Re: [HPAC] Darkening of Global Oceans - extremely important and profound paper

[John Nissen]

Hi Paul,

I think you are right about the paper being extremely important [1]. I've not heard about this darkening before.  I was somewhat misled by the title: so it's NOT about the albedo of the ocean surface, e.g. as observed from space; it IS about darkening within the water column -- in fact, the surface albedo could increase or decrease slightly (they talk about a colour change).

The darkening is significant: the photic layer where phytoplankton live has been reduced by over 10% across 9% over the global ocean surface.

The exact causes are not known, but the authors speculate: "A combination of nutrient, organic material and sediment loading near the coasts and changes in global ocean circulation are probable causes of increases in primary and secondary productivity that have reduced light penetration into surface waters."

The implications are potentially huge because of (i) the effect on phytoplankton growth and the ocean food chain, hence fish for human consumption; (ii) the reduced carbon uptake by the oceans, which had been absorbing around 1/3 of emissions (c.f. land absorbing around 1/6 of emissions); (iii) the feedback which leads to ocean stratification, which reduces the cooling effect of the oceans (over 90% of solar energy reaching the ocean surface is absorbed); (iv) the reduced DMS emissions by phytoplankton, which have a cooling effect through cloud physics (Lovelock gave this supportive feedback as an example of his Gaia principle).

If the darkening has its own positive feedback, such that photic zone contraction is accelerating, then there are severe implications for fish catches, especially in coastal waters.  Two billion people depend on fish for their protein.

But I doubt whether the darkening could be a key tipping point in the climate system, as you suggest it could be.  However, global warming is certainly an aggravating factor, and any additional tendency for ocean stratification must be a concern.  I would appreciate comments from those who are more versant with these matters than I am, such as Brian!

Cheers, John

[1] Davies and Smyth (Global Change Biology, May 2025)

Darkening of the Global Ocean

https://onlinelibrary.wiley.com/doi/10.1111/gcb.70227

On Thu, Jul 17, 2025 at 4:00 AM Paul Beckwith <paulhenr...@gmail.com> wrote:

https://youtu.be/gkv0eRGDjjE?si=DEFvZ2ejFA1LOFbA

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[Tom Goreau]

I had commented on this paper before.

 

The darkening referred to has nothing to do with albedo but with increasing turbidity from biomass in coastal waters due to land-based sources of nutrients and dust, and where upwelling is increasing.

 

By and large upwelling is DECREASING in most open ocean areas as they become more thermally stratified from the top down, reducing or preventing upwelling of nutrients to the surface.

 

Shifts in trophic chains at the lowest levels also affects bio-turbidity.

 

Thomas J. F. Goreau, PhD
President, Global Coral Reef Alliance

Chief Scientist, Biorock Technology Inc., Blue Regeneration SL

Technical Advisor, Blue Guardians Programme, SIDS DOCK

37 Pleasant Street, Cambridge, MA 02139

gor...@globalcoral.org
www.globalcoral.org
Phone: (1) 857-523-0807 (leave message)

 

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Innovative Methods of Marine Ecosystem Restoration

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On the Nature of Things: The Scientific Photography of Fritz Goro

 

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It’s much later than we think, especially if we don’t think

 

Those with their heads in the sand will see the light when global warming and sea level rise wash the beach away

 

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