Feature: Marine ecosystems may suffer irreparable harm unless carbon emissions further reduced

Authorities must reduce carbon emissions beyond targets agreed at the Paris climate talks or risk impacting the globe's most precious resource beyond repair, global marine scientists warn.

The global marine ecosystem is arguably the world's most precious resource, providing over 1 trillion U.S. dollars per year to the global economy via direct and indirect exploitation.

Since the industrial revolution, the world's oceans have absorbed approximately 30 percent of all carbon dioxide released into the atmosphere, playing a critical role in slowing the rate of global warming.

However, a price has been paid.

Increasing carbon dioxide emissions from industrialisation, the main cause of climate change, has led to lower sea-water pH and a reduction in carbonate ions needed to help protect keystone species from increased predation and sustain productive marine ecosystems.

Despite global leaders agreeing to reduce carbon emissions to limit human-induced global warning to a two-degree Celsius rise at the Paris climate talks in December (COP21), the ocean's chemistry will still significantly change.

"The rate at which the ocean is changing is directly proportionate to the atmospheric carbon dioxide levels," Dr Andrew Lenton, a senior researcher in the Oceans and Atmospheric department of Australia's chief scientific body, the CSIRO, told an online media briefing on Monday.

Lenton and 350 of the world's most influential marine scientists are meeting in Australia this week to discuss the prospects for understanding and monitoring the growing issue of the changing oceanic chemistry.

It's the first time the International Symposium on the Ocean in a High-CO2 World, held every four years, has been hosted in the southern hemisphere.

Combining the stress of ocean acidification and ocean warming, marine ecosystems are much more sensitive to the impacts than either one of those two stressors alone, but the tipping point -- the point where impacts of stress are irreversible -- is still being ascertained.

"Many (marine) species are sensitive below those COP21 targets, so from a species level, we can say yes we have significant impacts," U.S. National Oceanic and Atmospheric Administration (NOAA) senior scientist Dr Richard Feely said.

"We would be much better off if tried (for) a 1.5 degree (Celsius) change rather than a two-degree (Celsius) change, it would have significant improvements."

"But it's still a little bit harder for us to say what the overall ecosystem response would be (to ocean acidification), but in most cases we see a very significant impact below that two-degree level of COP21."

Ocean acidification has already reduced the ability of the marine's most productive ecosystems to grow and reproduce. The entire marine ecosystem is therefore impacted in terms of reproductive health, species composition, distribution, food web structure and nutrient availability.

What's expected globally from the dropping pH levels by the end of the century is already occurring on the U.S. west coast where localised upwelling -- brining carbon dioxide and nutrients up from the deep -- has combined with atmospheric carbon dioxide, Feely said.

"The impact of that has been quite striking," Feely said.

Ocean acidification has caused the shells of marine snails, a keystone species and primary food source of the U.S. west coast's salmon fishery, to dissolve, Feely said.

The weakening of the shells, which Feely said had increased by 30 to 40 percent since pre-industrial times, makes the marine snails more susceptible to predation, causing abundance loss, directly impacting the fishery.

"We are seeing these kinds of impacts directly related to changing (carbon dioxide) concentrations in species that are very important to the economic success in our region," Feely said.

Coral reefs themselves, one of the world's most productive ecosystems providing habitat and nursery grounds for many key commercial fisheries as well as economic benefits for tourism operators, have also seen their growth rate slow by 7 percent since the industrial revolution.

"(Now) one of the things that scientists are trying to do is work out how the barrier reef might be affected by ocean acidification in addition to other stressors such as temperature, nutrient run off and crown of thorns," University of Tasmania physiological ecologist Associate Professor Catriona Hurd said.

Macro invertebrates, the keystone of the ocean's food web, aren't the only organisms susceptible to ocean acidification.

Recent research suggests by 2050, fish and other marine animals will also experience the affects of hypercapnia, or "intoxication," much earlier than previously predicted.

Hypercapnia causes fish and other marine animals to loose their sense of direction due to high levels of carbon dioxide, impacting their ability to find their area of reproduction, therefore having large-scale implications on global fisheries and seafood security.

On a localised scale to minimise impacts, many area are replanting or reforesting seaweed beds which help raise the surrounding water's pH levels via photosynthesis, creating a "much more conducive habitat for things like shell-fish" to calcify, Hurd said.

Scientists are also looking to environmental engineering to increase the pH, essentially artificially changing water chemistry, which has shown promising results in localized areas.

There are also proposed large-scale schemes that seek to investigate adding large amounts of alkalinity into open oceans to act as a buffer to acidification-related impacts, Lenton said.

However, the key to stop increasing global ocean acidification and its associated impacts is to significantly increase uptake of renewable energy, keeping carbon locked in the ground and not dispersed through the atmosphere. Endit