An Ecosystem-Scale Flux Measurement Strategy to Assess Natural Climate Solutions.

TL;DR: In this paper, the authors propose three deployment types for ecosystem-scale flux measurements at various NCS scales to constrain wide uncertainties and chart a workable path forward: "pilot", "upscale", and "monitor".

Abstract: Eddy covariance measurement systems provide direct observation of the exchange of greenhouse gases between ecosystems and the atmosphere, but have only occasionally been intentionally applied to quantify the carbon dynamics associated with specific climate mitigation strategies. Natural climate solutions (NCS) harness the photosynthetic power of ecosystems to avoid emissions and remove atmospheric carbon dioxide (CO2), sequestering it in biological carbon pools. In this perspective, we aim to determine which kinds of NCS strategies are most suitable for ecosystem-scale flux measurements and how these measurements should be deployed for diverse NCS scales and goals. We find that ecosystem-scale flux measurements bring unique value when assessing NCS strategies characterized by inaccessible and hard-to-observe carbon pool changes, important non-CO2 greenhouse gas fluxes, the potential for biophysical impacts, or dynamic successional changes. We propose three deployment types for ecosystem-scale flux measurements at various NCS scales to constrain wide uncertainties and chart a workable path forward: "pilot", "upscale", and "monitor". Together, the integration of ecosystem-scale flux measurements by the NCS community and the prioritization of NCS measurements by the flux community, have the potential to improve accounting in ways that capture the net impacts, unintended feedbacks, and on-the-ground specifics of a wide range of emerging NCS strategies.

Summary

1. INTRODUCTION

• Stabilizing global temperature at 1.5 °C will require, in addition to rapid decarbonization, significant removal of carbon dioxide (CO2) from the atmosphere.
• The majority of IPCC emission pathways consistent with keeping global temperatures below 2 °C rely on biological CO2 removal practices,11 even though many of them are unproven at scale.
• They can be labor intensive, typically only focus on CO2, may miss important carbon pools, and have low spatial and temporal representativeness.
• Fortunately, eddy covariance measurement systems, de- ployed on what are known as flux towers, allow for the most direct observation of the net exchange−or flux−between ecosystems and the atmosphere at a management-relevant scale.
• The authors examine the role that ecosystem-scale flux measurements could play in evaluating, prioritizing, and implementing NCS.

2. SUITABILITY OF ECOSYSTEM-SCALE FLUX MEASUREMENTS FOR NATURAL CLIMATE SOLUTIONS

• While many conventional carbon inventories consider changes to only one or a few dominant pools of carbon,22 ecosystem-scale flux measurements have the ability to simultaneously measure changes in multiple pools.
• Many emerging NCS strategies involve changes to “hidden” and spatially heterogeneous carbon pools (e.g., soil), important above and belowground pools (e.g., perennial grasslands), hard to access biomass (e.g., in mangroves), or saturated sediments (e.g., in peatlands).
• NCS are typically designed to preserve or increase carbon stocks, but these ecosystem modifications also have the potential to alter emissions of other important GHGs, especially nitrous oxide (N2O) and methane (CH4).
• 40,41 Eddy covariance’s ability to measure all of the major trace gases exchanged by ecosystems make it a powerful option for understanding the entire GHG budget both for NCS strategies that intentionally modify CH4 and N2O regimes and for those that may unintentionally impact net fluxes.
• XXXX, XXX, XXX−XXX C Potential for Biophysical Feedbacks.

3. DEPLOYMENT OF ECOSYSTEM-SCALE FLUX MEASUREMENTS FOR NCS

• Deployment of ecosystem-scale flux measurements for NCS could fall into one of three (nonmutually exclusive) use-cases: “pilot”, “upscale”, and “monitor”.
• Research-driven “pilot” deployments can capture high quality new data about the performance of a particular NCS, under a specific management practice.
• Pilot eddy covariance deployment for NCS may utilize experimental techniques like space-for-time chronosequences (e.g., ref 33), paired control-treatments (e.g., ref 35) that may include ecosystem manipulation (e.g., ref 83), environmental gradients (e.g., ref 50) or even “natural” experiments to critically test the potential of a specific NCS strategy compared to business-as-usual.
• To achieve widespread and spatially explicit NCS quantification, gridded products derived from the current Fluxnet database100−102 could quantify the baseline potential carbon uptake.
• If every NCS project could be constantly monitored with ecosystemscale flux measurements, compliance schemes would be able to precisely quantify the year-to-year carbon performance of each project, compared to a nonintervention baseline, and align financial rewards accordingly.

4. CONCLUSION

• Climate change solutions that harness natural and working lands hold much promise and major uncertainties.
• As the most complete and direct method to measure ecosystem-scale fluxes, eddy covariance has an important role to play in prioritizing, measuring, and monitoring the implementation of NCS.
• The authors framework describes three deployment types that serve different needs and scales (Table 2).
• “Pilot” deployments, combined with complementary measurements, already allow for high precision emission reduction potentials for specific NCS activities.
• The need for accurate, affordable, and accessible accounting of the true impact of NCS, in the face of widespread ecosystem heterogeneity and ongoing climate change, is a key priority if NCS are to realize their potential.

Figures

Table 1. A Significant Portion of the Global Mitigation Potential for Natural Climate Solutions Could Come from Strategies with Hidden or Inaccessible Carbon Pool Changes, Non-CO2 GHGs, the Potential for Biophysical Feedbacks, and Interannual Variability and/or Successiona

Figure 1. We propose four criteria to guide decision making about where and how eddy covariance can be of most value for assessing emerging NCS strategies.

Table 2. Practical Deployment of Ecosystem-Scale Flux Measurements for NCS Can Be Grouped into Three Use-Cases

Figure 2. Ameriflux sites that report being managed (green colors) or disturbed (orange colors) have proliferated over the past decade, allowing potential for paired experimental designs that can provide insights into NCS performance. Sites are grouped by NCS ecosystem “type” (a−c), sized by number of sites (disturbed, managed, or “natural”) within 1 degree. Gray background points represent the climate envelope of Earth, with each point representing the mean climatic conditions of a 0.5° pixel.84 There are 94, 31, and 130 “managed” or “disturbed” sites in each category (forest, blue carbon, agriculture/grassland) out of 494 total Ameriflux sites. Ameriflux site data from https://ameriflux.lbl.gov/.

Full text

UC Berkeley
UC Berkeley Previously Published Works
Title
An Ecosystem-Scale Flux Measurement Strategy to Assess Natural Climate Solutions.
Permalink
https://escholarship.org/uc/item/3qn0j44k
Journal
Environmental science & technology, 55(6)
ISSN
0013-936X
Authors
Hemes, Kyle S
Runkle, Benjamin RK
Novick, Kimberly A
et al.
Publication Date
2021-03-01
DOI
10.1021/acs.est.0c06421
Peer reviewed
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University of California

An Ecosystem-Scale Flux Measurement Strategy to Assess Natural
Climate Solutions
Kyle S. Hemes,* Benjamin R. K. Runkle, Kimberly A. Novick, Dennis D. Baldocchi,
and Christopher B. Field
Cite This: https://dx.doi.org/10.1021/acs.est.0c06421
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ABSTRACT : Eddy covariance measurement systems provide
direct observation of the exchange of greenhouse gases between
ecosystems and the atmosphere, but have only occasionally been
intentionally applied to quantify the carbon dynamics associated
with specific climate mitigation strategies. Natural climate
solutions (NCS) harness the photosynthetic power of ecosystems
to avoid emissions and remove atmospheric carbon dioxide (CO
2
),
sequestering it in biological carbon pools. In this perspective, we
aim to determine which kinds of NCS strategies are most suitable
for ecosystem-scale flux measurements and how these measure-
ments should be deployed for diverse NCS scales and goals. We
find that ecosystem-scale flux measurements bring unique value
when assessing NCS strategies characterized by inaccessible and
hard-to-observe carbon pool changes, important non-CO
2
greenhouse gas fluxes, the potential for biophysical impacts, or dynamic
successional changes. We propose three deployment types for ecosystem-scale flux measurements at various NCS scales to constrain
wide uncertainties and chart a workable path forward: “pilot”, “upscale”, and “monitor”. Together, the integration of ecosystem-scale
flux measurements by the NCS community and the prioritization of NCS measurements by the flux community, have the potential
to improve accounting in ways that capture the net impacts, unintended feedbacks, and on-the-ground specifics of a wide range of
emerging NCS strategies.
1. INTRODUCTION
Stabilizing global temperature at 1.5 °C will require, in
addition to rapid decarbonization, signifi cant removal of
carbon dioxide (CO
2
) from the atmosphere.
1
This goal can
be achieved through a combination of engineered and
biological CO
2
removal strategies.
2
Earth’ slandsurface
currently removes more than a quarter of anthropogenic
carbon emissions through biological processes.
3
Intentionally
managing ecosystems for additional climate mitigation is a
critical component of many climate stabilization pathways.
Despite their prominence in international and subnational
climate change agreements and policy proposals,
4,5
and a
recent flurry of public and private investment,
6
there is an
incomplete understanding of the climate impacts and trade-offs
inherent in biological climate mitigation activities.
7,8
Natural climate solutions (NCS) provide climate benefits
through two broad pathways: avoiding greenhouse gas (GHG)
emissions that would otherwise occur from ecosystems, or
removing CO
2
from the atmosphere through photosynthesis
and sequestering it in biological carbon pools
9,10
(Figure 1).
The majority of IPCC emission pathways consistent with
keeping global temperatures below 2 °C rely on biological CO
2
removal practices,
11
even though many of them are unproven
at sc ale.
12,13
Compared to other more “engineered” CO
2
removal technologies that rely on capture (biological or direct
air) and injection of CO
2
into geological reservoirs,
12
NCS are
considered to be low cost,
2
immediately ready for large-scale
deployment,
14
and not reliant on substantial energy inputs.
15
They are also generally assumed to achieve environmental and
social cobenefits, although ecological and social trade-offs will
need attention as NCS projects scale.
16−18
Despite these high expectations, there are gaps in our
understanding of where, when, and how NCS strategies will be
effective components of a climate-solutions portfolio, especially
given the urgency with which activities must scale from current
levels (much less than 1 Gt CO
2
e) to the 10s or hundreds of
gigatons necessary for meaningful negative emissions.
16,19
For
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many types of NCS,
9, 20,21
traditional carbon inventory
approaches are incomplete or impractical. These traditional
approaches periodically measure the C stocks in biomass
22
or
soil,
23
in an effort to quantify rel atively smal l changes
associated with an NCS activity. They can be labor intensive,
typically only focus on CO
2
, may miss important carbon pools,
and have low spatial and temporal representativeness. In many
cases, current accounting relies on incomplete, or outdated
data.
24
A significant portion of the global potential (Table 1)
for NCS could come from ecosystem modifications that
require a more sophisticated understanding of the changes in
multiple ecosystem carbon pools and thus require a different
set of tools (Figure 1).
Fortunately, eddy covariance measurement systems, de-
ployed on what are known as flux towers, allow for the most
direct observation of the net exchange−or flux−between
ecosystems and the atmosphere at a management-relevant
scale.
25
By integrating carbon pool changes from the soil to the
Figure 1. We propose four criteria to guide decision making about where and how eddy covariance can be of most value for assessing emerging
NCS strategies.
Table 1. A Significant Portion of the Global Mitigation Potential for Natural Climate Solutions Could Come from Strategies
with Hidden or Inaccessible Carbon Pool Changes, Non-CO
2
GHGs, the Potential for Biophysical Feedbacks, and Interannual
Variability and/or Succession
a
a
Global mitigation potential 95% confidence intervals from ref 9.
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B

canopy, these data represent the combined atmospheric
sources and sinks of GHG, water, and energy exchange.
Organized into global and regional open-source data-sharing
networks,
26−28
eddy covariance flux data cou ld play an
expanded role in disentangling the benefits and trade-offs
associated with NCS implementation.
Ecosystem-scale flux measurement systems provide unique
opportunities to evaluate the performance of NCS. First,
sensors on flux towers m ake obse rvations continuously,
allowing for integration of annual, multiyear or even decadal
records of land-atmosphere exchange when combined with
statistical gap filling methods. The technique thus captures
variability over seasonal and diurnal cycles, as well as the
influence of interannual climate variability, management, and
disturbance events. Second, unlike most other approaches for
directly measuring GHG fl uxes, eddy covariance is non-
invasive; it does not introduce artifacts on ecosystems during
measure ment s that are potentially induced by chamber,
cuvette, soil cores, or destructive biomass sampling. Third,
eddy covariance is spatial ly integrative over ecologica lly
meaningful scales, capturing the myriad biotic and abiotic
processes conspiring to exchange mass and momentum with
the atmosphere. Its measurement footprint (typically 100−
1000 ha, depending on tower height and wind dynamics)
29
gives eddy covariance the power to provide data for a
reasonably sized management unit in real time, aggregating
across multiple fast and slow, apparent and hidden, above-
ground and belowground carbon pathways.
To date, ecosystem-scale flux measurements have been used
primarily to gain a richer process-based understanding of how
ecosystems work.
30
Despite serving as a “gold-standard” for
estim ates of land-atmosphere car bon exchange,
31,32
eddy
covariance flux measurement systems have only occasionally
been intentionally applied to quantify the mitigation potential
of emerging NCS strategies (e.g., refs 33−35), to scale regional
NCS portfolio performance, or to monitor compliance or
voluntary carbon sequestration projects (e.g., refs 36 and 37).
In this perspective, we examine the role that ecosystem-scale
flux measurements could play in evaluating, prioritizing, and
implementing NCS. We synthesize into four criteria the
characteristics of NCS strategies for which ecosystem-scale fl ux
measurements, as opposed to traditional inventory techniques,
add particular value (Section 2). Next, we develop a framework
for how ecosystem-sc ale flux measurements should be
deployed for diverse NCS scales and goals (Section 3) before
offering some concluding thoughts (Section 4). Together, we
see great potential to catalyze mutually beneficial solution-
based collaborations between the flux and t he NCS
communities.
2. SUITABILITY OF ECOSYSTEM-SCALE FLUX
MEASUREMENTS FOR NATURAL CLIMATE
SOLUTIONS
Nature of Carbon Pool Changes. While many conven-
tional carbon inventories consider changes to only one or a few
dominant pools of carbon,
22
ecosystem-scale flux measure-
ments have the ability to simultaneously measure changes in
multiple pools. This insight derives from eddy covariance’s
direct measurement of net ecosystem exchange (NEE), the
difference between gross photosynthetic uptake and ecosystem
respiration.
Many emerging NCS strategies involve changes to “hidden”
and spatially heterogeneous carbon pools (e.g., soil), important
above and belowground pools (e.g., perennial grasslands), hard
to access biomass (e.g., in mangroves), or saturated sediments
(e.g., in peatlands). These kinds of NCS ecosystems that are
not dominated by aboveground woody biomass encompass
between a quarter and a third of the global NCS potential
(Table 1, based on ref 9) but present challenges for traditional
inv ento ry approaches. Flux tower measurements can be
particularly useful because they integrate over multiple carbon
sources and sinks and can resolve relatively small changes in
carbon pools that would otherwise require extensive and
expensive sampling regimes. To characterize how suitable flux
measurements are for a specific NCS activity, it is important to
consider visibility (to satellites and airborne sensors), access
(to on-the-ground allometric biomass techniques), spatial
heterogeneity, detection limit (relative magnitude of changes
to the carbon pool), and location (aboveground, litter,
belowground, saturated) of the predominant pool changes.
Forest-NCS strategies like natural forest management or
reforestation, which primarily promote changes in above-
ground, long-lived woody carbon pools, make up the majority
of the global NCS potential (Table 1, based on ref 9). These
strategies may be most commensurate with traditional carbon
inventory approaches (Figure 1, left side). Extensive biometric
measurements have been compared to eddy covariance
measurements, yielding short-term disagreements, due to lags
between photosynthesis and tree growth , but multiyear
agreement.
38,39
(These biometric comparis ons qu antify
changes in carbon pools that are not always considered in
project-based carbon accounting of forest NCS, like soil,
detritus, and fine root litter.
22
) Even in forest-NCS, though,
valuation of ecosystem services beyond carbon removal may
warrant application of ecosystem-scale flux measurements.
Relevant Greenhouse Gases. NCS are typically designed
to preserve or increase carbon stocks, but these ecosystem
modifications also have the potential to alter emissions of other
important GHGs, especially nitrous oxide (N
2
O) and methane
(CH
4
). NCS estimates often fail to sufficiently account for
non-CO
2
trace gases that could enhance or degrade the net
climate impact.
40,41
Eddy covariance’s ability to measure all of
the major trace gases exchanged by ecosystems make it a
powerful option for understanding the entire GHG budget
both for NCS strategies that intentionally modify CH
4
and
N
2
O regimes and for those that may unintentionally impact
net fluxes.
In wetland restoration projects, for example, increased CH
4
emissions can be an unintended consequence of the ecosystem
modification
40
and can change the magnitude or the sign of the
net climate impact. Nitrous oxide, another potent trace gas that
can be measured by eddy covariance,
42,43
is highly intermittent.
In saturated, disturbed, and high-nutrient systems a substantial
fraction of the annual flux may occur in short bursts or in
particular “hotspots”,
44
which eddy covariance can uniquely
quantify. In fact, there is evidence that much of soil C
sequestration benefits could be offset by increased N
2
O
emissions,
41,45
emphasizing the need for understanding the
multi-GHG dynamics of NCS. Several NCS, especially in
agricultural ecosystems, are explicitly designed to reduce
emissions of CH
4
and N
2
O(Table 1). Ensuring that these
activities, like water table manipulation to reduce CH
4
emissions from rice cultivation,
35
do not come at the expense
of CO
2
or N
2
O is essential to a complete understanding of the
multi-GHG trade-offs.
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C

Potential for Biophysical Feedbacks. NCS are designed
to permanently reduce the amount of globally well-mixed
GHGs in the atmosphere, but they also lead to biophysical
impacts, including changes in albedo, energy partitioning, and
surface roughness. These biophysical changes, usually local to
regional, but potentially global in scale (Table 1), can enhance
or degrade the overall climate benefit of a particular NCS
activity or portfolio
46−49
and strongly impact ecosystems and
human communities within and in p roximity to NCS
landscapes. Eddy covariance and its associated instrumentation
are able to measure these types of impacts, such as changes in
surface and radiative temperature,
50,51
water use,
52,53
and
reflectance.
54
The biophysical feedbacks associated with afforestation and
reforestation
46,48,55
andsomecroplandmanagementap-
proaches
56−58
are reasonably well-observed, while feedbacks
associated with novel NCS strategies, like wetland restora-
tion,
59,60
are less well characterized. In some situations, like
reforestation in the temperate or tropical zones, associated
biophysical surface cooling enhances the overall climate
mitigation potential. In others, NCS activities may alter the
energy balance to cause local warming.
61
We currently lack
accounting mechanisms sophisticated enough to quantitatively
weigh the biophysical impacts of NCS activities against the
biogeochemical,
62
or to integrate biophysical effects into a
climate mitigation portfolio. Without accounting for these
unintended side effects, as is possible with ecosystem-scale flux
measurements, we risk scaling up NCS strategies that are self-
defeating or detrimental.
Interannual Variability. While many NCS estimates
assume steady-state conditions,
10,63
a long-term perspective
of the potential of NCS strategies must contend with the
nonstationary nature of carbon uptake over time scales of years
to decades.
64−67
Even in seemingly undisturbed or unmanaged
ecosystems, long-term trends in carbon uptake can result from
exogenous factors like CO
2
fertilization, nitrogen deposition,
and increased prevalence of weather extremes.
68,69
To count
on NCS strategies to provide long-term carbon removal
requires a sophisticated understanding of how NCS
ecosystems take up and release carbon at various time scales,
which has long been a core focus for the eddy covariance
community.
27
Because flux tower data are not autocorrelated beyond a
period of days to weeks,
70
it is possible to observe how
ecosystem processes respond to both short- and long-term
environmental changes.
71
This allows us not only to quantify
netreductionsinecosystemfluxes due to disturbance,
management, and succession in real-time, but also to gain
insight about the mechanisms by which these processes impact
gross ecosystem fluxes.
72−75
These kinds of ecosystem-scale
flux data can help train models and remote sensing products to
integrate permanence risks to NCS at a spatially extensive
scale.
Sets of eddy covariance towers (meso-networks) deployed
within a space-for-time experimental design have been used to
understand the carbon dynamics of postdisturbance succession
without having to wait many years for succession to play out at
a single site.
76,77
Constraining carbon recovery trajectories, like
in abandoned agricultural land restoring to forests in the
Table 2. Practical Deployment of Ecosystem-Scale Flux Measurements for NCS Can Be Grouped into Three Use-Cases
use-case goal metric implementor scale funding
pilot collect specific, high quality
data to help prioritize and
quantify the impact of spe-
cific NCS strategies
local to regional practice-
specific emission reduction
potentials that relate NCS
activity to fluxes
academic institutions,
foundations, govern-
ment agencies
site-level or meso-network observa-
tions contrasting land use/manage-
ment, or space-for-time treatments
research funding to establish
constraints and identify po-
tential unintended feedbacks
upscale facilitate spatially extensive
measurements of NCS per-
formance across broad
geographies
scaling functions that relate
spatially extensive, gridded
inputs, to observed NCS
fluxes
academic institutions,
government agen-
cies at multiple ju-
risdictional scales
regional to jurisdictional, based on
regional potential and uncertainty
in NCS portfolios. Multiple sites
needed for upscaling.
quality-control investments for
scaling and validating a
regional NCS portfolio or
compliance system
monitor quantify the climatic perform-
ance of a specific compliance
NCS project
annual net carbon and GHG
balance of NCS project
area
project implementor,
project aggregator,
land owner
project-scale, based on carbon and
ecosystem stratification within
project, and requirements of quan-
tification methodology
compliance requirements or
market-based incentives
Figure 2. Ameriflux sites that report being managed (green colors) or disturbed (orange colors) have proliferated over the past decade, allowing
potential for paired experimental designs that can provide insights into NCS performance. Sites are grouped by NCS ecosystem “type” (a−c), sized
by number of sites (disturbed, managed, or “natural”) within 1 degree. Gray background points represent the climate envelope of Earth, with each
point representing the mean climatic conditions of a 0.5° pixel.
84
There are 94, 31, and 130 “managed” or “disturbed” sites in each category (forest,
blue carbon, agriculture/grassland) out of 494 total Ameriflux sites. Ameriflux site data from https://ameriflux.lbl.gov/.
Environmental Science & Technology pubs.acs.org/est Perspective
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Environ. Sci. Technol. XXXX, XXX, XXX−XXX
D

Frequently asked questions

Q1. What have the authors contributed in "An ecosystem-scale flux measurement strategy to assess natural climate solutions" ?

In this perspective, the authors aim to determine which kinds of NCS strategies are most suitable for ecosystem-scale flux measurements and how these measurements should be deployed for diverse NCS scales and goals. The authors propose three deployment types for ecosystem-scale flux measurements at various NCS scales to constrain wide uncertainties and chart a workable path forward: “ pilot ”, “ upscale ”, and “ monitor ”. The authors find that ecosystem-scale flux measurements bring unique value when assessing NCS strategies characterized by inaccessible and hard-to-observe carbon pool changes, important non-CO2 greenhouse gas fluxes, the potential for biophysical impacts, or dynamic successional changes. Together, the integration of ecosystem-scale flux measurements by the NCS community and the prioritization of NCS measurements by the flux community, have the potential to improve accounting in ways that capture the net impacts, unintended feedbacks, and on-the-ground specifics of a wide range of emerging NCS strategies. 

Q2. What are the main effects of NCS?

NCS are designed to permanently reduce the amount of globally well-mixed GHGs in the atmosphere, but they also lead to biophysical impacts, including changes in albedo, energy partitioning, and surface roughness. 

Q3. What is the common use-case for NCS?

Deployment of ecosystem-scale flux measurements for NCS could fall into one of three (nonmutually exclusive) use-cases: “pilot”, “upscale”, and “monitor”. 

Q4. What is the way to measure the impact of NCS?

“Pilot” deployments, combined with complementary measurements, already allow for high precision emission reduction potentials for specific NCS activities. 

Q5. What is the way to measure carbon in the landscape?

Deployment of eddy covariance networks in tandem with high resolution remote sensing methods−near-surface, airborne, and satellite−facilitates quantification of landscape-scale carbon responses to management changes and ecosystem modifications. 

Q6. What is the key priority of the NCS community?

The need for accurate, affordable, and accessible accounting of the true impact of NCS, in the face of widespread ecosystem heterogeneity and ongoing climate change, is a key priority if NCS are to realize their potential. 

Q7. What is the role of eddy covariance in the planning of NCS?

As the most complete and direct method to measure ecosystem-scale fluxes, eddy covariance has an important role to play in prioritizing, measuring, and monitoring the implementation of NCS. 

Q8. What is the role of eddy covariance in the assessment of carbon pools?

Flux tower measurements can be particularly useful because they integrate over multiple carbon sources and sinks and can resolve relatively small changes in carbon pools that would otherwise require extensive and expensive sampling regimes. 

Q9. What is the main argument for continuous, direct measurement of project-based NCS performance?

Others have argued that continuous, direct measurement of project-based NCS performance can increase certainty and reduce invalidation risks. 

Q10. What are the critical jobs for ecosystem-scale gas exchange measurements?

XXX, XXX−XXXDsoutheastern U.S.51 or irrecoverable carbon hotspots78 like peatlands and other blue carbon ecosystems,33 are critical jobs for ecosystem-scale gas exchange measurements. 

Q11. What is the role of pilots in the NCS community?

Research-driven “pilot” deployments can capture high quality new data about the performance of a particular NCS, under a specific management practice. 

Q12. What is the role of eddy covariance flux data in the mitigation of climate change?

Organized into global and regional open-source data-sharing networks,26−28 eddy covariance flux data could play an expanded role in disentangling the benefits and trade-offs associated with NCS implementation. 

Q13. What is the common approach to quantifying carbon removal?

At lower levels of precision, quantification methodologies often take a conservative approach to credit allocation, resulting in fewer credits. 

Q14. Why is it possible to observe how ecosystem processes respond to short- and long-term environmental changes?

27Because flux tower data are not autocorrelated beyond a period of days to weeks,70 it is possible to observe how ecosystem processes respond to both short- and long-term environmental changes. 

Q15. What can be done to measure the net carbon balance of a NCS?

Site or meso-network measurement of the ecosystem-scale fluxes of NCS can offer critical insights into their performance, trade-offs, and unintended impacts. 

Q16. Why have eddy covariance measurements been used to quantify mitigation potential of emerging NCS?

To date, ecosystem-scale flux measurements have been used primarily to gain a richer process-based understanding of how ecosystems work.30 Despite serving as a “gold-standard” for estimates of land-atmosphere carbon exchange,31,32 eddy covariance flux measurement systems have only occasionally been intentionally applied to quantify the mitigation potential of emerging NCS strategies (e.g., refs 33−35), to scale regional NCS portfolio performance, or to monitor compliance or voluntary carbon sequestration projects (e.g., refs 36 and 37). 

Q17. What are the common types of NCS?

Forest-NCS strategies like natural forest management orreforestation, which primarily promote changes in aboveground, long-lived woody carbon pools, make up the majority of the global NCS potential (Table 1, based on ref 9). 

Q18. How much can a basic eddy covariance measurement system cost?

Even so, a basic eddy covariance measurement system can still cost multiple 10s of thousands of USD in shortstatured ecosystems, and substantially more where tall towers are required to extend above a forest canopy.