Research suggests even Antarctic ice sheets would be safe if we limited global warming to 1.5-2°Cby Rebecca Priestley
All’s not yet lost: scientists say we can prevent the melting of Antarctica’s ice sheets and catastrophic sea-level rise.
When Nature published new projections in March for sea-level rise caused by global warming, news headlines warned of a coming “climate catastrophe”, with “sea levels expected to rise twice as high as previously thought”. Yet one of the report’s authors, Rob DeConto of the University of Massachusetts, says the research also carried an alternative, more positive message.
DeConto, along with New Zealand scientist Tim Naish, was one of the speakers at a Scientific Committee on Antarctic Research (Scar) conference in Kuala Lumpur in August. According to the climate modeller, if the world adopts the aggressive mitigation strategy – to reduce greenhouse-gas emissions and keep global temperature increase to 1.5-2°C – agreed to at the United Nations climate change conference in Paris last year, we could largely be spared the melting of Antarctica’s ice sheets.
Under this future scenario, “we get very little sea-level contribution from Antarctica”, said DeConto. Climate catastrophe averted. Coastal cities saved. But it’s vital that we act now.
At the Paris conference, 195 countries agreed that global warming must be kept to no more than 2°C and that serious efforts must be made to limit it to 1.5°C. In early September, China and the US ratified the Paris Agreement. At press time, the total number of nations that had ratified was 29, responsible for 40% of global greenhouse-gas emissions. As nations sign up, they are expected to introduce policies to keep the global temperature rise within the 1.5-2°C range. For the agreement to come into force, at least 55 nations, accounting for at least 55% of global greenhouse-gas emissions, must ratify the treaty. And for global warming to remain below 2°C, more ambitious national targets are needed.
Ice sheet research a priority
The science that suggests the ice sheets would be safe if we limited warming to 1.5-2°C is new. At a Scar conference in Auckland in 2014, Naish, the director of the Antarctic Research Centre at Victoria University, said the Antarctic contribution to future sea-level rise was “one of the big uncertainties we still face”.
Since then, collaborations between climate modellers and geologists have revealed more about the “sensitivity of the polar ice sheets to very small changes in global average temperature”, says Naish. The Nature paper, written by DeConto and David Pollard of Pennsylvania State University, used a model that incorporates new features to account for the effects of surface-ice melt and ice-cliff instability and suggests that under a business-as-usual scenario – where we continue to increase carbon dioxide (CO2) emissions through to the end of this century – Antarctic ice melt alone could contribute up to 1m of sea-level rise by 2100 (ice melt from Greenland and thermal expansion of the oceans would increase total sea-level rise to 1.8m).
Another study, led by Nick Golledge from the Antarctic Research Centre and published in Nature in October 2015, differed by suggesting that under the same business-as-usual scenario, there would be more modest – but still significant – Antarctic contribution to sea-level rise of up to 40cm by 2100.
But what matters most, says Naish, is that the models agree about what happens up to the Paris treaty’s 1.5-2°C target. “They both show that if we can limit warming to 2°C, then we get minimal loss of Antarctic ice and contribution to sea-level rise.” But both models suggest there is a threshold somewhere close to 2°C above which there is significant Antarctic ice loss and sea-level rise.
Whereas DeConto and Golledge use computers to model climate and ice-sheet behaviour, Naish and other geologists focus on gathering geological evidence – in particular, sedimentary drill cores from the margins of the Antarctic ice sheets – to reveal the past climate, or palaeoclimate, of the frozen continent. This evidence is then used to test climate and ice-sheet models; if the models fit with reconstructions of past climate and ice-sheet changes, we can be more confident about their ability to accurately represent the future.
Research, including better models and more geological evidence from times in the past when the climate was warmer, is needed to identify more precisely where the threshold at which the Antarctic ice sheets start to melt lies. Another option, however, is to go all out to keep global warming below 1.5°C.
The theme of this year’s Scar conference, attended by more than 900 people from 41 countries, was “From the poles to the tropics: Antarctica in the global earth system”. As if to highlight the connection between what happens in the tropics and the fate of Antarctica’s ice sheets, during the week of the event Kuala Lumpur’s air pollution index was hovering between 50 and 60, a “moderate” but instantly noticeable level of pollution.
Local newspapers reported that it was the beginning of the annual “haze”, the result of smoke from land-clearing forest and peat fires blowing across the Strait of Malacca from Sumatra. Aside from being unpleasant, it was a reminder that deforestation, along with carbon emissions from industry, transport and fires, are all contributing to the CO2 problem that’s causing global temperatures to rise and ice sheets to start melting.
The conference included presentations from a range of Antarctic researchers – biologists, geologists, social scientists, astronomers and more – covering the six priorities for Antarctic research identified at a special Scar meeting in Queenstown in 2014. “The most important” of these research areas, says conference host Azizan Abu Samah, is to “understand how, where and why ice sheets lose mass”.
This is an area of research in which Kiwis are at the forefront. The conference was attended by 59 New Zealanders from universities, crown research institutes, Antarctica New Zealand and other agencies; Professor Christina Hulbe, from the University of Otago, gave the opening lecture. New Zealanders have a disproportionate amount of influence in Antarctic research, says Naish, because of our historical connection to Antarctica – as a staging post for Heroic Age expeditions, as one of seven countries with territorial claims in Antarctica, and as one of the first 12 partners of the Antarctic Treaty. “It’s always been a priority for our research and we’ve attracted really good researchers from around the world,” he says.
Time for action is now
Naish and DeConto arrived at the Kuala Lumpur conference from Geneva, where they’d been Scar representatives at a scoping meeting for a special report for the Intergovernmental Panel on Climate Change. The impetus for this special report, says Naish, came from the small island nations and African countries “who worry that 2°C” – the upper level of what was agreed in Paris – “is not a safe guardrail for them … it is important to know if there are some thresholds that come into play and cause dangerous climate change between 1.5 and 2°C.”
As part of the Paris Agreement, the world’s nations declared their “intended nationally determined concentrations”. “If you add up the current agreements, it gets you to about 2.7°C of warming by 2100,” says Naish. Given the current rate of emissions, if we want to keep the world below a 1.5°C increase, we all need to take action in the next five to 10 years, he says. “And this report will take three years to write. So then we’ll have two to seven years. And we’ll already be at 1.5°C.”
Records for the first half of 2016 show that global average temperatures are already 1.3°C above late 19th-century levels. As DeConto said in a Scar lecture, “we’re flirting with that 1.5°C warmer world already”.
New Zealand signed the agreement in April and Climate Change Issues Minister Paula Bennett says she expects the ratification process to be completed “within weeks”. Bennett says New Zealand’s target for 2030 – to reduce greenhouse-gas emissions to 30% below 2005 levels (equal to 11% below 1990 levels) – is “fair and ambitious and there is no intention to change or review it”. When you compare this target internationally, though, it is less than ambitious. California, for example, has agreed to a 40% reduction from 1990 levels by 2030, as has the European Union.
Crossing the threshold?
- More than 1000 parts per million (ppm) CO2 – no ice in Antarctica
- 400-1000ppm CO2 – West Antarctic Ice Sheet melts, East Antarctic Ice Sheet unstable
- Less than 400 ppm CO2 – stable Antarctic ice sheets
Palaeoclimate studies – analyses of Antarctica’s past climate and ice cover – reveal that 400ppm of atmospheric CO2 may be a key threshold for the stability of the Antarctic ice sheets. Although Nature papers by Rob DeConto and Nick Golledge hold out hope that the Antarctic ice sheet can be saved, now that we’ve topped 400ppm (Hawaii’s Mauna Loa Observatory is reporting 404ppm), many scientists believe the melting of the West Antarctic Ice Sheet, the smaller, marine-based expanse, resulting in up to 5m of sea-level rise, is inevitable.
In the 400-1000ppm range is another threshold, beyond which the East Antarctic Ice Sheet – a massive area up to 5km thick whose melting could raise sea levels by 50m – will also be lost. Finding this threshold is a focus of research efforts.
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