The evidence that climate change is manmade has become "unequivocal," the United Nations' Intergovernmental Panel on Climate Change reported Friday in its latest wholesale summary of climate science. Yet while humans are already suffering dire consequences of our actions, another entity is shielding us from the full fury of a planet in flux.
That entity is the ocean, IPCC scientists say, and it's absorbing the brunt of global climate change at a significant cost to its own health. Scientists have been saying this for years, but the evidence has grown stronger since the last IPCC report in 2007. The need to publicly explain the phenomenon has also grown, since a small chorus of science doubters has seized on a recent dip in the rate of surface warming as proof of "global cooling."
To be clear, the planet is still heating up, and slower warming doesn't equal cooling. Across Earth's surface, each of the last three decades has been warmer than all previous decades since 1850, according to the new IPCC report, and 1983-2012 was likely the warmest 30-year period for the Northern Hemisphere in the last 1,400 years. The first decade of the 21st century has been the warmest of all, and global average surface temperatures are now poised to rise by another 0.3 to 0.7 degrees Celsius between 2016 and 2035.
Still, Earth's land hasn't heated up nearly as much as it might have without 320 million cubic miles of seawater to soak up 93 percent of the heat trapped by industrial greenhouse gas emissions. Initially absorbed by surface waters, that heat is increasingly moving into the deep ocean, where it may seem "hidden" but remains dangerously capable of transforming climates. Aside from killing coral, shuffling ecosystems and changing ocean currents, excess warmth causes seawater to expand, helping raise global sea levels.
"As the ocean warms, and glaciers and ice sheets reduce, global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years," IPCC co-chair Qin Dahe says in a press release about the new report. On top of that, the oceans are also taking in about a quarter of our carbon dioxide emissions, preventing additional greenhouse warming but also gradually turning themselves acidic.
"Without the immense capacity of the ocean to absorb heat and carbon dioxide, we would be experiencing much more severe climate change impacts than we see today," says Trevor Manuel, minister in the presidency of South Africa and co-chair of the Global Ocean Commission, in a statement Friday. "The clear implication is that we need to redouble our efforts to make the ocean resilient in the face of climate change and acidification."
Here are some of the IPCC's key findings about oceans and climate change, as described in a press release issued Friday by the U.N. Environment Program:
- "It is virtually certain that the upper ocean (down to 700 meters) has warmed from 1971 to 2010. The deep ocean below 3,000 meters has also likely warmed since the 1990s, when sufficient observations became available. Ocean warming accounts for most of the change in the amount of incoming solar energy stored by the Earth, accounting for about 93 percent of it between 1971 and 2010. The global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation."
- "The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. The global mean sea level rose by around 19 centimeters from 1901 to 2010 due to increased ocean warming and melting glaciers and ice sheets. The rate of rise accelerated between 1993 and 2010, and it is very likely to increase further during the 21st century and beyond."
- "Seawater has become more acidic (its pH has decreased by 0.1) since the beginning of the industrial era due to humanity's carbon dioxide emissions; it will continue to acidify during the 21st century."
As land mammals, humans may seem like beneficiaries of the oceans' heat and CO2 absorption. Not only are we still at risk from the warmth that seawater doesn't soak up, however, but our fate is also inextricably linked to that of the ocean. Coastal cities must fend off rising sea levels and stronger hurricanes, for example, while swaths of humanity will suffer as economically important marine species relocate and die off.
"Disruption to ocean life results in less food for people — that's the stark reality," says David Milliband, president of the International Rescue Committee and a co-chair of the Global Ocean Commission, in Friday's statement. "With nearly a billion hungry people in the world already, we need to take every option we can for increasing our food supply sustainably, rather than allowing climate change to compromise it."
For a more in-depth, animated look at the IPCC's projections for surface-temperature changes over the 21st century, check out this video released by NASA: