EXPERT REACTION: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate

The report in my mind is perhaps the most comprehensive assessment of the impacts of climate change caused by humans, around the oceans in particular but also high mountains and polar regions and on sea level and extreme events, that we’ve ever had.

The assessed literature is about 7,000 citations. It has over 100 authors and probably has contributions from the research community, in oceanography and sea level and land surface, from another 250 to 300 people. It really has mobilised an extraordinary community of climate scientists to work on this report.

I think the really key message from the report is that there are options and ways and means to respond to climate change but there are limits in adaptation and consequently choices to be made around the sorts of emission pathways that society might choose as well. It’s very clear that the lower emissions pathway has many more benefits relative to a high emissions pathway.

It’s not just the physical oceanography that’s covered by this report. It’s also the impacts on ecosystems, impacts on the services that the environment provides humans in general and more generally for the wider community. These things, when you line them up and connect them up there’s a lot of evidence already that the climate system has impacted on those services.

Just to talk very specifically about my chapter… My chapter was very much about the changing oceans and the narrative of that chapter is from thinking about the physical change that is caused by humans: how it drives ecosystems and the response of those ecosystems, and how those ecosystems which support us humans, what we call human services and ecosystem services, are affected. Finally the report finishes up on topics around governance.

One of the key conclusions that comes out of this report is that it’s virtually certain that the global ocean is continuing to warm up. The rate of warming has doubled since 1993 so the ocean is continuing to act as a sink for all the excess heat trapped by greenhouse gases in the atmosphere. Marine heatwaves - which are a relatively new phenomenon in our understanding and thinking, although they’ve obviously existed before - they’ve clearly doubled since 1982 and amazingly the intensity of those heatwaves and the duration of those heatwaves is tending to increase as well. There’s some really nice examples of those heatwaves, which actually illustrated one of the underlying chapters, around the consequences for Tasmania - the 2015/16 spring drought and autumn flood, and also around the disease in oysters as well.  So these marine heatwaves are important.

The ocean is continuing to take up CO2, it’s continuing to acidify. We have a story of how that acidification, for the high emissions scenario, would lead to something like an additional 90% of ocean acidification.

Something that’s really new in this report and drawn out, it exists in the research literature: it’s very clear that there’s a coherent pattern of a change in oxygen concentration. Oxygen of course is what fish and life need to breathe. Below the light zone, deeper than 200 metres, there’s a distinct decrease in oxygen concentration. We think this is driven by the stratification of the ocean caused by ocean warming.

Something that’s particularly relevant to Australia is the note in this report about how the Southern Ocean is sucking up heat. Since the 1970s it’s actually stored 35-43% of the total heat in the upper ocean, and that share has actually increased since 2005 through to the present. The other thing that’s kind of curious is how the deep ocean is warming and particularly the emergence of that strong signal in the Southern Ocean.

It’s true that global sea level is rising and extremes in global sea level are changing rapidly. Perhaps the most important thing to note from this report is the acceleration in recent decades of sea level and that acceleration in sea level has been created by increased rates of loss from Greenland and Antarctica. The Antarctic and Greenland ice sheets, which are so far away from where most of us live, are having impacts, and an increasing impact, on the rise in sea level. In fact, those two components plus glaciers are now bigger than ocean thermal expansion.

The IPCC SROCC report confirms that sea level has been rising at an accelerating rate during the 20th and early 21st centuries.  Over recent decades, major contributions to this acceleration have come from more rapid loss of mass from the Greenland and Antarctic ice sheets.  These are in addition to the ongoing accelerations from ocean thermal expansion and glacier mass loss. 

The dominant cause of sea level rise since 1970 is our release of greenhouse gases. 

For strong mitigation scenarios (RCP2.6) the sea level rise in 2100 is projected to be 0.43 m (0.29–0.59 m, likely range). However for continued growth in emissions (RCP8.5) the projected sea level in 2100 is projected to be 0.84 m (0.61–1.10 m, likely range).  The projected rises for RCP8.5 are about 0.1 m larger than in the AR5 as a result of improved and larger estimates for the flow of ice from Antarctica into the ocean. 

These projected rises lead to a very significant reduction in the return period of extreme (high) coastal sea levels such that at many locations what is currently a 1 in 100 year sea level could happen at least once per year by 2100. 

The choices we make now about greenhouse gas mitigation have consequences not only for the 21st centuries but for many centuries to come.  For RCP2.6, the rise by 2300 may be limited to about 1 m but for RCP8.5 it is projected to be in the range 2.3 m to 5.3 m and continuing to rise rapidly for many centuries. 

Ocean and ice environments around the world are changing at an unprecedented rate, and many of these changes will accelerate into the future. Changes in these environments – which include sea level rise, ocean warming, melting ice and snow, and loss of oxygen in the surface ocean – have profound consequences for ecosystems and for human communities globally.

The oceans and cryosphere support services ranging from food supply and cultural values, through to tourism and coastal carbon sequestration. Findings from the SROCC report show that climate-driven changes in the physical environment are negatively impacting these services in all regions of the ocean, with the exception of the polar regions where there are mixed positive and negative impacts.

The polar regions are important on a global scale. The Southern Ocean in particular plays a key role in the earth’s energy budget, and about 90 per cent of all the ice on Earth is found in Antarctica. The IPCC-SROCC report includes even stronger evidence than before 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.

Ocean warming is also driving large-scale redistribution of marine species in the world’s oceans. In general, these species are moving away from the tropics, towards the poles. This means that the Arctic region and the Southern Ocean are gaining species, but also means that species which are highly adapted to cold, polar environments have little space to migrate to. We are already seeing very significant changes to ice and ocean ecosystems due to climate change.

We have never had such clear and almost real-time information on the state of climate and of the planet.

Report after report, including this latest IPCC special report, shows a consistent picture of climate change coming fast and at an accelerating pace. Some of the impacts are as expected and previously projected. For others, they are coming faster and punching harder than we had anticipated.

There is absolutely no doubt that we are at a critical point in time of intensification of climate change impacts, and we need to begin deploying strategies for climate change adaptation. And above all, greenhouse gas emissions need to peak immediately and come down at a rapid pace if we are to avoid the worse impacts of climate change.

This report echoes conclusions familiar from previous IPCC reports.

One new thing is that we can now say with some confidence that Arctic sea-ice decline is unprecedented in at least 1,000 years, which is some of the clearest evidence yet that human impacts on climate already dominate over anything natural.  

Also, this report examines expected changes out to the year 2300, when sea level rise is projected to be 3-4 metres without mitigation efforts, but could be kept under one metre with strong mitigation efforts.

There are possible responses to climate change that generate win-win options, where adaptation can reduce greenhouse gas emissions, as well as enhance social, economic and environmental outcomes.

But this requires informed, ethical, timely and strategic decision making. We are not in this space yet.

The report serves as a wake-up call to the world about the devastating consequences of failing to act to address climate change. We have no time to lose.

Australia depends on the ocean that surrounds us for our health and prosperity.

But that ocean is suffering from the effects of climate change that are playing out here and in the farthest reaches of our planet.

Globally, by 2050, more than one billion people will live on coastal land that is less than 10 metres above sea level, and will be exposed to combinations of sea level rise, extreme winds, waves, storm surges and flooding from intensified tropical cyclones.

Their future looks dire if we do not act to limit further climate change.

Australia’s coastal cities and communities can expect to experience what was previously a once-in-a-century extreme coastal flooding event at least once every year by the middle of this century – in many cases much more frequently.

But even if we act now, some changes are already locked in and our ocean and frozen regions will continue to change for decades to centuries to come, so we need to also make plans to adapt.

In Australia, adapting coastal communities to unavoidable sea level rise is likely a priority. There are a range of possible options, from building barriers to planned relocation, to protecting the coral reefs and mangroves that provide natural coastal defenses.