Residents in southern Australia may think about Antarctica only when a cold southerly wind affects their weather. But the importance of the Southern Ocean and Antarctica extends much further, especially in relation to climate change.
The Special Report on the Oceans and Cryosphere in a Changing Climate (SROCC), released today by the Intergovernmental Panel on Climate Change (IPCC), confirms years of accumulated evidence that the oceans and cryosphere are changing, and impacts will be far-reaching. But the SROCC goes further, highlighting recent research that shows the observed changes are happening faster than previously thought, and projections of future sea-level rise are more extreme.
CSIRO’s Dr Jess Melbourne-Thomas, Transdisciplinary Researcher and Knowledge Broker, is a lead author of the report’s polar regions chapter, and contributed to writing the report’s summary for policymakers. She says the report shows that ice around the world is melting, the oceans are warming and losing oxygen, and global sea level is rising.
“The SROCC is an update on the science since the IPCC Fifth Assessment Report (AR5) was published in 2014, and will help form the basis of the Sixth Assessment Report, due to be published in 2022,” she says. “It’s particularly important because of the rapid rate of change we’re seeing in the cryosphere, the importance of the oceans in regulating global climate, the threat of sea-level rise to coastal communities, and the dependence of the world’s population on ocean ecosystems.”
Scientists in the 1980s predicted that climate change would have significant impact on polar regions in the 21st century. These changes have now been confirmed, but the changes are occurring faster than expected, with ice loss from Greenland and Antarctic glaciers and ice sheets accelerating. Dr Melbourne-Thomas says the SROCC identifies there may be irreversible tipping points in these changes.
Metres of rising seas
Antarctica holds about 90 per cent of all the ice on Earth, a volume equivalent to 58 metres of sea-level rise. Dr Steve Rintoul from CSIRO’s Centre for Southern Hemisphere Oceans Research and a lead author on the AR5’s chapter on ocean observations, reviewed chapters of the SROCC and says a tipping point may be crossed in the coming decade or two that will commit us to metres of additional sea-level rise.
“Scientists have focused on West Antarctica, where rapid ice loss has been observed in recent decades, but most of the ice is in East Antarctica. The latest research reveals that East Antarctica is also exposed to warm ocean waters. This means the large East Antarctic ice sheet is also vulnerable to ocean warming and will contribute to future sea-level rise.”
He says there’s even stronger evidence now that ice sheets can change rapidly enough to contribute to sea-level rise on timescales of decades to centuries – researchers used to think it would take many centuries or thousands of years.
“The vulnerability of the Antarctic ice sheet to warming of the surrounding ocean is becoming clear. Since AR5 we’ve seen more evidence that Antarctica’s contribution to sea-level rise is accelerating as a result of warm ocean waters reaching the edge of Antarctica. The speed with which Antarctica and Greenland will contribute to sea-level rise is greater than we thought possible even five years ago.”
The problem, he explains, is ice sheets that rest on bedrock sloping up toward the coast are unstable: once retreat begins, it will continue until the ice is gone or the bedrock changes slope. This process is irreversible and will continue even if greenhouse gas emissions are reduced.
“Evidence that this marine ice sheet instability is already underway was published just after AR5, so that new science is included in the SROCC. There is evidence that there are tipping points, and we may already have passed those thresholds for some parts of the Antarctic ice sheet.”
Dr Rintoul says the more the Earth warms, the greater the risk of passing that tipping point threshold for other parts of the ice sheet.
“Metre-scale rises in sea level will take decades to centuries to occur, but we are committing ourselves to that sea-level rise within the next 20 years. If nations don’t take strong action to reduce emissions in the next decade, we will commit ourselves to warming that will cause several metres of sea-level rise. We can’t change our mind later on.”
Fast-changing poles affecting Australia
Antarctica and the surrounding ocean affect the Earth’s climate and weather. The Southern Ocean encircles the globe, connecting the Atlantic, Pacific and Indian oceans, with the world’s largest current, called the Antarctic Circumpolar Current, largely driving the resulting global circulation. The Southern Ocean also absorbs most of the extra heat and carbon dioxide released by human activities, slowing the pace of climate change.
“Sea ice formation in Antarctica is an important driver of the big ocean conveyor belt called the thermohaline circulation, where water sinking in Antarctica stores and distributes heat and carbon,” says Dr Melbourne-Thomas. “Substantial change in sea ice formation has implications for that mechanism, and could ultimately influence climate patterns in Australia.”
She says the SROCC describes an observed increasing intensity and frequency of marine heatwaves. “The concept of a marine heatwave only emerged recently, but they are now becoming common and are found worldwide. Australia has been hit by marine heatwaves that have caused bleaching on the Great Barrier Reef, caused loss of sea grass beds off Western Australia, and impacted kelp forests and the aquaculture industry in Tasmania.”
The rapid increase in marine temperatures is also causing species to move towards polar regions, which affects fisheries and has subsequent impacts on the marine resources that people depend on.
“The increasing rates of species redistribution driven by climate change that we’re seeing in the marine environment is a key theme coming out of the SROCC,” says Dr Melbourne-Thomas. “With a general trend of species moving from tropical regions to the poles, the Southern Ocean is likely to gain species and we may see new fisheries emerging as fish stocks move south. This has important implications for management.”
“However, Antarctic species adapted to cold habitats don’t have a lot of space to move into. For example, the habitat for Antarctic krill is contracting southwards, which affects krill populations and hence the numbers of penguins, seals and whales that feed on krill. These ecosystems are complex, so it’s not just the individual species moving; it’s the effect on food webs and ecosystem function in ways that we’re only just beginning to understand. So it can be difficult to say how it will play out.”
What can we do?
Dr Melbourne-Thomas says the SROCC summarises options to reduce the impacts on polar and ocean environments, and how to adapt to the changes already happening. “There are key findings regarding timescales for responses, and the scales of decision making. That will link to the specific assessments of risk and response in the Sixth Assessment Report.”
She says developing ecosystem models to explore adaptation options is one area where CSIRO science helps understand how we can respond. “Governments, industries and communities need to manage direct human impacts from fishing and coastal runoff along with these emerging climate change challenges. CSIRO scientists provide end-to-end ecosystem modelling, risk assessment and adaptation frameworks to assess different options under different scenarios, the costs and benefits of different kinds of actions, and the consequences of the choices we make. CSIRO is really ahead of the game in developing frameworks to help respond to these challenges.”
Dr Rintoul adds that another important aspect of CSIRO research is observing the climate system to track how it’s evolving. “This guides decisions as it gives confidence in understanding whether changes are natural or caused by human activity. CSIRO scientists are also studying how we can reduce emissions by changing energy systems and other aspects of how we live, and how we can adapt to the changes we can’t avoid.”
He says there is optimism that it’s not too late. “We have time to reduce emissions and the consequences of climate change that we want to avoid. But we do need to act quickly. If we do, there are many opportunities for the Australian economy that involve helping us and others transition to a low-carbon future.”
Steve Rintoul is a researcher in Project 2.4 Changing oceans and Australia’s future climate and Project 5.7 Tracking ocean change– ocean observations and models.