Oceanography | Early Online Release
the capabilities for testing this theory globally were in their infancy.
The consequences of fossil fuel consumption were of tremendous
importance societally, but at the time of Revelle and Suess’s procla
mation, that was not a part of public discussion and politics as it
is today. Decades of research followed as investigators sought the
means to conduct ocean-basin-scale measurements needed to
assess predicted trends. In 1999, Peter Brewer elegantly recounted
that history during the first annual Revelle Lecture (Brewer, 2000).
Capturing time-space variations in the ocean’s interior, at basin
scales, accurately, is no small challenge. For many years, the only
practical way to tackle this problem was by using crewed ships to
conduct hydrographic surveys. Despite the analytical and logisti
cal challenges, a picture of the exchange of CO2 between the atmo
sphere and ocean slowly emerged (Brewer, 2013). Decades of
work were required to establish the connection between the burn
ing of fossil fuels and the reality of human-driven climate change
and ocean acidification. Ironically, nearly 70 years after Revelle
and Suess issued their “geophysical experiment” proposition, we
now find ourselves scrambling to assess the promise and pitfalls
of artificially stimulating the ocean to absorb more CO2 to mit
igate a climate crisis of our own making (e.g., Coale et al., 1996;
Brewer, 2013; Bach and Boyd, 2021; NASEM, 2022; Levin et al.,
2023; S.M. Smith et al., 2024; Findlay et al., 2025, in this issue).
ENTER THE ROBOTS
As ocean sensor systems matured, so too did the platforms on
which they could be deployed. In addition to measurements
acquired manually, scientists and engineers developed the means to
automate air-sea CO2 flux measurements aboard ships, moorings,
and autonomous surface vehicles (ASVs; Friederich et al., 1995;
Chavez et al., 2017b, 2018). It was apparent that seasonality and
geographical location played an important role in when and where
there was a net flux of CO2 into or out of the sea (e.g., Takahashi
et al., 2009). Other sensor systems for autonomously acquiring bio
geochemical measurements, such as pH (Johnson et al., 2016) and
nitrate (Sakamoto et al., 2017), also evolved along with improve
ments for in situ quantification of oxygen and optical parameters,
all of which were deployable on autonomous underwater vehicles
(AUVs) and ROVs. Development and use of these biogeochemi
cal sensor suites were greatly aided by the availability of mooring
technology and well-established time-series studies that included
routine ship-based hydrographic surveys (e.g., Karl, 2010, 2014;
Chavez et al., 2017b). Now, after years of observations, the unmis
takable trend of rising CO2 in the atmosphere with concurrent
changes in ocean pH and temperature has emerged (Figure 1;
e.g., Thorne et al., 2024) along with complex biological and eco
system manifestations (e.g., Doney et al., 2020; Alter et al., 2024).
Thanks to a remarkable confluence of technologies and dogged
determination on the part of scores of visionary scientists and
engineers, it is now possible to observe ocean basin-scale car
bon cycling using a distributed fleet of profiling floats—robots—
that offer much more information at a far lower cost compared
to ship-based surveys (Figure 2a,b; e.g., Johnson and Claustre,
2016; Claustre et al., 2020; Schofield et al., 2022; Sarmiento et al.,
2023). A global fleet of floats now returns sensor measurement
data from remote regions of the globe in real time, and the infor
mation acquired is freely accessible to anyone nearly instantly via
the Internet (GO-BGC). This remarkable achievement has given
ocean scientists the equivalent of a medical doctor’s tool kit for
rapidly assessing a patient’s vital signs. As a result, we now know
that the Southern Ocean—one of the most inaccessible and diffi
cult places to work—plays a major role in ocean-atmosphere car
bon cycling and global climate modulation (Liniger et al., 2025).
FIGURE 1. Plots show time series from 1900 to the present of (a) atmospheric
carbon dioxide (CO2) measured from ice cores (black) and the Mauna Loa
Observatory (red) on the Big Island of Hawai‘i (Keeling et al., 2001; MacFarling
Meure et al., 2006). The trend in the partial pressure of surface ocean pCO2
(a measure of CO2 entering or exiting the sea) in Monterey Bay, California,
from the early 1990s is also shown (blue; updated from Chavez et al., 2017b).
(b) Surface ocean pH data from the early 1990s to the present are shown
here from the Hawai‘i Ocean Time-series (HOT) program (red; Karl and
Lukas, 1996) and Monterey Bay California (blue; updated from Chavez et al.,
2017b). Note that the pH scale is logarithmic. (c) The figure shows sea sur
face temperature anomalies (seasonal cycle removed) from the California
Current along the US West Coast (Huang et al., 2017). Clear trends are evi
dent for all of the measurements.