by Greg Asner
Greg Asner is a professor at Arizona State University and director of its Center for Global Discovery and Conservation Science. he also serves in numerous national and international programs including NASA, the U.S. State Department, the United Nations, and the U.S. National Academy of Sciences. Asner has published hundreds of scientific articles about his exploratory and applied research on ecosystems and climate change at regional to global scales.
In 1842, Charles Darwin published his first scientific monograph, not On the Origin of Species, but rather on The Structure and Distribution of Coral Reefs. During his voyage aboard the HMS Beagle, he theorized that the existence of coral reefs and atolls could be traced to geologic uplift and subsidence in the Earth’s crust. Much of what Darwin considered on that voyage set the geographic template for coral reef exploration and ecology for more than 150 years.
That’s all changing now, for it has become clear that coral reef habitats are being dramatically altered by human intervention. Species that have long inhabited reefs in the way that Darwin once understood are adapting or dying, and in some cases, departing and showing up in new locations.
As an ecologist with a long track record of mapping species and ecosystems around the world, I see clearly that the dramatic change in reef ecosystems has been outpacing our capacity to monitor it, necessitating a new approach, which we have developed through our Reefscape project. We are combining fins-on fieldwork with a capacity to map vast reef systems in record time through the use of survey aircraft and fleets of satellites to understand the what, where, and why of a changing coral geography across our oceans.
Five principal threats
In descending order, the five most pressing threats posed to coral reefs are marine heatwaves, coastal development, overfishing, ocean acidification, and sea level rise.
Whether on land or in the sea, heatwaves are being generated as the Earth’s climate redistributes added thermal energy (heat) created by the excess carbon dioxide, methane, and other greenhouse gases that we pump skyward from industrial, transportation, agricultural, and personal activities. Compared to urban heatwaves that can take scores of human lives, marine heatwaves are proportionally far more detrimental to corals, taking billions upon billions of lives each year.
The ever-changing geography of warming seawater is becoming clearer, thanks to international ocean monitoring systems. For example, we now know that marine heatwaves vary greatly in location, extent, and duration. We also know that nearly every heatwave causes coral bleaching: a process by which the coral animal loses its food source, a microalgal symbiont, which leads to coral starvation and death. Yet we have also discovered that some corals, even among species that often “die,” can actually make it through these heatwaves. These heat-tolerant corals become the genetic stock for the next generation on the reef. A big question is whether or not these survivors can continue if heatwaves get stronger and more frequent as predicted. We also don’t know whether surviving corals are able to send their larvae to new regions of the ocean, but it is possible given that coral larvae move on ocean currents.
The second and third greatest threats to reefs—coastal development and overfishing—are more regional in impact compared to marine heatwaves. Poor coastal development planning generates pollution and erosion on land that flows into the sea and onto reefs. Overfishing is a problem for reefs, both because some fishing practices physically damage coral structures and because coral needs fish to survive. While these two problems are found all over the world, the good news is that both are solvable through education and regulation. What we don’t know is how these problems contribute to a changing global coral geography.
Finally, there is ocean acidification and sea level rise, both of which will impact coral reefs in the long run. Studies reveal that the same carbon dioxide warming our atmosphere and oceans also seeps into seawater, making it more acidic and thus lowering ocean pH. This is making it harder for corals and other reef species to sustain and build their calcium carbonate structures, which dissolve at low pH. Meanwhile, rising seas generate more pollution from land to sea, especially during storm surge, which threatens reefs.
While these climate and coastal problems are clearly making life harder for corals, we still do not know how their geography will look a decade or more from now. It is for this reason that we have taken a multiscale approach to understanding what we’ve got on the planet today in order to project how it will change over time.
A matter of scale
My team and our collaborators are working at four interlocking scales to understand the changing geography of corals. In shallow tropical reefs down to about 20 meters of water depth, where a majority of corals live, we are combining fieldwork with airborne and satellite measurements to define the geography of corals.
With our partners in the Allen Coral Atlas, we are using the world’s largest fleet of satellites to map the extent of shallow tropical reefs. Then using our aircraft, the Global Airborne Observatory (GAO), we have developed a new and unique approach to mapping the location of live corals on those reefs, something that no satellite or drone can accomplish. The GAO is equipped with state-of-the-art laser, spectrometer, and ultrahigh-resolution multispectral imaging systems, super-computing, and the workspace for a crew of six. The GAO has a global reach, and with it, we’ve taken the airborne lab all over Latin America, southern Africa and Madagascar, Southeast Asia, the Caribbean, and the United States. Upon arrival in a new theater of operation, we map millions of hectares of reef areas, generating 3-D maps of the terrain (land or seafloor) along with the organisms living there. For coral reefs, the maps reveal the locations of live and dead corals in 3-D. Sometimes we can even map individual species.
Based on the combined satellite and aircraft imagery, we are able to deploy our survey divers to predetermined locations to get the most useful information on coral biodiversity, fish habitat, and other factors. This approach dramatically reduces the time and cost of fieldwork.
With the three levels of information, we are able to do a complete, wall-to-wall assessment of a reef system. This information is being used today for a wide variety of applications, including planning of new marine protection areas and efforts to restore coral reefs. Such detailed maps are playing a key role in our plan to win the fight in sustaining reefs under the five big threats.
Using this multiscale approach, we’ve learned a lot in the past few years. For example, coral bleaching events don’t leave reefs altogether “dead,” as some media outlets have reported. In reality, what we are seeing is that some coral species do quite poorly in certain regions, and those are the ones that may go extinct in coming years. Yet there is reason for hope; we are continually finding havens of coral life, areas that have eluded the big five threats. We also find that reefs with very little fishing pressure—sometimes due to protection and management efforts, but most times due to sheer inaccessibility—tend to fare better during marine heatwaves than those reefs with fewer fish. Why? Because herbivore fish nibble away at the macroalgae that overtakes bleached coral during heatwaves. Fish are critical lawnmowers that help keep the coral alive until the heatwave passes.
We also use our interlocking approaches to figure out where the most imperiled corals might be outplanted in efforts to restore reefs. This has traditionally been done in more of a farming mode, like planting trees in your backyard. Instead we are using our fieldwork, aircraft, and satellite trio to estimate where coral outplanting might have the best chance at survival. This is based on a set of criteria that we can map, such as seafloor type, water depth, ocean temperature, and 3-D habitat.
What about deeper reefs? Are they havens of survival below the marine heatwaves that pass through the surface ocean? This is where our fourth scale of exploration goes deeper, not just on coral reefs but on other potentially novel ecosystems. For example, we surveyed World War II shipwrecks in Bikini Atoll of the Marshall Islands and in Truk Lagoon of Micronesia. Shockingly, we found that approximately 30 percent of the world’s coral genera, a term that compiles groups of genetically similar species, are found on a few dozen military shipwrecks at depths of 50 to 60 meters in the ocean. These wrecks are deep enough to escape most surface ocean heatwaves, but they are not too deep to prevent the corals from getting their much-needed sunlight to grow. In short, shipwrecks are serving as temporary lodging for a lot of corals, and they too require improved protection and management.
Our Reefscape project is setting new standards for exploring corals wherever they may be found. In Darwin’s original monograph, he alludes to the fragility of coral reefs, giving us a glimpse into how an explorer and biodiversity theorist of his time viewed his world. I think he would be appalled to learn of the rate and extent to which we as a society have allowed coral reefs to decline since then. But evolutionary theory also tells us something important—and it too came from Darwin—that the engine of biological creation and diversification never stops. This holds true even under our watch.
Understanding the changing geography of corals will be critical to keeping them on this increasingly fragile, human-dominated world. Despite our efforts as professional scientists, a key part of the exploration rests in the hands of citizen scientists, divers, and explorers who understand the challenges, engage the possibilities, and communicate as we strive to unravel the mysteries of our changing global coral reefs.