are very difficult to adapt to – rates of one centimetre per year are not out of the question within the lifetime of our young people."

The team reviewed a wealth of evidence to examine the effect that the 1.5°C target would have on the ice sheets in Greenland and Antarctica, which together store enough ice to raise global sea levels by almost 65 metres.
The mass of ice lost from these ice sheets has quadrupled since the 1990s and they are currently losing around 370 billion tonnes of ice per year, with current warming levels of around 1.2°C above pre-industrial temperatures according to the latest Intergovernmental Panel on Climate Change (IPCC) report.

The authors argue that further warming to 1.5°C would likely generate several metres of sea level rise over the coming centuries as the Greenland and Antarctic ice sheets melt in response to both warming air and ocean temperatures.

This would cause extensive loss and damage to coastal and island populations and lead to widespread displacement of hundreds of millions of people.

Avoiding this scenario would require a global average temperature cooler than that of today, which the researchers hypothesise is probably closer to 1°C above pre-industrial levels or possibly even lower.

However, the researchers add that further work is urgently needed to more precisely determine a “safe” temperature target to avoid rapid sea level rise from melting ice sheets.

Policymakers and governments need to be more aware of the effects a 1.5°C rise in temperatures could have on ice sheets and sea levels, the researchers say.

Far from giving up on limiting climate change, Professor Stokes said it means we should work harder to limit the damage so we can eventually work hard to return temperatures to a level which would eventually see the ice sheets return to health.

He added: “We are not necessarily saying that all is lost at 1.5°C, but we are saying that every fraction of a degree really matters for the ice sheets - and the sooner we can halt the warming the better, because this makes it far easier to return to safer levels further down the line.

“Put another way, and perhaps it is a reason for hope, we only have to go back to the early 1990s to find a time when the ice sheets looked far healthier.

“Global temperatures were around 1°C above pre-industrial back then and carbon dioxide concentrations were 350 parts per million, which others have suggested is a much safer limit for planet Earth. Carbon dioxide concentrations are currently around 424 parts per million and continue to increase.”

However, co-author, Professor Rob DeConto, from the University of Massachusetts Amherst, USA, who specialises in computer simulations of Antarctica that reveal how the ice sheet might change under different warming levels, said places lost to rising sea levels could be lost for centuries.

Professor DeConto said: “It is important to stress that these accelerating changes in the ice sheets and their contributions to sea level should be considered permanent on multi-generational timescales.

“Even if the Earth returns to its preindustrial temperature, it will still take hundreds to perhaps thousands of years for the ice sheets to recover. If too much ice is lost, parts of these ice sheets may not recover until the Earth enters the next ice age. In other words, land lost to sea level rise from melting ice sheets will be lost for a very, very long time. That’s why it is so critical to limit warming in the first place.”

The research team combined evidence from past warm periods that were similar or slightly warmer than present, and measurements of how much ice is being lost under the present level of warming, together with projections of how much ice would be lost at different warming levels over the next few centuries.

Professor Andrea Dutton of the University of Wisconsin-Madison, USA, another co-author of the study, explained: “Evidence recovered from past warm periods suggests that several meters of sea level rise – or more – can be expected when global mean temperature reaches 1.5 °C or higher. Furthermore, this evidence also suggests that the longer those warm temperatures are sustained, the greater the impact on ice melt and resulting sea-level rise.”

Fellow study co-author Jonathan Bamber, Professor of Glaciology and Earth Observation at the University of Bristol, UK, has been measuring changes in ice sheets for several decades.

Professor Bamber said: “Recent satellite-based observations of ice sheet mass loss have been a huge wake-up call for the whole scientific and policy community working on sea level rise and its impacts. The models have just not shown the kind of responses that we have witnessed in the observations over the last three decades.”

Some countries have already had to adapt to climate change - with Belize moving its capital inland decades ago after a devastating hurricane.
Commenting on the research, Ambassador Carlos Fuller, long-time climate negotiator for Belize said: "Findings such as these only sharpen the need to remain within the 1.5°C Paris Agreement limit, or as close as possible, so we can return to lower temperatures and protect our coastal cities."

The research is published in the journal Communications Earth and Environment.