Carbon Brief - Roz Pidcock
Climate change is altering the world's watery expanses in a number of ways, with serious knock-on effects for the ocean's plants and animals, according to new research.
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| Shoal of spade fish in the Atlantic. Credit: leofrancini/iStock/Getty Images |
The study finds that with just 15
more years of current emissions, over half of the world's ocean will be
exposed to more than one source of stress, affecting everything from the
tiniest plants to the mightiest whales. By 2050, that figure rises to
around 86% of the ocean, say the authors.
The paper,
published
today in the journal Nature Communications, finds that if all countries
stick to their pledges to cut emissions, as part of the
Paris Agreement, this would relieve the pressure by giving organisms an extra 20 years to adapt or migrate.
But for Arctic species, there may be
no refuge from climate change, the paper warns, regardless of how
quickly we act to cut emissions.
'Stress factor'
The oceans are essential for human
existence, the paper begins, providing a primary source of protein for
one in seven of the global population.
But through our own activities, humanity is putting a strain on the ocean's ability to perform that function, in four main ways.
First, warmer water is more inclined to stratify into
layers,
restricting access to nutrients for plants living on the surface. At
the same time, excess CO2 absorbed by the ocean is making it
more acidic, with consequences for
marine life that build shells or skeletons.
Thirdly, warmer water reduces the
amount of oxygen
in the top layer of the ocean, the study explains. Finally, all of
these things affect the process by which marine plants produce food to live and grow, known as primary production.
Dr Stephanie Henson,
a specialist in biogeochemical oceanography at the National
Oceanography Centre in Southampton, who is the lead author on the paper,
tells Carbon Brief:
"Primary production is the base of the food web so the ultimate food source for everything in the oceans. If that food source is declining, that will obviously put stress on the marine ecosystems as well."
The study looks at how those four types of stressors interact in the future to give an overall "stress factor".
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| A calanoid copepod, a type of zooplankton. Species of warm-water copepod
in the North Atlantic have shifted northward in recent decades. Credit:
Proyecto Agua. |
Defining stress
First, the team need to define what
kind of conditions in the oceans count as normal and what is likely to
move marine life into stress mode. Henson explains:
"Ecosystems are adapted to the natural range of
variability that they experience. This means that organisms living in a
particular part of the ocean are able to survive the normal range of
winter-to-summer temperatures."
The scientists used those bounds of
the natural summer-winter cycle to define what was tolerable for a
particular region. Henson continues:
"Then we looked at where conditions under climate
change went outside that boundary of normal conditions – and stayed
there until the end of the century. That gave us our definition of when
things became unusual."
But it gets more complicated, the paper explains. The four different
stress factors rarely occur in isolation, sometimes
overlapping to amplify the overall impact on marine life.
A lowering of pH and oxygen concentration can make
corals and
crustaceans more sensitive to rising temperatures, for example. Sometimes stressors can interact in unexpected ways to trigger a
positive effect – all of which makes studying this topic in field or lab studies extremely challenging, the paper notes.
Today's study is a new attempt to streamline research in a lot of different areas, says
Dr Scott Doney,
department chair in marine chemistry at the Woods Hole Oceanographic
Institute, who wasn't involved in the research. He tells Carbon Brief:
"The study is novel because it brings together several different science threads to create a unified picture of ocean climate change."
Time of emergence
The team ran 12 different climate
models to try to pinpoint when the "signal" of each of the four
stressors – temperature, pH, oxygen and primary production – was likely
to emerge from the background "noise" of natural variability, marking
the shift to stressful conditions.
The results show that pH is the
fastest of the four stress factors to escalate. In fact, the team found
99% of the open ocean already is experiencing stress from water getting
more acidic.
The maps below from the paper shows
how each of the four stress factors pan out across the world. The red
colour spread across panel c shows how widespread exposure to stress
from acidifying oceans is already.
Similarly, rising temperatures are
already causing stress to organisms in the subtropics and the Arctic,
the paper explains. You can see this in panel a in the graphs below. On
average, the world's oceans will tip into temperature-related stress by
about 2034, assuming emissions continue to track as high as they are
now.
For primary production, that point is
reached later in the century, around 2052 (panel b). For oxygen, it is
later still, in 2070 (panel d). Henson tells Carbon Brief:
"Ocean acidification happens very quickly, the oceans
warm up relatively quickly while primary production and oxygen take
longer to affect ecosystems."
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| Multi-model average for the year when stress exceeds natural variability
for (a) sea surface temperature (b) primary production, (c) pH and
(d)oxygen content assuming a 'business-as-usual' scenario (RCP8.5).
Henson et al., (2017) |
Pace of change
Whether or not organisms will be able
to cope with the effects of climate change depends to some extent on
the speed of the change. As the paper explains:
"The more rapidly the system is pushed out of its
natural range of variability, the less time the organisms will have to
adapt or acclimate to the new conditions or migrate to more suitable
areas."
Another factor is whether more than
one rapidly-developing stressor occur together. The Arctic appears to be
a "hotspot of change", says the paper, experiencing very rapid changes
in pH, sea surface temperature and oxygen all at once.
For everywhere else, the changes are
slower. But by 2030, 55% of the world's oceans will encounter a "mosaic"
of stressors, rising to 86% in 2050. By 2100, nearly two-thirds of the
ocean (62%) will be stressed by all four factors together, the study
finds. Henson tells Carbon Brief:
"If organisms are negatively affected by climate
change, that effect is going to continue all the way up the food chain,
up from phytoplankton at the base, through fish and into animals like
dolphins and whales, seabirds and so one."
The impacts of stressed oceans will
be particularly important for coastal developing nations, which depend
heavily on the oceans for protein sources. Henson adds:
"There are areas around the Indian Ocean, for example,
that will be quite rapidly affected. So there is the possibility that
fishing countries along those borders will suffer."
The team looked at an alternative
scenario in which all nations signed up to the Paris Agreement succeed
in their pledges to reduce their emissions.
For those well-versed in the
Intergovernmental Panel on Climate Change's emissions scenarios, this
approximately corresponds to RCP4.5, implying warming by the end of the
century of between 2.6-3.1C.
This level of mitigation would mean
the pace of climate change in the oceans is much slower, says Henson.
She tells Carbon Brief:
"What the mitigation scenario shows us is that it
allows an extra 20 years or so for organisms to adapt and acclimate to
conditions."
Mitigation would mean that only 34%
of the ocean becomes exposed to multiple stressors within the next 15
years, compared with 55% under a "business-as-usual" scenario. By
2100, 30% of the ocean is affected by all four stressors in the
mitigation scenario, compared to 62% under "business as-usual".
The exception to the rule is the
Arctic, where mitigation does little to slow the emergence of multiple
drivers, the paper notes.
'One way street'
The study makes the implicit
assumption that organisms will suffer with prolonged exposure to
conditions outside of their natural ranges. But some
studies have shown species appearing to thrive in environments that are less than optimal, the paper notes.
Across most of the global ocean, however, the implications for marine life will be similar:
"When the environment changes sufficiently that new
conditions, or a new combination of conditions, emerge and persist, the
organisms must adapt, migrate to more favourable areas, or face
extinction."
Marine creatures that already
encounter big swings in natural variability may show the greatest
resilience to change, the study suggests. Others may be able to migrate
to new habitats. For example, a species of warm-water copepod in the
North Atlantic has
shifted northward in recent decades. But, as the paper notes, this tactic won't work for all species:
"Polar species are also particularly vulnerable as they
cannot shift their geographical range northward in response to emerging
drivers and so must either adapt to changing conditions or go extinct."
Human societies will need to adjust so that livelihoods are protected, as well as marine biodiversity. Doney tells Carbon Brief:
"The next steps are to develop better informed
strategies for protecting and managing marine ecosystems (for example,
fisheries, coral reefs, marine mammals and seabirds) in light of climate
stressors."
Henson agrees, telling Carbon Brief
that while her research defines the changes we're likely to see in the
oceans, scientists don't yet understand what the ultimate effect on the
marine ecosystem will be. She says:
"The marine ecosystem is so complex and there are so
many different organisms. It's interesting to think, does that mean some
will be winners and some will be losers in the future ocean? Right now,
we just haven't got the information to know how that will play out."
As the paper notes, climate change is
"essentially a one-way street", meaning that whatever changes occur in
the marine environment, they are "unlikely to be reversed".
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