Coral reefs are some of the most diverse and important ecosystems on earth. If you didn’t know that already you probably wouldn’t be here. For background on see these links (1, 2, 3). Also, reefs are beautiful and really cool (see below).

However, corals are very sensitive to changes in their environment. They are especially sensitive to temperature changes. Obviously climate change has had quite the impact on reefs already and climate phenomena such as El Nino do not help matters.

Corals are symbiotic, meaning that they rely on small algae like organisms living in their tissue for much of the energy that they need to survive. The symbiosis breaks down during stress (like when it gets really hot or cold). As a result, the coral turns white (because the algae that provide pigment are no longer in the coral). This phenomenon is called coral bleaching. We wrote about it in a past article. Check it out if you want more information!

Small-scale bleaching occurs pretty much every summer in various areas of the world. However, global scale bleaching events are much less common and much more deadly. In fact, in 1997-1998 the first global scale bleaching event occurred, leading to the mortality of 16% of the worlds coral reefs. This event lined up with the largest El Nino on record. El Nino conditions (the warm phase a climate pattern called the El Nino Southern Oscillation). basically means that there is a large patch of unusually warm water in the central and eastern Pacific Ocean (where there are a ton of corals). Additionally, global climate and weather patterns are effected causing cooling, warming, lots of rain, or no rain in various areas of the world.

The 1997-1998 El Nino bleaching event was the largest and worst bleaching event on record. Until now. Since 2014 we have been in a warm phase of the ENSO (basically, El Nino has been turning on since 2014, so it’s been hot in the oceans). As a result of this El Nino and the exacerbating impacts of climate change, we are again seeing a global coral bleaching event on the scale of the 1997-1998 event. In fact, due to the length of this El Nino, corals in some parts of the world have bleaching 2-3 times since 2014! Previous research has shown that this kind of repeat bleaching is extremely detrimental to corals (even relatively resilient ones) as it does not allow time for the corals to recover from one stressful event before another begins. This event has the world’s top coral researchers quite concerned as global reefs are bleaching and some of the most pristine areas of the Great Barrier Reef are seeing their worst recorded bleaching events One major concern is that El Nino events are hard to predict. They can occur as quickly as every 2 years, but the scale of each event is different. For example, there have been 5 El Nino events reported since 2000 and the first 4 did not have this kind of global scale impact on coral reefs.

We know that these El-Nino Bleaching events occur, we are still working on predicting them, and we know they can be really traumatic for coral reefs worldwide. So are all corals doomed due to these events? Is the end of the coral reef ecosystem on the horizon?

Short answer: if we don’t start drastically changing the way we live and use energy, probably.

Long answer: Every coral has a different level of thermal tolerance and corals respond to stress in plastic ways. It is likely that some reef-building corals will acclimatize or adapt to these stressful conditions and continue to survive. However, the ecosystems will start to look different (think patches of coral and lots of seagrass, algae, and sponges) and provide fewer or at least different services. What about so-called “Super Corals” (corals that are already living in very hot or stressful environments. Well, even super corals from Western Australia eventually bleaching if conditions are stressful enough.These corals remain healthier longer than other corals but if conditions are bad enough (bleaching events every year probably qualifies as bad enough) then these corals also can’t stay healthy and may eventually die off.

How can corals increase thermal tolerance?

–They can harbor more stress tolerant symbionts or acclimatize to current/future conditions in another way.

–They can adapt to future conditions (genetically, over more than 1 generation)

In fact, my lab is currently investigating how thermal history can impact coral health over time. To read more about our work see this post.

We are working as hard and as fast as we can on figuring out how corals can respond to these stress events and there are people all over the world doing the same thing. Every day our knowledge grows. Hopefully we can convince local, state, and federal governments to protect more areas and to keep emissions down in order to ensure the future of these key ecosystems. We still cannot predict exactly how “super” El Nino’s like the one occurring now (and the one in 1997) will impact corals worldwide or even when these events may occur but we are working on it. This merely points to the importance of scientific funding as worldwide priority and the ever-increasing need to support scientific research. If you want to help get involved with an NGO, learn how to reduce your carbon footprint, VOTE, and write to your congressman, president, or government officials and encourage them to fund renewable energy and scientific research. No one can change this world alone. We must all be in this together.