Phytoplankton loss could partly explain cloud albedo loss at low latitudes

[Clive Elsworth]

All

 

It seems to me that loss of oceanic DMS emissions from loss of phytoplankton productivity in tropical and subtropical regions might partly explain the loss of cloud albedo in these areas.

 

FWIW here’s an answer from ChatGPT:

 

Q. Given this paper: Secondary Marine Aerosol Plays a Dominant Role over Primary Sea Spray Aerosol in Cloud Formation: https://pubs.acs.org/doi/full/10.1021/acscentsci.0c00793, is it plausible that the loss of cloud albedo in tropical and subtropical regions could be strongly attributed to the decline of phytoplankton productivity in these regions?

 

A. Yes, it is scientifically plausible that declining phytoplankton productivity could contribute to reduced cloud albedo in some tropical and subtropical ocean regions, but I would not say current evidence supports it as a dominant explanation for the observed global low-cloud decline.

 

The paper you cited is important because it strengthens a link that had often been assumed rather than demonstrated: secondary marine aerosols produced from biologically emitted gases appear to be more important cloud condensation nuclei (CCN) than primary sea-spray particles in many marine environments. The authors found that secondary marine aerosols (sulfate, ammonium and organics) tracked phytoplankton biomass, while primary sea spray did not, and concluded that these secondary particles play a dominant role in marine cloud formation.

 

The causal chain you are proposing is:

Ocean warming → stronger stratification → lower nutrient supply → lower phytoplankton productivity → lower DMS/VOC emissions → fewer secondary marine aerosols → fewer CCN → reduced low-cloud reflectivity and coverage → additional warming.

 

Every link in that chain has some observational or theoretical support.

 

What is less certain is the magnitude.

 

Why the hypothesis is credible

In the subtropical oceans, cloud droplet concentrations are often aerosol-limited.

 

If phytoplankton-derived sulfur compounds such as DMS are important sources of CCN, then reductions in biological productivity could reduce cloud droplet numbers and cloud brightness. The ACS paper specifically argues that biological production of secondary marine aerosol exerts a stronger control on cloud-forming particles than sea spray alone.

 

Furthermore:

• warming generally increases upper-ocean stratification,
• stratification often reduces nutrient entrainment,
• oligotrophic subtropical gyres have expanded in recent decades,
• several studies have reported declining productivity in parts of the tropical and subtropical oceans.

 

So, the mechanism is physically coherent.

 

Why it is difficult to attribute current cloud loss primarily to phytoplankton decline

The observed recent reductions in low-cloud albedo are occurring in a system influenced by several competing drivers:

1. Reduced anthropogenic sulfate aerosols
• especially from shipping emissions.
• This mechanism directly removes CCN and has been observed in major shipping corridors.
2. Temperature-driven cloud feedbacks
• warmer SSTs can reduce low-cloud cover in subtropical marine stratocumulus regions.
• Some modeling studies suggest aerosol-cloud cooling effects weaken as the climate warms.
3. Changes in atmospheric circulation
• Hadley circulation shifts,
• trade-wind changes,
• ENSO variability.
4. Natural biological aerosol variability
• including DMS and other marine biogenic emissions.

 

At present, the literature generally attributes most of the recent global cloud-albedo decline to a combination of aerosol changes and cloud feedbacks rather than directly to declining phytoplankton productivity.

 

An intriguing possibility

Where your question becomes particularly interesting is that these explanations are not mutually exclusive.

Suppose Hansen is correct that warming is accelerating partly because low-cloud cover is declining.

 

One possibility is that:

• anthropogenic aerosol reductions initiated some warming,
• warming increased ocean stratification,
• stratification reduced biological productivity in certain regions,
• reduced biological productivity lowered marine biogenic aerosol production,
• cloud cover declined further.

 

In that case, phytoplankton decline would not be the initial cause but rather part of a positive feedback loop.

This feedback is scientifically plausible and, in my view, deserves more attention than it currently receives. However, I do not think the evidence is yet strong enough to conclude that it explains a large fraction of the observed cloud loss.

 

My assessment

If I were to assign qualitative confidence levels:

Statement	Confidence
Marine biogenic aerosols are important for marine cloud formation	High
Phytoplankton productivity affects CCN concentrations	High
Stratification can reduce productivity in many tropical/subtropical regions	High
Reduced productivity can reduce cloud albedo locally	Moderate to high
Recent tropical/subtropical cloud loss is substantially driven by declining phytoplankton productivity	Moderate
Recent global cloud-albedo decline is primarily caused by declining phytoplankton productivity	Low to moderate

 

So, I would regard your hypothesis as a scientifically credible candidate feedback mechanism, but one whose global contribution remains poorly quantified relative to the better-studied effects of anthropogenic aerosol reductions and SST-driven cloud feedbacks. A key unresolved question is whether observed declines in chlorophyll, DMS emissions, and low-cloud reflectivity can be shown to covary regionally and temporally strongly enough to establish causation rather than coincidence. That is an active research frontier rather than a settled conclusion.

--End of AI answer—

 

That makes a potential case for very diffuse OIF delivered by aerosols to enhance an MCB intervention. This could be relatively easy using ammonium sulfate aerosol CCN formed by gas-gas reactions and involving a trace amount of Iron(III) Chloride.

 

We have calculated that such a method would likely be much cheaper than seawater spraying using effervescent nozzles and result in far less CO2 emissions, owing to the enormous power requirement of effervescent sprayers at the scale needed. (That power is roughly equivalent to 2500 large container ships – and that’s only to halt ocean warming at the current Earth energy imbalance, not restore it).

 

Clive