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Opinion
Whales in the carbon cycle: can recovery
remove carbon dioxide?
Heidi C. Pearson ,
1,
*
,@
Matthew S. Savoca,
2,@
Daniel P. Costa,
3
Michael W. Lomas,
4
Renato Molina,
5
Andrew J. Pershing,
6,@
Craig R. Smith,
7
Juan Carlos Villaseñor-Derbez,
2,8
Stephen R. Wing,
9
and Joe Roman
10
The great whales (baleen and sperm whales), through their massive size and
wide distribution, influence ecosystem and carbon dynamics. Whales directly
store carbon in their biomass and contribute to carbon export through sinking
carcasses. Whale excreta may stimulate phytoplankton growth and capture
atmospheric CO
2
; such indirect pathways represent the greatest potential for
whale-carbon sequestration but are poorly understood. We quantify the carbon
values of whales while recognizing the numerous ecosystem, cultural, and
moral motivations to protect them. We also propose a framework to quantify
the economic value of whale carbon as populations change over time. Finally,
we suggest research to address key unknowns (e.g., bioavailability of whale-
derived nutrients to phytoplankton, species- and region-specific variability in
whale carbon contributions).
Whales and the oceanic carbon cycle
The ocean is an important carbon sink (see Glossary), absorbing approximately 22% of anthropo-
genic carbon emissions during 2010–2019 [1], some of which is exported to the deep sea [2]. A
better understanding of the role of its largest biota, the great whales,inthecarbon cycle is needed
to better inform ocean management and carbon dioxide removal (CDR) strategies. We discuss
the importance of whales to marine ecosystems and their role in the biological carbon pump
(BCP), assess their potential to contribute to carbon storage and carbon sequestration,and
consider how whale carbon services could play a role in climate-change mitigation strategies.
Whales and marine ecosystems
Blue (Balaenoptera musculus)andfin(Balaenoptera physalus) whales are the two largest animals
to ever exist on Earth [3,4]. The gigantism of filter-feeding baleen whales results from high energetic
efficiency facilitated by feeding on dense, high-energy prey patches with physical adaptations that
allow them to engulf large amounts of prey [5,6]. Their size and longevity allow great whales to exert
strong effects on the carbon cycle by: (i) storing carbon more effectively than small animals [7],
(ii) ingesting extreme quantities of prey [7], and (iii) producing large volumes of waste products
[8]. Considering that baleen whales have some of the longest migrations on the planet [9], they
potentially influence nutrient dynamics and carbon cycling over ocean-basin scales [10–12].
Industrial whaling is estimated to have reduced great whale biomass by 81% (L.B. Christensen,
MSc thesis, The University of British Columbia, 2006), resulting in the removal of an estimated
0.017 Gt carbon stored in baleen whale biomass [7]. In addition, depletion of the great whales
may have altered top-down forcing [13–15], which further impacted carbon-cycling dynamics
[8,16–18]. Whereas industrial whaling had the largest impact on great whale population
Highlights
As climate change accelerates, there is
increasing interest in the ability of whales
to trap carbon (i.e., whale carbon), yet it
is currently undetermined if and how
whale carbon should be used in
climate-change mitigation strategies.
Restoring whale populations will en-
hance carbon storage in whale biomass
and sequestration in the deep sea via
whale falls, though the global impact will
be relatively small.
Whale-stimulated primary productivity
via nutrient provisioning may sequester
substantially more carbon, though there
is uncertainty regarding the carbon fate
in these food webs.
Recovery of whale populations via re-
duction of anthropogenic impacts can
aid in carbon dioxide removal but its in-
clusion in climate policy needs to be
grounded in the best available science
and considered in tandem with other
strategies known to directly reduce
greenhouse gas emissions.
1
Department of Natural Sciences,
University of Alaska Southeast, Juneau,
AK, USA
2
Hopkins Marine Station, Stanford
University, Pacific Grove, CA, USA
3
Ecology and Evolutionary Biology
Department, University of California
Santa Cruz, Santa Cruz, CA, USA
4
Bigelow Laboratory for Ocean
Sciences, East Boothbay, ME, USA
5
Rosenstiel School of Marine,
Atmospheric, and Earth Science and
Miami Herbert Business School,
University of Miami, Miami, FL, USA
6
Climate Central, Inc., Princeton, NJ, USA
7
Department of Oceanography, University
of Hawai’iatMānoa, Honolulu, HI, USA
Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx https://doi.org/10.1016/j.tree.2022.10.012 1
© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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TREE 3074 No. of Pages 12
abundance [19] and body size [20], modern populations continue to face threats from fisheries
entanglement [21], climate-change-induced shifts in prey distribution and abundance [22],
noise pollution [23], ship strikes [24], marine debris [25], and, in some areas, continued commer-
cial whaling [26].
The BCP
The ocean’s carbon pump is a complex combination of biological, chemical, and physical
processes that control the transfer of carbon into the ocean interior where it is removed from
exchange with the atmosphere for long time periods [27,28]. The BCP dominates the total
sequestration of carbon in the ocean [29] and it can vary regionally by nearly an order of magnitude.
Carbon export generally decreases from the poles to the equator and nearshore to the ocean gyres.
The BCP can be further separated into the gravitational export pump and particle injection
pumps [27],whichinaggregateareestimatedtotransfer4–12 Gt C year
–1
to the deep sea [2,30,31].
Diel vertical migrations (DVMs) of mesopelagic fishes and zooplankton drive the mesopelagic-
migrant pump (one component of the particle injection pump), comprising the largest migration of
animals on Earth in terms of abundance and biomass [32]. Consequently, the DVM of mesopelagic
nekton can actively transport nutrients and organic matter between the epipelagic and deep ocean
[32,33]. DVM transport of carbon and nitrogen from the epipelagic, where many copepods,
euphausiids, and salps feed at night in shallow, phytoplankton-rich waters, to depths where they
excrete waste products can be a large component of carbon and nitrogen export [34,35]. Nonethe-
less, in some circumstances, the fluxes of organic matter and nutrients may be upward. For
example, Antarctic krill (Euphausia superba) can forage on the benthos and then migrate into
surface waters, releasing high concentrations of new and remineralized iron into the photic zone
[34,36,37]. Further, marine mammals (e.g., whales), seabirds, and large fishes forage extensively
on vertically migrating zooplankton and mesopelagic fish [38,39], releasing nutrient-rich
excreta into surface waters and stimulating primary production [40,41]. These connections comprise
a potentially important feedback for carbon export and storage.
Carbon persistence
The nomenclature of carbon persistence differs by timescale. Carbon storage refers to
short-term carbon retention in organismal tissues [42]. For longer-lived animals such as
whales, carbon storage can be on the order of decades to centuries. For organisms in the
great whale food web (phytoplankton, zooplankton, forage fish, and squid), carbon storage
is days to years.
Carbon sequestration, however, extends beyond the time horizon of carbon storage and it often
follows from more complex dynamics. For example, great whale excreta are highly enriched in
limiting nutrients, including nitrogen, phosphorus, and trace metals (e.g., iron) that baleen
whales recycle within the epipelagic (see Whale pump section; Figure 1A) [11,40,43–48]. In the
case of sperm whales (Physeter macrocephalus), the transport of these limiting nutrients from
the deep ocean can enhance phytoplankton production in the photic zone. A portion of this
whale-stimulated primary and secondary productivity is recycled within the upper-ocean detrital
food web. The amount that escapes recycling within the upper ocean can be exported below the
maximum mixing depth and sequestered on timescales of centuries to millennia. As such, deep
carbon export leading to sequestration is an important sink for CO
2
, yet the influence of whales on
deep carbon export and the BCP is largely unknown. Export efficiency (which can lead to long-
term sequestration) spans three orders of magnitude (range: 0.0001–0.24 [49,50]) due to
variability in productivity across the global ocean. It is also a process particularly difficult to
influence and measure.
8
Bren School of Environmental Science
& Management, University of California
Santa Barbara, Santa Barbara, CA, USA
9
Department of Marine Science,
University of Otago, Dunedin,
New Zealand
10
Gund Institute for Environment,
University of Vermont, Burlington, VT,
USA
*Correspondence:
hcpearson@alaska.edu (H.C. Pearson).
@
Twitter: @uasoutheast (H.C. Pearson),
@DJShearwater (M.S. Savoca), and
@sci_officer (A.J. Pershing).
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Direct and indirect carbon pathways of whales
Direct pathways
Biomass
In the pelagic ocean, animals comprise the vast majority of the biomass capable of storing
carbon from one year to the next [51]. Because whales are among the longest-lived animals,
they may constitute one of the largeststable living carbon poolsin the pelagic ocean. The singularly
large size of thegreat whales means that they interact with the carbon cycle in unique ways. While it
is obvious that whaling reduced the amount of carbon stored in populations of great whales, it is
not clear if whaling was a net source of CO
2
to the atmosphere.
Pershing et al.[7] proposed that because metabolic rates scale with individual biomass raised to a
power less than one (typically assumed to be ¾ [52]), the large size of whales makes them more
efficient at storing carbon. Using ¾ scaling, they calculated that removing one 92-ton blue whale
would provide enough uneaten krill to support seven minke whales (Balaenoptera acutorostrata)
or 1800 Adelie penguins (Pygoscelis adeliae). However, the new minke whales would have only
50% of the biomass of the missing blue whale, while the new penguins would have only 8% of
the biomass. This relationship suggests that the reduction of blue whales and other baleen
whales in the Southern Ocean has led to an ecosystem that maintains a lower total biomass of
living carbon and releases a greater amount of CO
2
for a given amount of ingested krill, than
prior to industrial whaling.
Whale falls
Large body size means that whales are particularly efficient at transferring carbon to the deep sea
via whale falls [7,53]. The overall reduction in the number of whales and, specifically, the loss of
larger species and larger individuals, has altered the flux of whale carbon to the deep sea and
caused extinctions of whale-fall specialists in the North Atlantic [20].
Indirect pathways
Whale excreta released in surface waters are rich in limiting nutrients. These nutrients, released
across both vertical and horizontal gradients, may stimulate carbon fixation by phytoplankton,
enhancing ecosystem productivity and potentially stimulating carbon storage, export, and
sequestration (Figure 1)[11,40,54,55]. Two indirect pathways are discussed next: the whale
pump and the great whale conveyor belt.
Whale pump
Buoyant fecal plumes released by whales can provide limiting nutrient concentrations three to
seven orders of magnitude higher than background seawater concentrations [40,45–47,56]
and can persist at the surface, allowing nutrients to leach into the surrounding seawater and
become bioavailable to phytoplankton [40,45,46,57]. Nutrient recycling and transport by whales
and other top predators may push planktonic food web structure toward systems dominated by
large-celled, quickly sinking diatoms and krill that could affect carbon export [44,58]. DVM by
whale prey (e.g., krill) and other nekton provides a particle injection pump that increases export
efficiency relative to gravitational flux of particles alone [27]. In locations of high productivity and
whale abundance, zooplankton fecal material, exuviae, and carcasses provide considerable
pulses of carbon to the deep sea, thereby demonstrating the importance of whale prey to
deep carbon export [50,59–61].
Whales unlock nutrients in prey through their feeding behavior and digestive processing. Specifi-
cally, allochthonous nutrients promote new primary production, carbon storage, export, and
potentially, sequestration, while autochthonous nutrients promote recycled primary production
Glossary
Additionality: carbon benefits,accrued
from a policy/management intervention,
exceeding the status quo.
Allochthonous nutrients: nutrients
originating outside a defined system.
Autochthon ous nutrien ts: nutrients
recycled within a defined system.
Biological carbon pump (BCP): sum
of all biological and physical processes by
which biologically derived organic carbon
is transported to the ocean depth.
Blue carbon: carbon naturally captured
by marine biota, originally used in
reference to coastal mangroves, salt
marshes, and seagrasses [83].
Capital breeder: animal that feeds and
stores energy before the breeding
season, relying on accumulated energy
stores to reproduce.
Carbon cycle: sum of processes
comprising the flux of carbon atoms
between the atmosphere, oceans, soils,
and sediments, and the organisms living
therein.
Carbon dioxide removal (CDR):
capture or retention of CO
2
,either
naturally or artificially, so that it is taken
from the atmosphere for a defined
period of time.
Carbon export: flux of organic carbon,
derived from CO
2
via photosynthesis,
from the upper water column to depth
(often considered below the maximum
annual surface mixed layer depth).
Carbon fixation: conversion of
inorganic carbon (CO
2
) into organic
carbon (i.e., photosynthesis).
Carbon persistence: tendency for
carbon to remainin a reservoir and out of
atmospheric contact.
Carbon sequestration: removal of
atmospheric organic carbon, generally
via photosynthesis, for geologic
timescales ≥100 years.
Carbon service: natural processes
that remove organic carbon, derived
from CO
2
via photosynthesis, from the
atmosphere.
Carbon sink: process that removes
more carbon (generally derived from
CO
2
via photosynthesis) from the
atmosphere than it emits.
Carbon storage: removal of carbon,
derived from CO
2
via photosynthesis,
from the atmosphere for <100 years.
Diel vertical migration (DVM): daily
movement of zooplankton, fish, and
cephalopods (e.g., squid, octopus) to
feed nocturnally in surface waters before
returning to depth during the daytime.
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(Figure 1A[62]). Deep-diving cetaceans, such as sperm whales, feed almost exclusively on
cephalopods living well below the surface mixed layer [63,64] and thus are presumed to release
allochthonous fecal nutrients at the surface (Figure 1A) [43]. In contrast, shallow-diving cetaceans,
such as baleen whales, often feed within the surface mixed layer [8,64] and thus are presumed to
generally release autochthonous nutrients at the surface [44].
The distinction between new versus regenerated nutrients has implications for carbon cycling
and, importantly, export calculations. Nonetheless, observations that overall ecosystem produc-
tivity has declined in regions with depleted whale populations (e.g., the Southern Ocean) suggest
that the role of whales in nutrient cycling is critical to ecosystem functioning and their associated
carbon sequestration potential [8,10,11,40,44].
Great whale conveyor belt
Many great whales are capital breeders, migrating from high-latitude, nutrient-rich feeding
grounds where they spend the summer to low-latitude, nutrient-poor breeding grounds where
they spend the winter (Figure 1B) [65]. Whales usually fast on the breeding grounds, metabolizing
lipid-rich blubber for maintenance and, in females, for lactation [10]. Allochthonous nutrient inputs
via carcasses, excreta (e.g., urine, placentas), and other byproducts (e.g., sloughed skin) derived
from nutrients ingested on the feeding grounds have the potential to stimulate new productivity,
carbon export, and carbon sequestration when they are released on the breeding grounds [65].
For species such as humpback (Megaptera novaeangliae), right (Eubalaena spp.), and gray
(Eschrichtius robustus) whales, breeding grounds tend to be shallow, with lower nutrient concen-
trations and higher densities of breeding whales. The nutrient subsidies that migratory whales
bring into these systems also support fish, scavengers, and benthic macroinvertebrates [66].
Quantifying the carbon roles of whales
To date, five studies have quantified the role of the great whales in direct or indirect carbon
pathways (Table 1). However, we caution that current estimates do not represent the total direct
and indirect carbon contributions from all great whale species across all oceans. Specifically,
global biomass and whale-fall carbon values did not consider sperm whales [7], the whale
pump was examined for just five great whale species in the Southern Ocean [8,43,44], and
examination of the great whale conveyor belt was limited to Southern Hemisphere blue whales
[10]. Thus, the values in Table 1 represent a fraction of the total contribution of the great whales
to the carbon cycle. It should be noted that some estimates report wide confidence intervals
spanning multiple orders of magnitude. We have a higher level of confidence in the direct carbon
pathway estimates, such as biomass and whale falls, than the indirect carbon pathway estimates.
Nonetheless, the great whale populations studied to date (Table 1) currently produce an estimated
2.0 × 10
–3
Gt C storage [7], 6.2 × 10
–5
Gt C year
–1
sequestration, 2.2 × 10
–2
Gt C year
–1
fixation
[8,10], and 4.0 × 10
–4
Gt C year
–1
export [43].
As industrial whaling devastated great whale populations and their associated carbon benefits by
an order of magnitude or more (Table 1), these values should be considered a lower bound
because the full carbon values of the great whales have yet to be restored. This may provide an
opportunity to enhance CDRfrom the great whales. Still, the currently known carbon contributions
from the great whales are small compared with carbon stored in all marine animals (~1.4 Gt C [67])
or exported via the BCP [2,30,31].
Recently, an outsized role for great whales in carbon fixation, storage, and sequestration was
suggested (e.g., carbon contribution of a single blue whale valued at $1.4 million [68,69]).
These estimates, based on assumptions beyond our understanding of whale ecology and
Earth system model: model that
simulates carbon movement through
atmospheric, terrestrial, and marine
environments.
Epipelagic: open ocean waters from
the surface to approximately 200 m
depth.
Export efficiency: portion of primary
productivity (PP) sinking from the upper
water column (oftentimes defined as
below the maximum annual surface
mixed layer depth), quantified as export
flux/PP.
Gravitational export pump:
downward flux of carbon in the ocean
due to gravitational sinking of biologically
derived particles.
Great whales: baleen and sperm
whales.
Limiting nutrient: nutrient required by
phytoplankton for photosynthesis but
typically present in small quantities and
which limits phytoplankton rate
processes and/or standing stocks.
Mesopelagic-migrant pump: type of
particle injection pump through which
vertically moving biota can directly, by
grazing, and indirectly, by boundary
layer drag, move particles from the
surface ocean to depth.
Nature-based solution (NBS): action
that conserves and sustainably
manages ecosystems that also
promotes biodiversity and human
well-being.
Particle injection pump: physical
processes that actively transport
biologically derived carbon to ocean
depths.
Pelagic: open ocean waters.
Photic zone: sunlit portion of the upper
water column where photosynthesis
occurs.
Surface mixed layer: uppermost,
unstratified layer of the water column.
Top-down forcing: ecosystem effects
arising from upper-trophic level
changes.
Whale fall: whale carcasses thatsink to
the deep sea.
Whale-fall sp ecialists: fauna relying
on deep-sea whale carcasses to
complete their life cycles.
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Figure 1. Great whales’direct and indirect nutrient and carbon cycling pathways. (A) A hypothetical Southern Ocean, demonstrated by two endmembers
(deep-diving sperm whale, Physeter macrocephalus, left, vs. shallow-diving mysticete, right) that share the same direct carbon pathways (i.e., biomass carbon, whale falls) but differ
in their indirect carbon pathways (i.e., the whale pump). Sperm whales, feeding below the mixed layer, release new nutrients at the surface, stimulating new primary production,
carbon export, and carbon sequestration. Mysticetes, feeding within the mixed layer, release regenerated nutrients at the surface, stimulating recycled primary production and
carbon export. Arrow thickness indicates relative magnitude of the carbon flux. Arrow color indicates direct (orange) versus indirect (white) pathways. (B) The great whale conveyor
belt, an indirect carbon pathway depicting nutrient and carbon fluxes between humpback whale (Megaptera novaeangliae) feeding and breeding grounds in the North Pacific
Ocean. Nutrients released on the breeding grounds are assumed to be new nutrients to the system, thus stimulating new primary production, carbon export, and carbon
sequestration. Nutrients released on the feeding grounds will follow a pathway similar to the mysticete in panel A. While not shown, direct carbon pathways (see panel A) will
also occur on both the feeding and breeding grounds. Illustrations by Alex Boersma.
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biological oceanography, give values for whales that do not acknowledge the complexity and
spatiotemporal variability inherent to whale ecosystem effects (Box 1). We recommend an
economic framework for quantifying the carbon values of great whales that is grounded in
whale ecology and biological oceanography (Box 2). Earth system models may aid in quantifying
indirect carbon pathways by simulating the impact of whale-derived nutrients on phytoplankton
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Figure 1 (continued).
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6Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
and zooplankton communities and their associated carbon export. As data become available, the
economic framework can be populated to estimate whale carbon values for different whale popu-
lations and the predicted impact of conservation interventions.
Can great whale recovery be a nature-based solution to climate change?
Two-thirds of signatories to the Paris Agreement have committed to nature-based solutions (NBS)
to meet the goal of limiting global air temperature rise to ≤1.5°C above pre-industrial levels [70,71].
Protection of blue carbon ecosystems is a nature-based climate mitigation and adaptation solution
declared in the nationally determined contributions (NDCs) of 45 nations [72]. Whereas these NDCs
pertain to coastal blue carbon, quantification of carbon values in other marine biota has expanded
the concept to include noncoastal or oceanic systems [42,73–79]. For marine vertebrates, how-
ever, based on currently available data, there remains significant uncertainty about their carbon
sequestration potential and the ability to effectively manage their role in CDR remains debated
[80]. As more data become available, it is likely this level of uncertainty will decrease.
There have been several studies examining the impact of whales on the carbon cycle that hold
promise for including whales in blue carbon schemes (Table 1), though whether they achieve the
criteria outlined by Lovelock and Duarte remains to be seen [73]. Although it is clear that great
whales are capable of CDR through direct pathways (Table 1), the many unknowns in the indirect
pathways (Figure 1,seeOutstanding questions) preclude quantification at this time of the full
magnitude of whale-CDR required for integration into NBS calculations (e.g., sensu [80,81]) or
NDCs. For comparison, coastal blue carbon systems sequester 0.08–0.2 Gt C year
–1
[82] and
marine vertebrates (including great whales) assessed so far sequester 0.03–0.05 Gt C year
–1
(though this covers a limited number of species across limited regions) [54]. The global potential
for CDR via marine ecosystem recovery is likely to be <0.3 Gt C year
–1
[65].
Table 1. Estimated carbon (C) fate (fixed, stored, exported, sequestered) in pre-industrial whaling and modern-day great whale populations
Mechanism Species Region Pre-whaling C estimate
a
(N) Modern C estimate
(N, year)
C fate Refs
Biomass 8 Baleen whale taxa
b
Global 1.1 × 10
–2
Gt C (2.9 × 10
6
) 2.0 × 10
-3
Gt C
(1.1 × 10
6
, 2016)
Stored
c
Updated
from [7]
d
Whale falls All baleen whales Global 3.6 × 10
–4
Gt C/year
(2.9 × 10
6
)
6.2 × 10
-5
Gt C/year
(1.1 × 10
6
, 2016)
Sequestered Updated
from [7,95]
e
Whale pump Blue whale Southern Ocean 1.3 × 10
–1
Gt C/year, range:
5.2 × 10
–2
Gt C/year to
2×10
2
Gt C/year (2.4 × 10
5
)
2.8 × 10
-3
Gt C/year,
range: 1.2 × 10
-3
to
4.5 × 10
-3
Gt C/year
(5.2 × 10
3
, 2012)
Fixed [44]
Whale pump Blue, fin, humpback, and
Antarctic minke whales
Southern Ocean 2.2 × 10
–1
Gt C/year; range:
2.7 × 10
–2
to 1.5 Gt C/year
2.2 × 10
–2
Gt C/year;
range: 2.7 × 10
-3
to
1.5 10
–1
Gt C/year
Fixed [8]
Whale pump Sperm whale (Physeter
macrocephalus)
Southern Ocean 2.4 × 10
–3
Gt C/year
(~1.2 × 10
5
)
4×10
–4
Gt C/year
(1.2 × 10
4
, 2001)
Exported [43]
Great whale
conveyor belt
Blue whale Southern
Hemisphere
1.4 × 10
–4
Gt C/year
(3.3 × 10
5
)
5.1 × 10
–7
Gt C/year
(1.2 × 10
3
, 2001)
Fixed [10]
a
All C values are gross and do not account for the amount of C respired by great whales.
b
Minke (Balaenoptera acutorostrata), Antarctic minke (Balaenoptera bonaerensis), sei (Balaenoptera borealis), Bryde’s(Balaenoptera brydei), blue (Balaenoptera musculus),
fin(Balaenoptera physalus), bowhead (Balaena mysticetus), gray (Eschrichtius robustus), right (Eubalaena spp.), and humpback (Megaptera novaeangliae)whales.
c
While some great whales can live >100 years, most known lifespans of these species are <100 years; thus, the C is considered stored.
d
Values calculated per Pershing et al.[7] using updated pre-whaling and modern population estimates from Smith et al.[20].
e
Pershing et al.[7] was the first attempt to calculate C export by whale falls. They used a conservative assumption that 50% of rorqual and gray whale carcasses are exported
while only 10% of right and bowhead whale carcasses are exported. Smith and Baco [95] suggested that a high proportion of whale deaths occur during migration. Even for
North Atlantic right whales (Eubalaena glacialis), a coastal species with a robust monitoring network, the observed number of carcasses is much lower than the estimated
number of whale deaths. For this paper, we used higher percentages: 90% for rorquals and gray whales, 50% for right and bowhead whales. This increases the export flux
estimates by a factor of 1.8 compared with the original values.
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A key component of blue carbon is the ability to manage and protect ecosystems for climate-
change mitigation [73,83]. The full CDR role of great whales (and other organisms) will only be
realized through robust conservation and management interventions that directly promote
population increases (e.g., bycatch mitigation, including entanglement reduction; vessel speed
and noise reductions in whale hotspots; creation of marine protected areas). We emphasize
that for whales to aid in CDR, additionality needs to occur [84].
Noting this, natural enhancement of the oceanic carbon sink via whale recovery could be an effec-
tive ‘low-regret’[85] CDR strategy with less risk, longer permanency, and higher efficiency than
geoengineering solutions (e.g., direct carbon injection into the ocean interior [86]). While the artificial
enhancement of the BCP may be ‘leaky’, with effects persisting for a few decades to approximately
150 years [86], the lifespans of great whales range from at least 50 to 200 years [87,88]. When
carbon persistence via reproduction is considered [54], whale recovery has the potential for
long-term self-sustained enhancement of the ocean carbon sink by increasing the standing
stock of stored carbon and carbon sequestered in the deep sea via direct pathways and enhancing
the BCP via indirect pathways (Figure 1). Indeed, ecosystem functions (including carbon services)
of whales have already been formally recognized by resolutions to the International Whaling
Commission and the Convention on Migratory Species [66].
Concluding remarks
Understanding the role of whales in the carbon cycle is a dynamic and emerging field that may
benefit both marine conservation and climate-change strategies. However, while whales are ca-
pable of some degree of CDR, there are numerous knowledge gaps (see Outstanding questions)
[66] that must also be addressed by policy makers seeking to incorporate whale conservation
Outstanding questions
How effective are whales at creating
and maintaining primary productivity
hotspots leading to enhanced
carbon sequestration? Quantification of
cetacean foraging depth with respect
to the annual mixed layer depth is
needed to determine the contribution
of fecal nutrients to carbon recycling
versus export. The proportion of
carbon derived from whale-stimulated
productivity that persists in the food
web versus exported, and potentially
sequestered, likely varies by location
and season.
How bioavailable are whale-derived nu-
trients? Phytoplankton uptake rate of
whale-derived nutrients depends on
numerous poorly understood factors
(e.g., whale defecation depth, persis-
tence of whale excreta in the photic
zone, nutrient leaching rate, organic
ligand presence, variation by whale
and phytoplankton species, microbial
loop remineralization). Estimates from
previous studies should be empirically
tested; incubation experiments may
help.
What is the carbon flux from cetaceans
to the atmosphere? Carbon flux from
cetaceans to the atmosphere via
respiration is needed to provide
cetaceans’net capacity for CO
2
removal. Few studies have included
estimates of CO
2
loss to the
atmosphere via respiration. Empirical
data on breathing rates, respiratory
tidal volume, and CO
2
exchange rates
in the lungs are needed.
How does cetaceans’impact on the
carbon cycle vary by species and
region? Cetaceans’carbon effects
likely have a patchy spatiotemporal
distribution and spatial variability in
sequestration timescales prohibits
a single model of whale-mediated
carbon sequestration for the global
ocean. Caution must be exercised
when extrapolating findings across
species or regions, especially if both
direct and indirect pathways are not
included.
Box 1. Monetizing whale carbon
Whales provide numerous ecosystem services such as tourism in the form of whale watching, enhanced primary productivity,
and carbon sequestration, alongwith culturalbenefits such as education, aesthetics, and existence value [89]. Recent attempts
to monetize whales have garnered attention by valuing an ‘average’whale at $2 million for carbon-capture and other services
[68,69]. As the authors of several foundational papers cited in these reports, we feel the scientific support for this valuation is
lacking [90,91]. Here, we present five concerns regarding the monetary valuations of carbon dioxide removal (CDR) by whales
as proposed by Chami et al.[69].
1. 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 [66]. Whales are absent
from vast areas, with no indication that they could increase phytoplankton productivity by 1%.
2. Organisms’lifespan influences the duration of carbon storage. Whereas whales can store carbon for decades or lon-
ger, phytoplankton and zooplankton typically store carbon for days to months (see ‘Carbon persistence’). Such short-
term storage is generally not considered applicable for carbon-mitigation schemes, which seek storage for ≥100 years
[65]. Carbon must be transported into the deep ocean, or buried in sediments, to achieve centennial sequestration
times [86]. One method used to calculate export efficiency assumes that 10% of net primary production is exported
and 10% of that is sequestered below 1000 m, but export and sequestration efficiencies vary considerably by location
[65,86]. Regional variation in carbon-storage time and efficiency could reasonably reduce the estimated CDR resulting
from whale stimulation of phytoplankton production by 100-fold.
3. Baleen whales often feed and defecate in the mixed layer, yielding primary production from nutrient regeneration [92].
Although surface-feeding whales can play an important role in nutrient recycling, they would have little effect on new
production, carbon export, or CDR, further reducing the values proposed.
4. The model relies on overly simplistic assumptions about whale population growth. Age and size structure should be
considered due to influences on carbon-storage duration (Box 2).
5. The most advanced work on whales and carbon sequestration (see ‘Whale falls’) was not included.
Whales contribute to vital ecological functions and ecosystem services, including CDR in whale falls, yet recent valuations
go beyond our current understanding of their role in the carbon cycle [68,69]. To be viable for CDR strategies, carbon
removal by whales should be measurable and long-term, similar to the Verified Carbon Standard on land (verra.org). Field
experiments and models (e.g.,Earth system models) couldprovide the requisitedata, but recently proposed monetary values
should not be implemented in CDR or other policy strategies before such scientific documentation.
Trends in Ecology & Evolution
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8Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
strategies as part of their climate policy. Persistence of carbon stored in whale bodies and
exported to the deep sea as whale falls via direct pathways are the best resolved and most
precise, yet the largest carbon benefits may result from the indirect pathways, which are also
currently the least understood. Elucidating these key unknowns, including realizing the full poten-
tial for whales to sequester carbon, presents rich opportunities for further study. Achieving this
goal will require interdisciplinary collaboration between marine ecologists, oceanographers,
biogeochemists, carbon-cycle modelers, and economists.
Box 2. A bioeconomic framework for carbon valuation of a whale population
A simple framework to establish the carbon value of whales requires at least three components: (i) species-specific whale population dynamics, (ii) carbon dynamics, and (iii) eco-
nomic valuation. The first component addresses how a whale and its offspring can capture carbon o ver their lifetimes. The second component addresses the processes by which
whales directly and indirectly contribute to the fixation, storage, and sequestration of carbon. Both of these components will require species- or population-specific parameters.
The third component aggregates these dynamics and connects them to the social cost of carbon. This intergenerat ional sequestration potential allows us to quantify the total and
marginal carbon sequestration potential of whale populations. This framework allows for clear comparisons of different population trajectories and can also accommodate more
complex modeling approaches that tackle parameter uncertainty and complex processes. A visual representation of this framework is shown in Figure I, with a hypothetical
business-as-usual (BAU) scenario and two alternative policy interventions.
Model
We use bold symbols to denote matrices and vectors and Greek letters to denote parameters in the model. When relevant,we use subindices to denote specific times or
age classes.
Component 1: species-specific population dynamics
The discrete-time, density-dependent model of an age-structured population builds on [93] and is given by:
Ntþ1¼NtþδM−IðÞNt½I
N
t
is a α× 1 vector of age-specific population sizes at time t. Parameter αdenotes the terminal age class of the population. The scalar δinduces density dependence in
the form δ¼KN
K, where Nis the total population size (or total mass) and Kis the carrying capacity. Finally, Mis an α×αLeslie matrix and Iis the identity matrix. Indexing
age class with the letter i, and letting μ
i
and σ
i
be the age-specific fecundity and survival, respectively, Mis given by:
M¼
μ1σ0μ2σ1…μασα−10
σ00…00
0σ1…00
00…σα−10
2
6
6
4
3
7
7
5½II
Component 2: carbon dynamics
The stock of whale carbon at time tis then given by:
Ct¼∑a
i¼1NitMiCb
|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
Inbody carbon
þ∑a
i¼1NitMiCp
|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
Stimulated
þ∑a
i¼11σi
ðÞNitMiCs
|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
Whalefall
½III
C
b
,C
p
,andC
s
represent the per-kilogram parameters of in-body storage of carbon, carbon storage via productivity enhancement, and sequestration due to death and
sinking, respectively. These parameters can be obtained from theoretical and field studies [7,8,94]. Finally, M
i
is the mass-at-age.
Component 3: valuation
The present value of whale carbon over a horizon of time Tis given by:
V¼∑T
t¼1
θtCtψ
1þρðÞ
t½IV
θ
t
represents the social cost of carbon ($ per ton CO
2
), ρis the social discount rate, and ψis the molecular weight ratio of CO
2
to C. Using a BAU scenario, where whale populations
evolve with no additional interventions, allows us to estimate the current value of whale carbon, V
bau
. Simulating different interventions, like reducing or increasing mortality
(e.g., mitigating entanglement or increasing fisheries activity in whale hotspots, respectively), one can calculate the net value of said intervention, V
pol
. The difference, V
pol
−V
bau
,
is the relative value of carbon sequestered under the intervention. Figure I shows a comparison of two hypothetical alternative recovery paths and their value in relation to BAU.
Trends in Ecology & Evolution OPEN ACCESS
Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx 9
Given the high uncertainty in many of the processes described earlier, and for many natural
climate solutions in the ocean, we advocate for calculations of nature-based carbon sequestration
that explicitly tackle this uncertainty. Importantly, considering the severe and escalating nature of
the climate crisis, we feel it necessary that any consideration of whale recovery as a climate-
change mitigation strategy be carefully assessed and considered in tandem with other mitigation
strategies, particularly those that directly reduce greenhouse gas emissions. NBS grounded in a
whole-ecosystem perspective, which includes natural abundances of species in protected ecosys-
tems, will be essential to these approaches, as will better understanding of the ecological functions
provided by whales and other marine species [65]. We suggest that the precautionary principle be
applied to promote recovery of whale populations as a holistic ecosystem goal. This is likely to
garner multiple benefits to combat the biodiversity and climate crises of the Anthropocene, including
enhanced ecosystem health, productivity, and resilience [8,10], with CDR being a co-benefit[84].
There is power in the totality of actions taken concomitantly to meet the climate challenge.
Acknowledgments
We thank Whale and Dolphin Conservation for constructive feedback,especially Vicki James and Ed Goodall, and for funding
to support the graphics and publication fees. We also thank Kristen Krumhardt and two anonymous reviewers for helpful
feedback. M.S.S. was supported by the National Science Foundation (PRFB 1906332) and MAC3 Impact Philanthropies.
Trends
Trends
in
in
Ecology
Ecology &
Evolution
Evolution
Figure I. Stylized representation of the proposed bio-economic framework. Business-as-usual (BAU) represents the BAU scenario, while Pol1 and Pol2
represent recovery paths that are slower and faster at recovering the population, respectively. (A) Three population trajectories, (B) whale carbon associated with
each trajectory, (C) discounted value of carbon through time, (D) the difference value relative to BAU, and (E) the net present value (NPV) of each policy.
Trends in Ecology & Evolution
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10 Trends in Ecology & Evolution, Month 2022, Vol. xx, No. xx
Declaration of interests
No interests are declared.
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