INSIGHTS | PERSPECTIVES
sciencemag.org SCIENCEultimately constrains the production of bio-
mass at all levels of the food chain.
On the basis of a prior analysis of histor-
ical data, a recent study hypothesized that
the annually cumulated PP in seasonally
ice-free regions is determined primarily by
the input of nutrients (essentially nitrate)
brought to the surface through vertical mix-
ing ( 2 ). Despite the increase in underwater
light from shrinking sea ice, growing fresh-
water inputs more strongly stratify the AO.
This likely will lower the nutrient supply
and thus PP, unless other physical changes
reverse this trend ( 3 ).
Ocean color can be detected from space
and analyzed to estimate the concentration
of chlorophyll a at the sea surface (a proxy
for phytoplankton biomass) and the rate
at which solar energy hits the AO surface.
When the atmosphere is cloud free, these
first-order determinants of PP, together with
sea surface temperature, are monitored daily
by several satellites over the entire world
ocean. Such ocean color data have been
available uninterruptedly since 1998.
From the late 2000s on, several studies
based on ocean color data have reported
a substantial increase in the annual PP
of seasonally ice-free Arctic waters ( 4 , 5 ).
Ecologists have ascribed this increase pri-
marily to a decrease in the icepack during
summer and early fall, combined with the
lengthening of the ice-free season. As sea ice
decreases, open waters are exposed to much
more sunlight, which promotes photosyn-
thesis and phytoplankton biomass produc-
tion. It is unclear, however, how much of
this increase results from new production
that can be transferred to the food chain
versus “useless” recycled production.
Lewis et al. used satellite observations of
AO color to determine the response of an-
nual PP to climate change during the past
two decades, the longest Arctic PP time
series available. The authors observed a
progressive increase in PP that over time
reached 57%, which far exceeds previous es-
timates. These findings confirmed the role
of sea ice shrinking as a driver of PP, but
only during the 1998 to 2008 period. Lewis
et al. then showed that the increase in PP
between 2009 and 2018 was driven primar-
ily by an increase in chlorophyll a concen-
tration, although the authors documented
substantially different regional trends.
The standing stock of phytoplankton de-
pends on the balance between gains from
growth and losses from grazing by small
animals. However, the stock also gener-
ally correlates with nutrient concentra-
tions. A key conclusion that can be drawn
from Lewis et al.’s study is that the budget
of inorganic nutrients might be increasing
rather than diminishing in specific AO sec-
tors over the time periods studied. Nutrient
input might intensify at the Pacific and
Atlantic boundaries, whereas the reduc-
tion in sea ice cover might expose the AO to
stronger atmospheric forcings that promote
more vertical mixing.
Lewis et al. also found that PP did not
change in several areas. Therefore, it re-
mains unknown whether overall AO-wide
PP will continue to increase if the amount
of sea ice keeps shrinking. The rest of this
story will stem from longer time series
and a better grasp of the physical phenom-
ena that control nutrient concentrations.
Although satellite observations provide rel-
atively long time series and a means for ex-
ploring the AO at a large scale, information
from AO satellite images comes with limi-
tations—for example, restriction to surface
properties and low spatial and time resolu-
tion, especially for optical sensors in thick
cloud cover. Furthermore, scales relevant
to PP span millimeters to the ocean basin
(spatial) and seconds to decades (time).
Satellites provide no observation under
sea ice, where much occurs that is equally
important to PP. Indeed, massive phyto-
plankton blooms were recently observed
under sea ice and are thought to be more
common than previously anticipated ( 6 ).
The ongoing thinning of sea ice and increas-
ing occurrence of melt ponds, which act as
skylights at the ocean’s surface in spring,
might make blooms even more prevalent
( 7 ). Emerging approaches for capturing the
various scales important to PP include au-
tonomous sampling platforms with sensors
for exploring the medium to large scales in
ice-free and ice-covered waters.
Much remains to be discovered about
the small scales, such as those relevant to
microalgae living in the inner interstices
of sea ice. To address current and future
changes of PP in the AO and their effects
on the marine trophic network, researchers
require new devices to explore tiny compo-
nents and an integrated observation system
to capture and connect all relevant scales. jREFERENCES AND NOTES- K. M. Lewis, G. L. van Dijken, K. R. Arrigo, Science 369,
198 (2020). - J. E. Tremblay et al., Prog. Oceanogr. 139, 171 (2015).
- W. K. W. Li, F. A. McLaughlin, C. Lovejoy, E. C. Carmack,
Science 326, 539 (2009). - K. R. Arrigo, G. L. van Dijken, Prog. Oceanogr. 136, 60
(2015). - S. Bélanger, M. Babin, J.-É. Tremblay, Biogeosciences 10,
4087 (2013). - K. R. Arrigo et al., Science 336, 1408 (2012).
- K. E. Lowry, G. L. van Dijken, K. R. Arrigo, Deep Sea Res.
Part II Top. Stud. Oceanogr. 105, 105 (2014).
ACKNOWLEDGMENTS
M.B. thanks M. Lizotte, A. Randelhoff, J.-E. Tremblay, and M.-H.
Forget for valuable editing and scientific comments.10.1126/science.abd1231ELECTROCHEMISTRYFuel cells that
operate at
300° to 500°C
High-conductivity
electrolytes are needed for
electrochemical systems
By Meng Ni1,2 and Zongping Shao3,4R
educing the operating temperature
of ceramic fuel cells (CFCs) from
800° to 1000°C to the 300°-to-500°C
range would improve efficiency, seal-
ing, durability, and cost while still
maintaining favorable electrode re-
action kinetics as compared with those of
low-temperature fuel cells such as polymer
electrolyte fuel cells. Developing stable elec-
trolytes with a low ionic resistance and neg-
ligible electronic conductivity, however, is
challenging. In principle, reducing the elec-
trolyte thickness can reduce the resistance.
Fabricating an ultrathin electrolyte re-
quires advanced techniques that inevitably
make mass production difficult and costly.
Developing new high-conductivity electro-
lyte materials is another way to address this
problem. On page 184 of this issue, Wu et
al. ( 1 ) report a fuel cell with a distinct, high
proton conductivity electrolyte.
Conventional CFCs have an oxygen-ion
conducting electrolyte (OCFC) that usually
operates at above 800°C to allow for fast
oxygen-ion transport. Because the proton
has a lower barrier for diffusion, protonic
CFCs (PCFCs) are generally accepted to be
more promising than conventional OCFCs
for operation at reduced temperatures. In
addition, because the fuel-diluting H 2 O
is produced in the cathode chamber of
PCFCs, the expectation is for higher fuel
utilization and higher theoretical perfor-
mance. The authors’ electrolyte consists of
a NaxCoO 2 /CeO 2 composite, which shows a
conductivity of 0.1 to 0.3 S cm–1 at 370° to
520°C. The authors believe that the proton(^1) Research Institute for Sustainable Urban Development
and Research Institute for Smart Energy, Hong Kong
Polytechnic University, Hong Kong SAR, China.^2 Department
of Building and Real Estate, Hong Kong Polytechnic
University, Hong Kong SAR, China.^3 State Key Laboratory
of Materials-Oriented Chemical Engineering, College of
Chemical Engineering, Nanjing Tech University, Nanjing,
China.^4 WA School of Mines: Minerals, Energy, and Chemical
Engineering, Curtin University, Perth, WA 6845, Australia.
Email: [email protected]; [email protected]
138 10 JULY 2020 • VOL 369 ISSUE 6500