Thursday, November 12, 2015

Deep Ocean: Climate change’s fingerprint on this forgotten realm

From overfishing to mineral mining, we humans subject our oceans to myriad stressors. Many hundreds of metres below the surface, the deep sea isn’t untouched by climate change either.

With the global oceans getting warmer and more acidic as they absorb heat and carbon dioxide, habitats and food webs are changing with them, a new perspective in the journal Science explains. The paper is part of a new special issue, looking at the oceans and climate change.

But while the consequences of biodiversity loss will affect us all, the deep sea exists in a “policy vacuum”, the authors argue. Next month’s climate summit in Paris needs to serve as a forum to get the deep ocean more attention on the world stage, they say.

Prof Nadine Le Bris, a marine chemist and co-author on the new paper, tells Carbon Brief:

So far, the attention has been mostly directed on terrestrial environments and quite recently coastal areas. There are still big gaps, particularly the open ocean and ocean floor ecosystems.

Feeling the heat

Most of the habitable space on earth is not on land, today’s paper begins. More than 90% of it is in the deep ocean.

Over the time we’ve been industrialising, the oceans have absorbed about a third of the carbon dioxide that’s gone into the atmosphere. That has slowed the pace of atmospheric warming to some extent, but it has come with consequences.

Carbon dioxide dissolves in seawater to form carbonic acid. With a typical pH ranging between an alkaline 7.8-8.4, this extra acid has pushed the pH of seawater down by 0.1, on average.

The oceans have also taken up more than 90% of the heat trapped by greenhouse gases in the atmosphere, raising temperatures at the surface by 0.1C per decade since at least the 1970s.

But some of the heat finds its way much deeper. Seawater at about 700m has warmed by about 0.015C per decade since the 1970s, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC). Measurements from 700m down to 2,000 metres suggest that section of the ocean has warmed in the last decade or so, too (though the data doesn’t extend before that).

The amount of oxygen in seawater has dropped in many parts of the global ocean, leading to the expansion of what are known as “oxygen minimum zones.” This is partly because warmer water holds less oxygen, but the main reason is that warming makes ocean layers less inclined to mix, meaning less oxygen is transported from the surface to deeper layers.

Read more:

Author: Roz Pidcock

Image: Deep water corals provide habitat for other species (Lacaze-Duthiers canyon, Western Mediterranean Sea, 520 m depth). Credit: LECOB_Chaire UPMC-Foundation Total