Maine Climate Office News

Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part II)

The Solubility Pump: An Oceanic Cooling System

In a paper published earlier this week in Nature Geoscience, oceanographer Galen McKinley and her research team reveal that rising North Atlantic ocean temperatures are dampening the ocean’s ability to absorb atmospheric carbon dioxide. They present data from the past 29 years, long enough to overcome natural oscillations in the ocean system and expose a real trend. Since the ocean is one of the most important global carbon sinks, absorbing a quarter of all atmospheric carbon emissions, these findings have significant implications for the Earth’s carbon cycle and future climatic system.

But why does a warmer ocean reject carbon dioxide? For the same reason that drinking a lukewarm carbonated beverage is generally underwhelming. Lacking the crispness that we crave in such refreshments, these drinks have been subjected to gas laws that make it hard for warm water to keep gas in solution. As a result, they lose their bubbles to the atmosphere and become ‘flat’.

Even if kept cool, beverages eventually lose their fizz, and their appeal, to the atmosphere. This is because, according to Henry’s Law, gases constantly try to attain equilibrium with their surroundings. Beverages maintain carbonation over some time because manufacturers pump beverage containers to the brim with carbon dioxide. Upon opening, gas bubbles immediately begin to escape and equilibrate with the atmosphere.

The ocean, unlike carbonated beverages, naturally contains less carbon dioxide than the ambient atmosphere and thus absorbs atmospheric carbon dioxide. This natural carbon sequestration mechanism is called the Oceanic Solubility Pump, and is an important cooling knob on the global thermostat discussed in last week’s post. But, as McKinley’s team reveals, this pump weakens in a warming climate system and, in some regions, may begin to hose carbon back into the atmosphere.

Famously, carbon dioxide warms our atmosphere. As the carbon-grabbing tendrils of the ocean begin to loosen with warming waters, the following positive feedback loop takes hold:

More carbon dioxide accumulates in the atmosphere, thereby

1. increasing atmospheric and ocean temperatures,
2. further weakening the ocean’s grip on atmospheric carbon dioxide,
3. further raising atmospheric carbon dioxide concentrations,
4. leading to further warming.