Eloise Gibson is Newsroom's environment and science editor. She's written for the New Zealand Herald, Stuff.co.nz, The Listener, and BBC Future.com. Twitter: @eloise_gibson.

environment

IPCC: Ocean’s future depends on emissions

The ocean has protected us from experiencing even worse effects from global warming, but changes to fisheries, coasts and cyclones are beginning to bite. What happens next depends on us, says the latest IPCC special report.

The state of the ocean will enter “unprecedented territory” this century, and it will take an unprecedented social transformation to stop things getting worse from there, according to the latest IPCC special report.

The Intergovernmental Panel on Climate Change’s Special Report on the Ocean and Cryosphere is out, drawing on more than 6,000 studies, reviewed and synthesised by a panel of 104 scientists from 36 countries.

The fate of the Antarctic ice sheet and the Southern Ocean – two areas of intense research and monitoring by New Zealanders – feature heavily in the report’s gloomier findings, regarding ocean heating around Antarctica and the potential for surprise runaway ice melt.

The report’s key messages are that we’ve already locked in significant changes to ocean levels, cyclones, fish stocks, glaciers and beaches, but we can avoid more extreme changes by acting fast. That would require "unprecedented" social change, though.

The cryosphere - what is it and why is it news?

The “special” IPCC reports are designed to give governments more focused and updated information in between full assessment reports.

The good thing about the IPCC’s gargantuan, collaborative full assessment reports is that the findings are detailed and based on a solid consensus between hundreds of experts globally.

The downside is that each report takes many years to prepare and reach agreement on. The last full IPCC report (known as AR5, or the fifth assessment report) was finalised in 2014. The next one, AR6, isn’t due until 2022.

Since each report relies on a rolling body of hundreds of individual research papers, some of the studies relied upon will be several years old by the time an assessment report comes out, and even older by the time there is a new assessment eight or so years afterwards.

Special reports are designed to help fill the gaps, and they focus on issues governments are particularly interested in.

This is the third and final of three special reports slotted in between AR5 and AR6: the others were on land use and keeping global warming to 1.5C.

Oceans and the cryosphere (icy and snowy parts of the world) was an obvious candidate for an update, since polar ice melting has been accelerating and the science around Antarctica’s potential decline was not as solid as researchers would have liked back when the previous full report, AR5, came out.

One of the significant changes since 2014 is that scientists now feel confident enough in their models to increase the upper bounds of melting from Antarctica, if emissions keep going as they are until the end of the century.

A third of Antarctica’s ice sheet sits below sea-level, making it vulnerable to widespread collapse as the world around it warms. The new report highlights the need for more work to understand the risk, given Antarctica alone could raise the sea by several metres, not including melted ice from Greenland and the world's glaciers.

However, the latest report concludes there is still a chance to keep sea level rise to within 1m by 2300 – still a costly level for New Zealand, but manageable compared to what will happen if emissions stay on their current, high track.

Our ocean saviour

If you’re worried about the effects that global heating is having already, give thanks to the ocean next time you’re passing: it could have been much worse for us land-dwellers.

The ocean works as an enormous buffer against global warming. Over 90 percent of the extra heat humans have added to the climate system with greenhouse emissions has gone into the ocean, the report says. And while this has saved global temperatures from rising as much as they otherwise would have, it has also warmed the ocean.

A startling proportion (an estimated 45-62 percent since 2005) of ocean warming has been concentrated in the Southern Ocean, which surrounds Antarctica and its vast ice shelves like a chilled-but-slowly-heating bath.

Alongside a general trend of ocean warming, marine heatwaves globally have doubled in frequency since the early 1980s and grown more intense, too, the report says.

Not only is the ocean absorbing physical warmth, it is doing double duty as a climate change sponge by mopping up much CO2 from the air, stopping the gas from warming the climate. An estimated 20-30 percent of people’s carbon dioxide emissions since the 1980s have been sucked in by the ocean.

But the extra heat and CO2 is changing the water.

The added CO2 has acidified the top layer of water, most likely causing oxygen levels to drop, the report says. The ocean's changing composition can cause problems for shell-making creatures, who need a comfortable PH level to build their hard bodies. Meanwhile warming is combining with nutrient and sediment run-off from land to create the conditions for more algal blooms.

The ocean will keep absorbing more heat, but how much it takes on depends on emissions, the report says.

By 2100, the ocean will take up 2 to 4 times more heat than it did between 1970 and the present, if global warming is limited to 2°C. That rises to up to 5 to 7 times as much, if emissions aren't curbed.

Ice melt and sea level rise today

The ocean has helped us by taking heat out of the atmosphere, but in the process the ocean has expanded, because warmer water takes up more space.

Thermal expansion of the ocean has contributed almost as much to sea level rise today as melting ice has (though ice still leads).

Meltwater and thermal expansion together pushed sea levels up 3.6mm a year in the decade to 2015, a rate the report says is “unprecedented over the last century.”

That rate is accelerating.

Around 10 percent of Earth’s land area is covered by glaciers or ice sheets, and as they melt they add water to the ocean.

Glaciers are smaller and have melted quicker to start with, but, as they shrink, accelerating sea level rise is being driven by the much larger ice masses near the poles. Melting on the surface of the Greenland ice sheet, and the rapid thinning and retreat of glaciers draining the West Antarctic Ice Sheet drove much of the recent acceleration, the report says.

Ice loss from the Antarctic ice sheet likely tripled in the decade between 2007 and 2016, compared to the previous decade, while ice loss from Greenland doubled over the same time.

“Acceleration of ice flow and retreat in Antarctica...[was] observed in the Amundsen Sea Embayment of West Antarctica and in Wilkes Land, East Antarctica”, says the report’s summary.

“These changes may be the onset of an irreversible ice sheet instability.”

Then there’s melting sea ice. (This doesn’t directly raise sea levels in the same way as land ice, because sea ice is already floating in the ocean. It does, however, worsen warming by making the sea's surface darker and more heat-absorbent). The trend for Antarctica isn't clear. But in the north, things are changing rapidly: “Between 1979 and 2018, Arctic sea ice extent has very likely decreased for all months of the year…sea ice changes in September are likely unprecedented for at least 1000 years,” the report says.

Future melting

The report lays out some stark differences between the changes we'll see to oceans at 1.5C of warming, compared with 2C, or over 4C.

For example, if global warming is stabilised at 1.5C above pre-industrial levels, the Arctic ocean would only be ice-free in September – the month with the least ice – once in every hundred years. For global warming of 2C, this formerly icy ocean would be ice-free up to one year in three, the report says.

The report also highlights that significant amounts of sea level rise and ocean warming is already locked in, no matter how much people cut emissions. Those changes will need planning-around to help people cope, it says.

Sea level rise could reach around 30-60 cm by 2100, even with the sharpest cuts to greenhouse gases, the report says.

If emissions keep rising as they are, seas will likely be 60-110cm higher by 2100 (by which point temperatures would likely have risen about 4.3C). That includes a bigger potential contribution from Antarctica than what was previously projected in AR5, because there’s now more certainty about how the ice sheet could react.

There's still “major uncertainty” about the future of Antarctica, and when it could tip into instability, the IPCC authors say. “Considering the consequences of sea level rise that a collapse of parts of the Antarctic Ice Sheet entails, this... merits attention.” (This is an effort New Zealand scientists are already working on).

Under the high-emissions, business-as-usual, scenario, glaciers will be all but gone in much of the world by the century's end, it says: “Regions with mostly smaller glaciers (e.g., Central Europe, Caucasus, North Asia, Scandinavia, tropical Andes, Mexico, eastern Africa and Indonesia) are projected to lose more than 80 percent of their current ice mass by 2100."

Beyond 2100

The report gives a sense that it will not be long before the oceans reach unchartered territory, at least on human timescales.

“Over the 21st century, the ocean is projected to transition to unprecedented conditions,” the report says.

But the really big variations in how high the seas could go will come after 2100.

Even if nations meet the 2C (or less) Paris target, sea level rise will continue beyond 2100, regardless, because of the warming (and melting) past emissions have already committed us to. Other oceanic changes will also continue, including sea warming.

Having taken a while to get started, compared to glaciers, the Greenland and Antarctic Ice Sheets are projected to melt increasingly quickly throughout the 21st century and beyond. The rate and magnitude of change will worsen further in the second half of the 21st century if emissions kept going as they are, the report says.

That would result in several centimetres a year of sea level rise -- compared with millimetres per annum today -- and a multi-metre rise over the longer term.

As well as rising, the ocean will continue to warm throughout the 21st century. With un-tamed emissions, by 2100, the top 2km of ocean is projected to take up 5-7 times more human-induced heat than it has already, while marine heatwaves would grow 50 times more frequent than they were in pre-industrial times, the report says. They'd also be ten times more intense.

Cyclones and coastal hazards

Followers of sea level news will have seen studies showing extreme sea levels (the levels typically reached when a huge storm hits on top of a high tide) are projected to get much more frequent in places like Wellington with only modest amounts of sea level rise.

The projections in the latest IPCC report say this will be a global issue: local, super-high sea levels that historically occurred once a century are projected to occur annually at many locations by 2050, even under the best emissions scenarios, especially in tropical regions.

The reason for more regular super-surges is that rising seas boosts the high tide line, meaning it takes a smaller storm surge on top of high tide to reach the same distance inland as a whopper storm surge would today. Today's whopper surge, coming on top of the new high tide levels, will reach even further inland.

The latest IPCC report says extreme sea levels will be exacerbated by increases in tropical cyclone winds and rain, and increases in extreme waves in some places. All this is especially risky for small island states, for obvious reasons.

The proportions of tropical cyclones reaching Category 4 and 5 are projected to increase with even a 2C global temperature rise, and the impact on coastal communities will be further worsened by higher sea levels – even if cyclones don’t become more frequent.

Shrinking buffers

The report talks about how people need to start adapting to oceanic changes, to avoid the worst costs and impacts. The smaller the impacts, the more likely they are to be manageable using natural coastal defences, such as mangrove planting and stronger dunes, it says.

Unfortunately, some of the natural barriers to coastal flooding – such as wetlands – are disappearing just when they are most needed.

Nearly half of coastal wetlands have been lost globally over the past 100 years, the report says, as a result of human pressures, sea level rise, warming and extreme climate events. Efforts to replant mangroves and other plants are succeeding in some places, but overall these natural barriers are at risk.

Fewer fish

Fisheries is one area where the IPCC's news is not all bad, since fish stocks are projected to rise in some places, especially in colder latitudes.

However the overall trend for kaimoana, corals and other sea creatures is not good.

The report says fishers are already seeing smaller catches in many regions because fish and shellfish stocks have dropped due to the direct and indirect effects of global warming (over-fishing is of course involved, too).

Fish and other marine species have been moving from the equator towards the poles in response to lower oxygen levels in the water, warming water and sea ice loss in their natural habitats -- expanding the range of some species that can now move closer to the Arctic and Antarctica, but shrinking the habitats of others, including the Southern Ocean habitat of Antarctic krill, an important staple food for penguins, seals and whales.

Declining fish stocks will be felt most in the tropics, the report says: “A decrease in global biomass of marine animal communities, their production, and fisheries catch potential, and a shift in species composition are projected over the 21st century in ocean ecosystems from the surface to the deep seafloor under all emission scenarios."

Ocean acidification and marine heatwaves will add to the stress, hurting some shellfish and crustaceans and bleaching coral, it says.

Then there are projected increases in harmful algal blooms, which are already getting more frequent because of a mix of climate impacts and nutrient run-off from land.

Wholesale changes?

The report comes with a note from IPCC leaders emphasising the enormous social and structural changes to energy and others aspects of life that are needed to duck the worst scenarios -- and what a difference they could make, still.

The ocean's changes will be felt on land, even by people far away from the shore, said Hoesung Lee, Chair of the IPCC, announcing the report’s release. “The open sea, the Arctic, the Antarctic and the high mountains may seem far away to many people,” he said. “But we depend on them and are influenced by them directly and indirectly in many ways – for weather and climate, for food and water, for energy, trade, transport, recreation and tourism, for health and wellbeing, for culture and identity.”

Lee said: “If we reduce emissions sharply, consequences for people and their livelihoods will still be challenging, but potentially more manageable for those who are most vulnerable.”