INSIGHTS | PERSPECTIVES
1530 23 DECEMBER 2016 • VOL 354 ISSUE 6319 sciencemag.org SCIENCE
life. However, the increase in nutrient input
would need to be sustained for at least a
thousand years to produce a change in phosphate levels sufficient to bring on a full-scale
OAE. Whole-ocean anoxia is thus not an immediate global concern. If sustained for long
enough, the deoxygenation occurring today
could nevertheless have lasting negative consequences for the global environment.
On yet longer time scales, the paradox
noted by Redfield may be explained by another feedback. A sustained increase in phosphate entering the ocean would increase not
only anoxia but also marine productivity,
causing more carbon to be buried in sediments. Carbon burial is the source of free
oxygen because burial is the only process by
which photosynthetically fixed carbon can
escape reoxidation on a geologically short
time scale. The resulting atmospheric oxygen increase would start to be significant on
time scales of 100,000 years or more, eventually alleviating the ocean anoxia (see the
figure). Rising molecular oxygen might also
limit forest vegetation on land because of
the increased prevalence of wildfire; given
that forests increase the rates of weathering
of continental rocks, limiting them would
provide a negative feedback on the supply of
phosphorus to the oceans, bringing the system back to a new steady state (14).
Atmospheric oxygen and ocean phosphorus are thus linked in a network of multiple
feedback loops. Negative feedbacks help to
explain the longevity and stability of atmosphere and ocean composition, but some
feedbacks are of opposite sign and may at
times destabilize the Earth system, as during
OAEs. The role of these positive feedbacks in
sustaining OAEs remains an open question,
however, as does a complete description of
the underlying causes of modern-day deoxygenation; conceivably, natural feedbacks may
act to amplify the effects of global change on
ocean oxygen concentrations. j
1. B.J.W.Millsetal.,Geology 44,1023(2016).
2. I. J. Glasspool etal., Nat.Geosci. 3, 627 (2010).
3. H.C.Jenkyns, Geochem.Geophys.Geosyst. 11,Q03004
4. L. M. E. Percival et al., Geology 44, 759 (2016).
5. L. M. E. Percival et al., Earth Planet. Sci. Lett.428, 267 (2015),
6. IPCC, Climate Change 2013: The Physical Science Basis.
Working Group I Contribution to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change,
T. F. Stocker et al., Eds. (Cambridge Univ. Press, Cambridge,
7. “Ocean ventilation in a warming world,” Meeting at the Royal
Society of London, 12 to 13 September 2016, J. G. Shepherd,
A. J. Watson, A. Oschlies, P. Brewer, Conveners.
8. A.C.Redfield, Am. Sci.46,205(1958).
9. B.Sell etal., EarthPlanet.Sci.Lett. 408,48(2014).
10. S. C. Turgeon etal.,Nature 454, 323 (2008).
11. I.C.Handoh, T.M.Lenton,Global Biogeochem.Cycles 17,
12. K.Ozaki etal.,Earth Planet.Sci.Lett. 304,270(2011).
13. F. T.Mackenzie etal., Chem.Geol. 190,13(2002).
14. N. M. Bergman etal., Am.J.Sci.304, 397 (2004).
10.1126/science.aaj2321 P H
By Elizabeth A. Fulton1,2
Human intuition often does not lead to thoughtful long-term decision- making (1). In partial remedy to this, a long history of storytelling conveys messages about what is worth remem- bering, worth doing, or best avoided.
Stories also help to shape visions of what is
possible and desirable (2). Scientific models
are used in the same way. For example, the
modeling study by Cheung et al. (3) on page
1591 of this issue indicates that making the
effort to keep global temperature increases
to 1.5°C would minimize impacts on marine
food security and resilience. The work is an
important addition to the discussion of the
costs and benefits of keeping the average
global temperature rise to less than 2°C.
A temperature change of 2°C does not
seem like a lot, especially given much wider
daily temperature ranges in many locations.
However, in terms of global mean temperature, it marks a point where climate change
would have broad-scale effects on many locations and aspects of life, ultimately leading to a less productive world with reduced
capacity to support basic human needs (4).
This was the motivation for the Paris Agreement, adopted by consensus on 12 December
2015 and ratified on 4 November 2016. The
aims at the heart of this agreement highlight
the basic tension in today’s public policy
decision-making: how to achieve economic
development, meet basic human needs, and
limit environmental impacts as the global
population grows toward 9 billion by 2050.
The agreement aims to hold the global average temperature rise to less than 2°C—and
ideally less than 1.5°C—above preindustrial
levels while also increasing adaptive capacity,
resilience, and food security.
Meeting these targets seems like a daunting task. The International Energy Agency
has concluded that a $16.5 trillion USD
investment would be needed to meet the
energy-related climate pledges of the agreement by 2030 (5). Nevertheless, models show
the value of action and the costs of inaction.
For example, Cheung et al.’s findings indicate
that meeting the agreement’s temperature
target may also be the best means to deliver
on food security and achieve resilient communities and economies.
The authors use models of changing species distributions, based on the physiological and habitat needs of each species and
their capacity to move to suitable habitats
as conditions change, to explore the potential future abundance and catch of 892 species currently fished around the world. They
show that a global temperature rise of 1.5°C
is enough to reduce fisheries catches on a
global scale. As the temperature rise climbs
from 1.5° to 3.5°C, global catch losses jump
from 2.5 to 8.0%. Modeled changes in species
diversity are even greater as species migrate
or go locally extinct: Species turnover more
than doubles from 8.3 to 21.6% as the temperature increase rises from 1.5° to 3.5°C.
1CSIRO Oceans and Atmosphere, Hobart, TAS 7001, Australia.
2Centre for Marine Socioecology, University of Tasmania,
Hobart, TAS, Australia. Email: email@example.com
A stitch in
Acting now to limit global
temperature change will
help to ensure global food