+By Berit Anderson
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Why Read: For humans around the world who spend all their days on land, the ocean is "out of sight, out of mind." It is easy to forget that the health of our children and grandchildren - the air they will breathe and the food they will eat - is directly dependent on the health of the ocean, which currently absorbs 30% of global CO2 emissions.
Today, research on phytoplankton - the workhorses of that CO2 absorption - suggests that up to 50% are at risk of transforming from carbon sinks into carbon emitters, with the entire phytoplankton population already becoming significantly less productive. The urgency to reduce emissions and improve ocean health has never been greater. Read on for a compilation of research on these critical oxygen producers and what we can do to ameliorate the situation.
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Marine phytoplankton play a central role in supporting life in the oceans and profoundly affect global biogeochemical cycles.
- Research Article,
Advancing Earth and Space Sciences (4/30/21)
Last year, I asked a colleague - an expert in oceans and marine biology - whether we shouldn't be worried about the fact that the ocean is currently responsible for 30% of global carbon capture and very clearly at risk of destabilization.
Is there not a risk, I wondered, of a mass phytoplankton die-off due to rising temperatures, acidification, or some unholy combination of the two? And if that happened, what would be the impacts?
Would it throw off the equilibrium of ocean health enough to cause a mass extinction event of marine flora and fauna? How much of the ocean's plankton population would be affected?
At the time, my colleague seemed unconcerned about that particular outcome, reminding me that phytoplankton can survive in a wide range of temperatures, from the arctic to the tropics.
I left that conversation feeling somewhat comforted (I deeply trust this person's perspective and intelligence) but also disquieted. It didn't seem they were taking the threat - something renowned oceans expert Roger Payne had warned Future in Review attendees about years before - seriously enough.
Perhaps, for personal reasons, it was easier for them not to.
Oceans Have a (Bad) PR Year
This year, the ocean is having its climate moment. It just may not be the one we'd hoped for.
Just a few weeks ago, on September 24, the Planetary Health Check 2025report was released, detailing that seven out of nine planetary boundaries have now been breached - including, for the first time, ocean acidification.
In the eport's introduction, ocean expert Sylvia Earle wrote:
This paints a grave picture - not just for marine ecosystems, but for the entire Earth system that depends on a healthy ocean. Recent global discussions - whether on the future of deep-sea mining, or on the proposed cuts to Earth system science funding - remind us that without robust, transparent, and long-term ocean observation systems, we simply cannot conduct the kind of scientific assessments this report aims to provide. Knowledge is power - but only if we commit to gathering it.
The 2025 Global Tipping Points report, published in advance of COP30 just a few weeks later, warns that ocean warming is already pushing coral past its thermal tipping point as well, stating:
Already at 1.4°C of global warming, warm water coral reefs are crossing their thermal tipping point and experiencing unprecedented dieback, impairing the livelihoods of hundreds of millions who depend on them. Parts of the polar ice sheets may also have crossed tipping points that would eventually commit the world to several metres of irreversible sea-level rise affecting hundreds of millions.
Cultivating Strategic Imagination
These reports are essential to understanding the current state of world oceans, but they are also primarily backward-facing. They tell us where we have been and where we are now. Not, with any real clarity, what our future will hold.
For that we must use our strategic imagination, combining what we know from these reports with scientific patterns, strategic foresight, and analysis of emerging economic, cultural, and technological trends to identify unforeseen feedback loops and consequences.
One of the people most known for this type work is bestselling science-fiction author Kim Stanley Robinson, whose 2020 novel The Ministry for the Futureexplores the impacts of our continued trajectory toward a 3-degree future.
A longtime SNS member himself and multi-time FiRe speaker, Stan joined other members last week for our monthly virtual Spark salon, where we discussed the challenges and opportunities guiding our climate future.
Spark salons are typically off the record to allow our members to ask and receive honest, unmeasured answers to their most burning questions; but in this case, we thought it in the best interests of everyone to share his responses to my opening questions about our current climate challenges and solutions. Our attendee Q&A remained off-record. You can watch the full recorded portion here.
In assessing our likely future, one framework Stan advocates for is Romain Rolland's 1920 coinage "Pessimism of the intellect; optimism of the will."
In other words, let's look at the future with our intellects assuming a worst-case scenario, then use our willful optimism to find solutions. In this week's issue, I am applying that framework to the impacts of rising ocean temperatures on phytoplankton.
Pessimism of the Phytoplankton Intellect
Let's start with the stakes. Phytoplankton are responsible for about 40% of the ocean's total carbon capture. That means they are the workhorses absorbing about 12% of global CO2. In other words, if all phytoplankton disappeared, we might expect to see a 12% increase in global CO2.
How will increasing ocean acidification and temperature increases affect phytoplankton, and thus the air we breathe in the future? One six-yearinvestigation, released by PNAS in March of this year, concluded that ocean acidification in oligotrophic tropical and subtropical oceans reduced the production of phytoplankton by 10%:
When extrapolated to all affected low-chlorophyll ocean regions, this translates to an estimated 5 billion metric tons loss in global oceanic primary production, which is about 10% of the total carbon fixed by the ocean each year.
Ocean acidification isn't good for phytoplankton.
Scientists generally report that this decrease is due to a decline in nutrients at the surface as a warming ocean becomes more stratified between the warmer surface, where phytoplankton grow, and the colder waters below.
From NASA Earth Observatory:
The effect is most obvious in the part of the world's oceans that scientists describe as the permanently stratified ocean, bounded by black lines in the images [shown here above]. "Permanently stratified" means that rather than being well-mixed, there is already a distinct difference in the density of warmer, fresher water at the surface and colder, saltier water deeper down. Seasonal stratification occurs in other parts of the ocean, but in the labeled area, the stratification exists year-round.
One study on phytoplankton heat thresholds, published in Advancing Earth and Space Sciences (4/30/21), found that chlorophyll actually increased up to around 14°C, beyond which it began to decrease. In other words, it's only at above 14°C that phytoplankton generally become less effective at carbon capture.
Where in the world are ocean-surface temperatures above 14°C?
According to NOAA's Sea Surface Contours map from earlier this week, the answer is: In quite a lot of places. Those shown here in the green, yellow, orange, and red bands are above 14° Celsius.
Source: NOAA
In addition, warming has been found to shift the phytoplankton communities toward a higher proportion of small-sized species. According to a research article published in the Royal Society, "[a] direct effect of warmer temperature on phytoplankton populations has been also described, as a significant increase in the proportion of small-sized species under higher thermal conditions has been evidenced in both freshwater ecosystems and in the marine domain."
The smaller species themselves aren't necessarily bad news - they have a higher surface area to volume ratio and therefore can capture more carbon by volume than larger species. Unfortunately, it's the larger species that tend to sink to the bottom of the ocean when they die, sequestering more carbon for the long term.
"Our results could have major implications for understanding how the blooms affect regional carbon biogeochemistry," said OSU College of Science microbiology researcher Steve Giovannoni, who worked on one of the key studies in this area. "[... The] multispecies blooms we describe can have lower carbon export efficiencies than the models typically allow for."
The six-year PNAS study on phytoplankton and ocean acidification also found that small species of phytoplankton are most affected by ocean acidification.
Here's where things get a bit darker.
Research from 2023 found that temperature increases always transform mixotrophic phytoplankton from carbon sinks to carbon emitters.
What is mixotrophic plankton, or mixoplankton? Mixotrophy is a function of many species of plankton that can switch back and forth between a photosynthesis phase, in which it uses carbon for photosynthesis, and a predation phase, in which it derives carbon from predation. As stated above, in the former case, it absorbs CO2. In the latter, it emits it.
And as reported in a research article by D.J. Wieczynski, H.V. Moeller, and J.P. Gilbert:
We find that warming switches mixotrophic systems between alternative stable carbon states - including a phototrophy-dominant carbon sink state, aphagotrophy-dominant carbon source state and cycling between these two. Moreover, warming always shifts this mixotrophic system from a carbon sink state to a carbon source state, but a coordinated increase in nutrients can erase early warning signals of this transition and expand hysteresis.
What percentage of phytoplankton are mixotrophic? Oh, only 50% or so. And at what temperature is that shift likely to take place?
Model simulations showed a shift from net carbon absorption to net emission between 20.7°C (69.26°F) and 22°C (71.6F), though the precise threshold is dependent on other conditions.
Wait, wait. Let's look at that ocean temperature chart again. That's still anywhere in the green-yellow, yellow, orange, and red bands - a vast part of the world's oceans.
In very rough estimates, about 50% of phytoplankton in about 60% of global oceans making up about 30% of global carbon capture could already be shifting from carbon sinks to carbon emitters.
Oceans currently absorb 90% of Earth's excess heat, and their temperatures are increasing much faster than expected. That structural shift would be terrible news for global average temperatures, which would in turn heat up oceans even faster, further decreasing their capacity to absorb carbon and increasing sea-level rise.
SNS members of the world, please prove me wrong on this analysis. I would love to be wrong. But I am also deeply worried that I am at least 50% right.
Optimism of the Phytoplankton Will
So things would feel quite dire in the plankton world. But it's worth pointing out that there is still a lot we don't know.
Phytoplankton may adapt to changes in the ocean, as coral has - shifting and evolving into deeper bands, where the temperature is cooler. There may be other species of plankton or other ocean plants that thrives in warmer temperatures, filling-in areas previously dominated by phytoplankton.
The Schmidt Ocean Institute has been discovering a wealth of deep-sea coral now thriving off the coast of Uruguay - including multiple discoveries of new species.
We are certainly not helpless to deal with all this.
We have the tools to keep a closer eye on the oceans than we have been - to deploy sensors and studies of the ocean's surface temperature and acidification, as Sylvia Earle called for in the Planetary Health Check report. We also have the technology to track and fight illegal international fishing fleets, which stress already at-capacity ecosystems.
Most important, we are already moving extremely quickly to build out renewable-energy infrastructure around the world in the form of solar and storage - a platform that provides the basis to reduce global emissions by as much as 89% through the electrification of sectors including transportation and manufacturing. At Future in Review, we call this work "Plan A" - because it is the fastest and cheapest way to decarbonize today, and because there is no Planet B.
We can maximize land-based carbon sinks, protecting reserves of forest and grassland and integrating regenerative agriculture into unused and fallow fields.
As Jane Goodall said in her Famous Last Words interview - a conversation recorded before her death last week to be released only posthumously:
I want to make sure that you all understand that each and every one of you has a role to play. You may not know it, you may not find it, but your life matters, and you are here for a reason.
And I just hope that reason will become apparent as you live through your life. I want you to know that, whether or not you find that role that you're supposed to play, your life does matter, and that every single day you live, you make a difference in the world. And you get to choose the difference that you make.
I want you to understand that we are part of the natural world. And even today, when the planet is dark, there still is hope. Don't lose hope. If you lose hope, you become apathetic and do nothing. And if you want to save what is still beautiful in this world - if you want to save the planet for the future generations, your grandchildren, their grandchildren - then think about the actions you take each day.
Your comments are always welcome.
Sincerely,
Berit Anderson
[email protected]
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