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  • Writer's pictureDr. Hansi Singh

A Massive Global Coral Bleaching Event is Underway

A Tragic Consequence of Warming Oceans on a Warming Planet

The oceans are extremely warm today, certainly warmer than they have been in a few thousand years, and possibly warmer than they have been in a few million years. In a future post, we'll go in-depth on why the oceans are so warm this year – in a nutshell, it's not only climate change, but also the super El Nino that has developed over the tropical Pacific, possibly combined with recent shipping fuel regulations that have decreased emissions of sunshine-reflecting sulfates.

But whatever the factors, our oceans are breaking records this year, and not in a good way.

These ultra-warm ocean waters are having enormous impacts around the world. Warm waters can intensify storms by providing more water vapor, particularly for hurricanes (also known as typhoons) and atmospheric rivers, exacerbating severe weather conditions. Warmer waters occupy more volume than colder waters, contributing to coastal sea level rise. This can lead to the inundation of coastal communities during storm surges and the salinization of coastal drinking water sources. Warming can also decrease oxygenation of ocean waters, both because warmer water holds less oxygen than colder water (Henry's Law) and because warming of the upper ocean can slow down the ocean circulation, decreasing the rate at which oxygen-depleted water from the deeper ocean is ventilated (i.e., exposed to air) at the ocean surface. As a result, fisheries around the world are impacted by ocean warming – juvenile fish may be less likely to survive to adulthood, decreasing fish stock, or fish may migrate away to where ocean conditions are more favorable. Because the oceans are the primary source of protein for over 1 billion people, impacts of ocean warming on fisheries is a global concern.

Coral Reef Biodiversity

However, the impacts of ocean warming are most immediate and apparent for corals, the colorful invertebrates that form the foundation of reef ecosystems worldwide. Corals are the keystone species for biodiversity in the tropical oceans. In the tropics, generations of corals with calcium-rich skeletons grow atop each other in succession, and the reefs created by their bodies serve as the scaffolding for complex, diverse ecosystems of mammals, fish, and invertebrates. Corals are cnidarians, closely related to jellyfish and anemones. For their nutritional requirements, they rely on zooxanthellae, single-celled photosynthetic microorganisms with which they are symbiotic. The coral provides a safe home for zooxanthellae, while the zooxanthellae provide food for the corals using energy from the sun. This is a mutually-beneficial relationship, and has enabled corals to thrive for millennia.

Heat Stress on the Great Barrier Reef 2024
NASA Earth Observatory image by Michala Garrison, using data from the MUR SST project & reef info from GBRMPA

But when tropical ocean waters warm, things can go terribly wrong. Corals under heat stress tend to expel their life-giving zooxanthellae. Since the zooxanthellae are responsible for the bright colors of corals, these corals now look white, or 'bleached'. Bleached corals aren't dead, but they aren't well – they're starving. If ocean waters cool to safe temperatures, the corals will take up zooxanthellae again, and can recover. But if ocean water temperatures remain warm for too long, the corals will die. And when corals die, reef ecosystems collapse. The colorful fish and invertebrates disappear, and fuzzy algae coats the coral skeleton. What remains is a graveyard, a shadow of the colorful, biodiverse, vigorous ecosystem that once existed.

This coral bleaching process is occurring globally due to record-high ocean temperatures. Unfortunately, such events have occurred before, though not as severe or extensive as the current one. In 2014 - 2015, a major warm temperature anomaly over the Pacific Ocean, known as 'The Blob,' led to extensive bleaching of reefs around Hawaii and other island states in the West Pacific, and the death of many reefs. But none of these past events have been so severe or extensive as the global coral bleaching event underway today with ocean temperatures so anomalously warm around the world.

One important question is why corals are so sensitive to temperature fluctuations, given that these tropical reefs have existed over the last several million years in the tropics. Surprisingly, temperatures in the tropics are extremely steady, and year-to-year variability in temperatures is relatively low. So even though temperatures in the extratropics (i.e., away from the equator) have fluctuated extensively as the Earth transitioned between major ice ages and warm intervals over the last million years, the tropics have remained relatively untouched. This is one reason why, for example, biodiversity in tropical rainforests is so great – these ecosystems have remained stable over the last several million years, allowing evolution to work its magic and create a diverse menagerie of species. But the downside to this stability is that the animals in these ecosystems have not adapted to temperature volatility. Because most coral species have never been exposed to such warm temperatures before, they lack the genetic diversity that might allow for more rapid adaptation, making them extremely vulnerable today.

Our planet's climate is in a state of flux, brought on by the build-up of atmospheric greenhouse gasses that is creating a warmer, more volatile world. Now, more than ever, the impacts of climate change are becoming more and more apparent, making it clear that decreasing emissions to zero is the only way to move forward. But it's also clear that precise environmental information is crucial for helping us find new ways to protect the life-giving ecosystems that are our heritage as stewards of this planet.


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