Could whales be a solution to climate change? Probably only a very small one.

The numbers don’t live up to some of the hype.

This question is a bit of a fun one. But it’s also important because the idea that whales could be a big climate solution comes up quite a lot. Google “whales climate change” or “whales climate solution” and you’ll find a lot of articles touting the benefits.

A few research papers have also made some bold claims, suggesting that a carbon market for whales could be worth as much as $1 trillion (or $2 million per whale).¹

Someone recently asked me — after stumbling on the post below — if whale restoration programs were an underrated solution, so I thought it would be useful to go through the numbers here.

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The two main ways that whales can capture carbon

Whales are probably a net carbon sink. What’s debated is how big this sink is.

There are two key ways they can store and sequester carbon. Below is my very simple sketch of each.

The first is called whale falls. As large mammals, whales store carbon in their bodies. If they die and decompose close to the surface, this carbon is ultimately released to surface waters (and probably into the atmosphere too). But, many whales sink to the deep ocean when they die, and if they decompose there, that carbon tends to stay there for a long time.

The second is called the whale pump.² Whales move from deeper waters to the surface of the ocean, and by doing so, they can often recirculate important nutrients that are depleted in surface waters. For example, they might feed at depth and excrete (i.e. poo) out waste closer to the surface. This waste is often rich in nutrients such as iron, nitrogen, and phosphorus.

This transport of nutrients can often enhance the growth of phytoplankton. They suck up CO₂ through photosynthesis and can then export that carbon to the deep ocean when they die or are eaten.

Our confidence in the estimated carbon potential of whale falls is much better than the whale pump. Let’s take a look at the potential of each.

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Whale falls: dead whales on the seafloor

We're interested in a couple of numbers: how much carbon whale falls currently sequester and what their future potential is if we were to get serious about restoring whale populations.

One of the leading papers in this area is from Pershing et al. (2010).³ They estimated that current whale populations export around 28,000 tonnes of carbon — or 102,000 tonnes of CO₂ — to the deep ocean each year through whale falls.⁴

Of course, this was much bigger before we decimated whale populations. The chart below shows estimates of how whale populations declined over the 20th century. The number of Blue Whales, for example, fell by almost 99%.

Let’s then imagine that we took whale reintroduction seriously and managed to restore populations to their pre-whaling levels. How much carbon could they sequester?

Pershing et al. (2010) estimated 190,000 tonnes of carbon — or 690,000 tonnes of CO₂ per year.

These estimates are sensitive to assumptions about what percentage of whales sink to depth, and how much CO₂ is released closer to the surface. For example, Pershing et al. (2010) estimated that only 50% of rorqual and grey whales, and 10% of right and bowhead whales sunk to depth before decomposition, but some think that this percentage could be higher. An update of these figures, assuming that more whales fell to the seafloor, came up with an estimate of around 1.3 million tonnes of CO₂ per year.⁵

Based on the range of estimates in the literature, I think restoring whales to their pre-whaling populations could sequester 225,000 to 1.5 million tonnes of CO₂ per year from whale falls.⁶

That might seem like a huge range, but once we put it into context, it doesn’t make much difference. These numbers are small. Like, really small.

Globally we emit around 36.8 billion tonnes of CO₂ from fossil fuels and industry every year. Whale falls — if we restored populations back to their pre-whaling size — would be equal to 0.0006% to 0.004% of global emissions. Barely a dent.

Or if we consider my colleague’s comparison of whales and tree sequestration: globally we still emit 3.6 billion tonnes of CO₂ from land use change every year. Those land use emissions are basically the loss of carbon from trees (and to a lesser extent, ecosystems such as wild grasslands and peatlands). So whale falls could “offset” 0.006% to 0.04% of these emissions. But they’re not a match for keeping trees standing, let alone planting more.

I’ve summarised this in the chart below, taking the higher estimate of 1.5 million tonnes for whale falls.

Another way to quickly sense-check these numbers is to assume that the average whale stores around 10 tonnes of carbon — or the equivalent of 30 to 40 tonnes of CO₂ in its body.⁷ Before whaling, there were possibly around 4 to 5 million whales. We could therefore say that as much as 200 million tonnes of CO₂ was stored in whale populations at any given time.⁸ Only a fraction of those whales will die each year, and only a fraction of their carbon will sink to depth.

It seems to check out that the yearly magnitudes we’re talking about here are probably in the low millions of tonnes.

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Whale pump: recycling nutrients to increase phytoplankton

So, whale falls are not going to save us. Nor are they going to make a $1 trillion carbon market. Maybe most of the whale climate solution comes from the whale pump?

This is where the evidence gets a bit sketchy.

Estimating the carbon stored in whales or from whale falls is relatively easy. We need to make some assumptions about the number of whales we have, what they weigh, and how many make it to the deep ocean when they die. But we can get a reasonable range; certainly, order-of-magnitude estimates we can be reasonably confident in.

It is much more difficult to calculate the extra phytoplankton growth and productivity whales generate by recycling nutrients.

Some studies in this area suggest that the carbon potential for the whale pump is roughly similar to that of whale falls. One paper estimated that sperm whale populations remove around 875,000 tonnes of CO₂ from the atmosphere via this mechanism.⁹ This is for current whale populations; if we were to restore them to pre-whaling levels, carbon sequestration could be around an order of magnitude higher. Let’s then call it 9 million tonnes of CO₂. This study was also for sperm whales in southern oceans only, so the global total might be somewhere in the 10 to 20 million tonnes range. Yes, this is a bit hand-wavy, but we’re going for order-of-magnitude estimates here.

A couple of more recent studies caused a stir when they suggested that the whale pump could capture 700 million to 1.7 billion tonnes a year.¹⁰ These were the papers that put the monetary value of whales at $1 trillion.

These figures rely on the assumption that whales would increase the primary productivity of global phytoplankton by 1% to 2%. One key problem, though, is that there is no empirical evidence to support this, and the authors of these two papers give no real justification for it (more info in the footnote).¹¹

These numbers have been criticised by a number of researchers. As a review by Pearson et al. (2024) highlights:

“Whales are proposed to increase primary productivity globally by 1%, with no supporting empirical data. Whales can increase primary productivity, but the magnitude of this varies by species, location, and season. Whales are absent from vast areas, with no indication that they could increase phytoplankton productivity by 1%.”

The authors go on to list several other concerns about the underlying assumptions. One of the other key ones is that even if whales do increase the productivity of phytoplankton, we don’t have a great understanding of how much of that carbon actually falls to the deep ocean. It could be the case that a lot of it decomposes near the surface, where it’s not a carbon sink at all.

In short, our understanding of how whales contribute to nutrient recycling — and how much carbon that might save us — is far too limited to justify billion-tonne claims.

Of course, we don’t have to disregard these papers completely. They add something to the overall scientific literature. But it’s important to note that they are pretty far from the scientific consensus on this, and we should be cautious about putting those findings into practice, by investing money and deploying solutions (or non-solutions).

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We should bring back the whales, but not for the climate benefits

The decimation of the world’s whale populations across the 20th century was tragic. I would love to see a comeback. And there are many environmental and ecological benefits to doing so.

But we shouldn’t do it for climate reasons. Or at least, we shouldn’t expect them to be a major climate solution. Yes, they might store a bit of carbon and that’s a nice bonus. But I think their impact is likely to be small.

Overstating whales' climate potential is not helpful and could be damaging. Creating a $1 trillion carbon removal market for whales with almost no evidence that whales could actually remove $1 trillion worth of carbon would be irresponsible. We should be putting that money into technologies and solutions that we know can reduce emissions.

It might be different if we were talking about a few hundred million, or even a few billion dollars. At that scale, there is more scope for experimentation on solutions that we need to test and understand better.¹² But $1 trillion? That’s not money to mess around with.

Even beyond the actual tonnes of carbon sequestered, it threatens the reputational risk for climate action as a whole. Imagine the headlines if we created a $1 trillion carbon market for whales, and then had to admit that it achieved almost nothing.

I’m far from the first to make this point.¹³ Last year, a group of leading biodiversity scientists published a paper in Nature Climate Change: Resisting the carbonization of animals as climate solutions.¹⁴ They look at the state of the research in much more detail, but reach similar conclusions:

“Ultimately, we strongly endorse trophic rewilding as an approach to protecting and restoring biodiversity. While rewilding may support climate mitigation in some circumstances, we caution that unverified and inflated economic valuations, selective media reporting, and a narrow management focus on carbon could result in perverse outcomes and reputational risk from failed carbon capture, and distract from the urgent need to reduce fossil fuel emissions. We cannot afford to make this mistake: ecosystems, humans and wildlife could suffer as a result.”

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1. Chami, R., Fullenkamp, C., Berzaghi, F., Español-Jiménez, S., Marcondes, M., & Palazzo, J. (2020). On valuing nature-based solutions to climate change: a framework with application to elephants and whales.

2. There are several mechanisms by which whales can potentially increase biological uptake of carbon in the ocean. You might hear them referred to as the "whale conveyer belt" or "whale bioturbation".

For simplicity, I'm going to just refer to the whale pump.

3. Pershing, A. J., Christensen, L. B., Record, N. R., Sherwood, G. D., & Stetson, P. B. (2010). The impact of whaling on the ocean carbon cycle: why bigger was better. PloS one, 5(8), e12444.

4. This study focuses on species of baleen whales. It does not include sperm whales. Here’s why:

"We excluded sperm whales (Physester macrocephalus) from our calculations. This species tends to feed at great depth. The prey available at these depths likely derives a portion of its nutrition from the organic matter sinking from the photic zone. Thus, by feeding on these animals and returning to the surface to respire, sperm whales could potentially counteract the export of carbon which we aim to calculate. Determining whether sperm whales represent a net upward or downward flux of carbon is an interesting calculation, but one that is beyond the scope of this study."

5. Pearson, H. C., Savoca, M. S., Costa, D. P., Lomas, M. W., Molina, R., Pershing, A. J., ... & Roman, J. (2023). Whales in the carbon cycle: can recovery remove carbon dioxide?. Trends in Ecology & Evolution, 38(3), 238-249.

6. Note that the 1.5 million tonnes estimate comes from Durfort et al. (2022) estimate is for the Southern oceans only. It's therefore an underestimate of global sequestration using its methodology.

But at a maximum, the global estimate would be twice as high, and the order of magnitude would not change. It makes little difference to the final result.

Durfort, A., Mariani, G., Tulloch, V., Savoca, M. S., Troussellier, M., & Mouillot, D. (2022). Recovery of carbon benefits by overharvested baleen whale populations is threatened by climate change. Proceedings of the Royal Society B, 289(1986), 20220375.

Pershing, A. J., Christensen, L. B., Record, N. R., Sherwood, G. D., & Stetson, P. B. (2010). The impact of whaling on the ocean carbon cycle: why bigger was better. PloS one, 5(8), e12444.

Pearson, H. C., Savoca, M. S., Costa, D. P., Lomas, M. W., Molina, R., Pershing, A. J., ... & Roman, J. (2023). Whales in the carbon cycle: can recovery remove carbon dioxide?. Trends in Ecology & Evolution, 38(3), 238-249.

7. This varies a lot by species, but this seems like a reasonable average.

8. 40 tonnes multiplied by 5 million whales is 200 million tonnes.

9. They estimated that the carbon removed was around 400,000 tonnes per year. But this was partially offset — at 160,000 tonnes — by whale respiration.

So the net carbon removal was 240,000 tonnes.

Converting this to CO2 gives around 875,000 tonnes.

Lavery, T. J., Roudnew, B., Gill, P., Seymour, J., Seuront, L., Johnson, G., ... & Smetacek, V. (2010). Iron defecation by sperm whales stimulates carbon export in the Southern Ocean. Proceedings of the Royal Society B: Biological Sciences, 277(1699), 3527-3531.

10. A 2019 paper by Ralph Chami and colleagues estimated that the whale pump — if restored to pre-whaling levels — could sequester 1.7 billion tonnes of CO2 per year.

https://www.imf.org/Publications/fandd/issues/2019/12/natures-solution-to-climate-change-chami

A 2020 follow-up by the same lead author assumed a lower figure of 700 million tonnes.

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3686168

11. After waving away some of the estimates from the studies I referenced above (for being too low), the authors say the following:

“Given the limited number of studies and the variation in their estimates of whale impact on primary production, caution is warranted when using their results. Nonetheless, attributing one percent of current phytoplankton production to the current whale population appears, given the current state of the research, to be a reasonable initial estimate of the impact of whales on primary production.”

So, caution is warranted for other researchers’ results, but picking a relatively random number for your own study is apparently fine.

12. Maybe research that improves our understanding of whales' role in carbon cycling would qualify here. No, we shouldn't be spending billions on it, but it could be worth much smaller investments.

13. Many researchers have raised concerns about the uncertainty of the role that whales play in the carbon cycle. To mention only a few:

Meynecke, J. O., Samanta, S., de Bie, J., Seyboth, E., Prakash Dey, S., Fearon, G., ... & Mackey, B. (2023). Do whales really increase the oceanic removal of atmospheric carbon?. Frontiers in Marine Science, 10, 1117409.

Pearson, H. C., Savoca, M. S., Costa, D. P., Lomas, M. W., Molina, R., Pershing, A. J., ... & Roman, J. (2023). Whales in the carbon cycle: can recovery remove carbon dioxide?. Trends in Ecology & Evolution, 38(3), 238-249.

Trembath and Wang (2019). Negative-Emission Whales. Breakthrough Institute.

14. Duvall, E. S., le Roux, E., Pearson, H. C., Roman, J., Malhi, Y., & Abraham, A. J. (2024). Resisting the carbonization of animals as climate solutions. Nature Climate Change, 14(9), 892-895.