Rapid increases in human-induced carbon emissions are causing the oceans to suffer in profound ways, and the problem is getting worse.
In a new review published in the journal Current Biology, Andrew Brierley of St. Andrews University in Scotland and Michael Kingsford of Australia's Cook University write that if warming continues at these levels — not seen in 56 million years — the planet will experience a 4.6-foot rise in sea levels by 2100 and a near-total disappearance of the ice-covered polar seas by 2020.
The loss of most coral reefs by 2040 is also looking increasingly plausible. Skyrocketing emissions are making the world's seas dangerously acidic, and ocean acidification is irreversible for tens of thousands of years.
"Given how essential the oceans are to how our entire planet functions, it is vital that we intervene before more tipping points are passed and the oceans go down the sort of spiral of decline we have seen in the world's tropical forests and rangelands," the authors say.
The study, Impacts of Climate Change on Marine Organisms and Ecosystems, also details why unchecked climate change is pushing the Earth's fisheries to the edge of collapse and threatening human food security.
For every 1-degree Celsius increase in temperature, ocean oxygen levels are said to decline by about 6%. "It is not unreasonable to expect ... warming of 5.5 degrees by the end of this century," the authors warn.
If that nightmare were to come true, the oceans would be starved of vital oxygen. Areas in the seas that are already low in oxygen would grow by at least 50%. The losses in fisheries would be devastating.
The oceans presently provide about 16% of human animal-protein food and contribute about 63% in financial terms to global ecosystem services, the study says.
Brierley and Kingsford compared current greenhouse gas emissions with those forecast by the UN Intergovernmental Panel of Climate Change (IPCC) in their 2007 report. Back then, the IPCC predicted a "worst-case scenario" that would cause CO2 in the atmosphere to increase by 2 parts per million (ppm) each year.
We are now exceeding that. The latest review affirms that atmospheric CO2 concentration — now at 385 ppm — is rising at a rate of 2.5 ppm a year.
It has generally been believed that a CO2 level of 450 ppm is the danger point beyond which catastrophic and irreversible climate change might occur. That assumption is changing.
Research by 10 of the world's leading climate scientists published in November 2008 in The Open Atmospheric Science Journal made the case that the planet would probably need an ultimate target of 350 ppm to stave off climate disaster.
To get there seems politically impossible. Governments would need to enact ultra-sharp emissions cuts now, including a total phase-out of dirty coal over the coming two decades, on top of reforestation and other measures to retain soil carbon.
NASA climate change scientist James Hansen, co-author of the November report, has said we have no choice but to act with that kind of fierceness,
if "humanity wishes to preserve a planet similar to that on which civilization developed."
But what of the world's oceans? Is it too late to preserve them?
Surface waters of the seas naturally absorb CO2 that accumulates in the atmosphere above them. The problem is that as more CO2 gets concentrated, the more acidic the oceans become. And ocean acidification is now up 30 percent since the industrialized revolution. As the Current Biology study notes:
Over the past 200 years, the oceans have absorbed approximately half of the anthropogenically-generated CO2 and at present a further approximately 1 million tonnes of CO2 diffuse in to the world ocean per hour.
What to do?
"In the face of such terrifying changes, even large scale interventions such as establishment of very large networks of Marine Protected Areas are unlikely to be effective," Kingsford says. "On a global scale, an immediate reduction in CO2 emissions is essential to minimize future human-induced climate change."
Their findings, and an accompanying plea for global warming mitigation, are supported by other peer-reviewed ocean research.
In a statement released on June 1, the national science academies of 69 nations, including those from the U.S. and China, warned the whole world that the oceans are reaching a point where they can no longer buffer the effects of CO2. They write:
"If atmospheric CO2 is stabilized at 450 ppm, only a very small fraction (~8%) of existing tropical and subtropical coral reefs will be surrounded by [waters favourable to coral growth], and at 550 ppm, coral reefs may be dissolving globally. Cold water corals are also vulnerable and are likely to be affected before they have even been fully explored. By 2100, it has been estimated that 70% will be in waters unfavourable for growth."
The national science academies issued their own appeal for action, in advance of the December climate change conference in Copenhagen. They called for cuts in greenhouse gas emissions of 50% below 1990 levels by 2050, followed by steeper cuts thereafter:
"Even with stabilisation of atmospheric CO2 at 450 ppm, ocean acidification will have profound impacts on many marine systems. Large and rapid reductions of global CO2 emissions are needed globally by at least 50% by 2050."
At the G8 meeting in Italy in July, the G8 countries accepted a so-called aspirational goal of cutting emissions 80% by 2050. In a setback to industrialized nations, though, developing country leaders have so far refused to endorse the 50% by 2050 target being asked of them.