Coral Reef, © USFWS Pacific

Corals and Climate Change: It’s a “MAAD” World

Daniel Thornhill, Coral Reef Marine Scientist

©Richard Ling

©Richard Ling

Whether it’s polar bears losing their icy habitats in the Arctic or corals bleaching in the tropics, climate change has drastically disrupted the lives of wildlife throughout the world. As our global climate continues to warm, wildlife species will respond with a “MAAD” set of options. Some will Move to a new home where conditions are less stressful. Others will adjust the way they live to Acclimate to the new conditions. Across generations, wildlife populations may even Adapt to the changing planet through natural selection. Too often when these responses are not an option, wildlife will Die as a result of climate change.

These changes are already happening on coral reefs. Reefs are home to more species than any other marine habitat on our planet, but this diverse environment is incredibly fragile. Reef-building corals — the very foundation of these shallow ocean ecosystems — are a delicate partnership between the coral polyps and microscopic algae. When temperatures become too hot, this partnership falls apart — a problem known as coral bleaching. Bleaching causes corals to starve, sicken and eventually die. This has already caused massive die-offs of corals throughout the world, leading the National Oceanic and Atmospheric Administration to propose 66 species of corals as threatened with extinction under the Endangered Species Act.

Despite this grim forecast, death is not the only option for corals. Recently, my colleagues and I investigated how corals respond to climate change. One of our findings is that average yearly temperature determined the northern limits of a coral’s range; if temperatures were too cold, corals could not grow. This is a bit of good news for corals — as warm temperatures push coral reefs away from the equator, many coral species will be able to shift north (or south in the Southern Hemisphere) from one generation to the next. Of course, other conditions must be right in order for this to happen. There must be enough light for the corals’ microscopic algae to thrive, hard surfaces for corals to attach themselves to, currents to move coral offspring to new places, and enough of the dissolved components of seawater that corals need to build their skeletons.

Coral Great Barrier Reef

©Toby Hudson

We will need to take action on many levels to address climate change, from lowering greenhouse gas emissions  to planning ahead for a warmer world. Determining where wildlife can live helps us understand both how animals will naturally respond to climate change, and how we can give them the best chance at survival in an era of rising temperatures. Our study is an initial step in understanding these responses in corals.

Coral reefs are huge, immobile structures, but the corals that build them aren’t so static from one generation to the next. If we take action on climate change and learn what reefs need to survive, we can protect corals for generations to come.

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