COP28: Why 2 °C is too high for the polar ice sheets and sea level rise

This article was published as part of our COP28 campaign. To find out more about our delegates and work to tackle one of the most pressing challenges of our time, visit our COP28 webpage. 

Written by: Professor Chris R Stokes, Department of Geography

On the 12 December 2015, 196 countries adopted the Paris Climate Agreement at the 21st Conference of the Parties (COP21). The overarching goal of this landmark agreement is to limit the increase in global average temperature to “well below 2 °C” and to pursue efforts to “limit the increase to 1.5 °C above pre-industrial levels”. For context, the pre-industrial baseline refers to the average temperature for the period 1850-1900, which is the earliest period of sufficiently complete global temperature observations.  

Fast-forward a few years and the latest Intergovernmental Panel on Climate Change (IPPC) report, published in 2021, concluded that the mean global surface temperature had now increased to 1.1 °C above pre-industrial levels.  

The IPPC’s Summary for Policymakers also warned that global surface temperatures will continue to increase until at least mid-century. The summary also warned that global warming of 1.5 °C and 2 °C will be exceeded in the 21st Century unless there are urgent and deep reductions in greenhouse gas emissions. To limit warming to 1.5 °C will probably require emissions to peak before 2025 and to decline by 43% by 2030. Or, to put it bluntly, we are now in the intensive care unit to try and keep “1.5 alive”. 

Smaller Glaciers and Sea Level Rise 

As a glaciologist – someone who studies glaciers – much of my recent research has been targeted at measuring the response of glaciers to global warming, including their current and future contribution to sea level rise. For smaller mountain glaciers and ice caps – the ones we might visit on student fieldtrips to Iceland or Norway – this is a relatively straightforward task. You don’t even have to visit them. We have a multitude of different satellites in orbit that enable us to measure their surface area, elevation and even the flow speed of the ice every few days.  

Unfortunately, the outlook is bleak. One recent study projects that we will lose 50% of the Earth’s glaciers if global warming reaches +1.5 °C and that this would increase to 80% for a +4 °C scenario. Given that glaciers are a critical water resource for 1.9 billion people worldwide, the loss and damage would be catastrophic. In contrast, the impact on sea level would be relatively small. Although there are a staggering 274,531 individual glaciers on Earth, they only store the frozen equivalent of around 32 cm of sea level rise.  

Vast polar ice sheets and sea level rise 

From a sea-level perspective, we are worried about the vast polar ice sheets in Greenland and Antarctica, which together contain a volume of water equivalent to 65 (sixty-five!) metres of sea-level rise. When you consider that 230 million people live within one metre of sea level, it’s clear that even small changes in the polar ice sheets will have profound impacts on coastal communities. 

To further complicate matters, these huge continent-sized ice sheets in Antarctica and Greenland do not respond in a predictable (linear) manner. Instead, they are very sensitive to temperature thresholds that, once crossed, could lead to a rapid acceleration in sea level rise. This non-linear response is due to several self-reinforcing feedbacks that, once triggered, would become essentially irreversible on human time-scales.  

Determining the precise temperature thresholds for both the Greenland and Antarctic ice sheets is now a topic of huge scientific and societal importance. Worryingly, the latest measurements show that they added around 21 mm to sea level between 1992 and 2020, with the rate of ice loss increasing from 105 billion tonnes per year in the 1990s up to 372 billion tonnes of ice over the last few years. Indeed, some scientists have argued that this represents the first signs of an instability that may be irreversible, particularly in West Antarctica.  

Our work in Durham has mostly focussed on East Antarctica, which is the largest ice sheet on Earth, storing 52 m of sea level. This ice sheet has often been viewed as less vulnerable than the West Antarctic or Greenland ice sheets and some of the early computer modelling showed it would require a temperature increase of 17 °C to cause major deglaciation. Our recent work has shown that there are already worrying signs of mass loss from parts of East Antarctica, driven by ocean warming, and that its temperature threshold may actually be as low as 2 °C, emphasising the importance of the Paris Climate Agreement. 

The Road to COP28 – why 2 °C is too High 

There is now a clear consensus amongst scientists that the lower limit of the Paris Climate agreement – 1.5 °C – should be the target if we are to avoid a dangerous acceleration in sea level rise from the world’s ice sheets this century and beyond.  

Recently, I’ve been working with the International Cryosphere Climate Initiative (ICCI) to get this urgent message across to climate negotiators and high-level policymakers.  

At COP27 in Sharm El-Sheikh, I was privileged to witness a broad coalition of 20 governments, led by the two polar nations of Chile and Iceland, join together to sign a new declaration. The declaration calls for emergency-scale reductions in greenhouse gases to keep alive the possibility of limiting warming to 1.5 °C and protect as much of the Earth’s cryosphere as possible.  

A further sign of progress at COP27 was that the word ‘cryosphere’ appeared in the Cover Decision of a COP for the very first time, which recognised “the impact of climate change on the cryosphere and the need for further understanding of these impacts, including of tipping points”.  

I would argue that we already know these impacts and that exceeding 1.5 °C is likely to trigger multiple tipping points and not just in relation to ice sheets. 

At COP28, I will once again be sharing the latest research on the Antarctic Ice Sheet and stressing the need for urgent action. Picking up some of the themes from the ICCI’s State of the Cryosphere Report from 2022, the message is simple: we cannot negotiate with the melting point of ice.  

Find out more  

The cover image depicts Geography students on the ‘Arctic’ fieldtrip to northern Norway in Sept 2023, standing in front of a typical mountain glacier – the marker post in the foreground shows the position of the glacier in 2010 and it has retreated over 300 m since then. 

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