The Future Remains Bleak For Corals – But Not All Reefs Are Doomed

Christopher
Cornwall, Te
Herenga Waka — Victoria University of Wellington
and Orlando
Timmerman, University
of Cambridge

A recent report
on global tipping points warned that coral reefs face
widespread dieback and have reached a point from which they
cannot recover.

But in our new
research, we show this might not be the case for some
reefs if corals can gain tolerance to rising temperatures,
or if we can cut greenhouse gas emissions and restore reefs
with heat-tolerant corals at scale.

Nevertheless,
the outlook likely remains bleak.

Coral reefs provide habitat for thousands of
other species in tropical oceans. They deliver economic
value through fisheries and tourism and provide shoreline
protection from storm surges and extreme weather by
dampening the impact of waves.

However, coral
reefs are vulnerable to the effects of climate change. Our
study combines previously published assessments of climate
impacts on different coral reefs and reviews the scientific
consensus to examine how long reef structures could persist
as climate change intensifies.

Ocean warming,
acidification, darkening and deoxygenation all threaten the
persistence of coral reefs. Ocean warming brings marine
heatwaves, which are the leading cause of mass coral
bleaching that has led to a
global decline in coral cover.

Corals are animals that house microalgae
within their tissues that provide sugar in exchange for
nitrogen. When temperatures become too hot, corals expel
these symbiotic microalgae, leaving behind white skeletons.

Ocean acidification reduces the ability of corals
to build their skeletons through a process called
calcification. Warming, darkening and deoxygenation can also
reduce calcification.

Coral reefs are built by adding calcium
carbonate, coming mostly
from corals but also coralline
algae and other calcareous seaweeds. But as the
ocean’s pH (a measure of acidity) is reduced, processes
called bio-erosion
and dissolution act to remove calcium carbonate.

Our meta-analysis examined how climate change
affects the calcification and bio-erosion of coral reefs and
we then applied these results to a global
data set of reef growth.

There is no
scientific consensus on which organisms will build future
coral reefs. We explore four most likely
scenarios:

1. Present-day extreme
reefs represent the future of coral reefs. These are
locations where temperatures are already warmer, waters are
becoming more acidic and oxygen has dropped to conditions
similar to those expected at the end of the century. These
reefs are dominated by coralline algae and slow-growing
heat-resistant corals.

2. Presently degraded reefs
take over future reefs. These reefs are dominated by
bio-eroders such as sponges and sea urchins and have low
coral cover.

3. Corals can gain
heat tolerance to an extent that keeps pace with low to
moderate greenhouse gas emissions scenarios. Under these
scenarios, only about 36% of global corals would be lost and
there would be a moderate
reduction in growth. These heat-tolerant reefs are
dominated by faster growing corals with symbiotic microalgae
that can evolve heat tolerance.

4.
Reefs where restoration practices include using
heat-tolerant corals that can then disperse to other
regions. These restored reefs would have lower coral cover
in remote regions lacking restoration or with unsuccessful
restoration practices. This kind of reef restoration would
need to cover half of global coral reefs to maintain net
growth – an unlikely scenario.

We found coral
reefs transition to net erosion under all scenarios, even
under low to moderate greenhouse gas emissions, meaning they
are dissolving or being eaten faster than they can grow.
Only reefs with heat-tolerant corals could prevent this from
occurring.

The next step for the scientific
community is to determine which reefs can persist in the
future using global efforts to combine information. The
major issues is that we are missing measurements from large
parts of the Pacific, and we do not know how deoxygenation
or coastal darkening will impact coral reefs. The processes
of reef bioerosion and dissolution are also poorly
described.

Although the climate has been altered
to the point of threatening
the future survival of coral reefs, their fate is not
doomed yet if we act now.

Another question is how
long reef structures will persist after living corals are
removed. We do not have an answer yet. It will take global
efforts to rapidly obtain these measurements to better
manage and protect coral reefs before climate change
intensifies.

It is up to governments everywhere,
including New Zealand, to better support these initiatives
before it is too late.

Christopher
Cornwall, Lecturer in Marine Biology, Te
Herenga Waka — Victoria University of Wellington
and Orlando
Timmerman, Doctoral Candidate in Earth Sciences, University
of Cambridge

This article is
republished from The
Conversation under a Creative Commons license. Read the
original
article.