Describe the ocean as a major carbon sink, explain the solubility and biological carbon pumps and the role of blue carbon, and explain how a warming ocean affects carbon uptake

What this dot point is asking

QCAA wants you to explain how the ocean takes up and stores carbon, the mechanisms that move carbon from the surface to the deep, the role of coastal blue carbon, and how warming changes the ocean's ability to absorb carbon. This sits between the productivity biology of Unit 3 and the acidification chemistry of Unit 4.

The ocean as a carbon sink

A carbon sink is a reservoir that takes up more carbon than it releases. The ocean is the largest active sink, holding vastly more carbon than the atmosphere and absorbing a large fraction of the carbon dioxide humans emit each year. This uptake has slowed the rise of atmospheric carbon dioxide and the pace of climate change, but it comes at the cost of ocean acidification (covered in its own dot point). Two physical and biological processes drive the uptake.

The solubility pump

Carbon dioxide dissolves in seawater, and cold water dissolves more gas than warm water. In cold polar regions, surface water absorbs carbon dioxide and, being cold and dense, sinks as part of the thermohaline conveyor, carrying dissolved carbon into the deep ocean where it is stored for centuries. This physical process is the solubility pump. Because it depends on cold water, a warming ocean weakens it.

The biological pump

Phytoplankton in the sunlit surface fix carbon dioxide into organic matter by photosynthesis. When they and the animals that eat them die, or produce sinking waste, some of that carbon sinks out of the surface layer toward the deep sea, where it is decomposed or buried. This transfer of carbon from surface to depth by living things is the biological pump. It depends on surface productivity, so it links directly to the Unit 3 productivity and plankton material.

Blue carbon

Coastal vegetated ecosystems, mangroves, seagrass meadows and saltmarshes, capture and store carbon at high rates in their plant matter and especially in their waterlogged soils, where low oxygen slows decomposition. This stored carbon is called blue carbon. Per unit area these systems lock away carbon far faster than most land forests. The flip side is that clearing or damaging them releases that stored carbon, so protecting Australian mangroves and seagrass is now recognised as a climate strategy as well as a biodiversity one.

How warming reduces carbon uptake

Climate change weakens the ocean sink in several ways:

• Warmer surface water dissolves less carbon dioxide, slowing the solubility pump.
• Warming makes the surface layer more stratified (layered), so fewer nutrients mix up from below, reducing phytoplankton growth and the biological pump.
• Loss of mangroves and seagrass to development releases stored blue carbon.

So the ocean's great service of absorbing carbon is being undermined by the very warming it has been buffering, a feedback that makes future warming harder to slow.

Australian context

Australia holds globally significant blue carbon stores in its extensive mangroves and seagrass meadows, including those of Moreton Bay and the Great Barrier Reef lagoon. The cold Southern Ocean south of Australia is one of the world's most important regions for the solubility pump, drawing down large amounts of carbon dioxide into the deep sea.