20,000 years ago
sea level was
400 feet lower
than today.
The missing water
was on the land
in the form of ice
glaciers, covering
Canada, Chicago,
Detroit, etc.
That means
any coral living
in the first
400 feet of the
ocean's depth
must have
dried up and died
20,000 years ago.
But the
coral reefs
managed to
fully recover
as our planet
warmed in the
past 20,000 years.
Corals also face huge
damage from cyclones
( hurricanes ), coral-eating
crown-of-thorns starfish,
and El Niño (Pacific ocean
heat releases) related to
coral bleaching:
Most reefs
fully recover
in 7 to 30 years.
A huge 1998
El Niño event
caused about
95% mortality
for 12 coral reefs --
but 6 of the 12 reefs
fully recovered
within 7-12 years,
and within 16 years,
total coral cover
had increased
to 135% to 305%
of pre-bleaching values !
The 6 other coral reefs
continued to recover
at a slower rate.
Cyclones cause
about half of lost
coral cover, plus
about 40% from
crown-of-thorns
starfish feeding,
and about 10%
from warm water
bleaching.
Coral growth
is typically
fully restored
within 10 years.
Remnants
of living tissues
can expand,
regenerating
tissue that covers
dead skeletons
( "The Phoenix Effect" )
Bleaching is not
always lethal.
When over 90%
of the coral
on the Palmyra Atoll
experienced bleaching,
there was no loss of coral
on the reef flats, and only
a 9% loss on the fore reefs.
Reef building corals
depend on energy from
photosynthesizing
symbiotic algae.
During the winter,
coral increase their
symbiotic algae,
as lower light reduces
photosynthesis.
Each summer, as light
intensity increases,
they expel symbionts.
Bleaching
is an extreme
of that behavior.
After bleaching,
coral can replace
their symbiotic algae
within days, or months,
with no resulting
mortality.
Coral can even get
an instantaneous
genetic upgrade
by acquiring new
symbiotic algae.
Acquiring different
symbiotic algae
must have allowed
coral to adapt
to dramatic
temperature
changes as
the Ice Ages
came and went.
During El Niños,
there's extra
warm water
and cloud cover.
Local sea levels
dramatically fall.
-- Falling sea levels
expose the Great
Barrier Reef coral
to drying winds,
and shallower bays,
that heat more rapidly.
-- With less clouds,
the Great Barrier Reef
is exposed to
more sunshine and
more heat waves.
There's a very
strong connection
between ocean heat
waves, and El Niños.
And Coral bleaching
correlates well
with El Niños.