Tropical coral reefs are magical places, yet if all the world’s reefs were placed together they would occupy a relatively small area—less than half the size of France. These most diverse of marine ecosystems punch above their size, however, supporting up to a third of all marine species and acting as home to between hundreds of thousands or possibly millions of reef-associated organisms—even marine biologists just don’t know how many. This tremendous diversity and spectacular array of different life forms has been an endless source of fascination. In 1802, while undertaking the first circumnavigation of Australia, Captain Matthew Flinders described a reef in the southern Great Barrier Reef as “equalling in beauty and grandeur the most favourite parterre of the curious florist.”

How things have changed. Looking underwater in the northern sector of the reef in 2016, he would have seen no vibrant colors but a ghostly white scene of dead and dying corals, rapidly being overgrown by seaweeds. This was part of a protracted mass coral bleaching event, the third global event since 1998, which has affected many of the world’s coral reefs since mid-2014 and is still ongoing as of late 2016. So what has gone wrong in the intervening two centuries for the Great Barrier Reef, and why are more than 60 percent of the world’s tropical coral reefs considered to be under immediate and direct threat from local causes? And why should humanity care what happens?

The answers lie with people. People are responsible for both the local degradation of reefs due to over-exploitation and pollution, and the global-scale consequences of interfering with the Earth’s climate system. And it is people who will be affected by these consequences. And it is people who could protect these reefs.

A little background on tropical coral reefs. Big enough to be seen from space, reefs are complex and dynamic ecosystems, uniquely defined by both their biological components and the geological structures they create. Although organisms on reefs have evolved over the past 40 to 50 million years, today’s reefs formed only within the past 8,000 to 10,000 years—the blink of an eye on a geological time scale—when the sea level rose about 125 meters to its current level after the height of the last ice age. Corals are marine animals without backbones (invertebrates) forming colonies of many individual polyps which extract calcium and carbonate from seawater to construct hard skeletons for support and protection. The foundation of tropical coral reefs is a special, mutually beneficial relationship, or symbiosis, between the coral animal and single-celled, photosynthesizing algae known as zooxanthellae.

These microscopic plants provide corals with their color, obtaining protection and essential nutrients from their coral host—which, in turn, gains photosynthetic products from the algae. This additional source of cheap food gives corals the extra energy needed to produce their calcium carbonate skeletons faster than the natural forces of biological and physical erosion, thus building the extraordinary variety of coral growth structures that are the backbone of a reef. These ecosystems are not just about hard corals: They provide food and shelter for an enormous number of other living things including sponges, soft corals, fish, echinoderms, mollusks, seaweeds, seagrass, marine mammals, turtles, and innumerable species of microbial organisms.

Coral reefs are not found everywhere in the tropical oceans but are instead confined to warm, shallow, well-lit, clear, low-sediment, and low-nutrient waters. The chemistry of the seawater is also an important control on the distribution of coral reefs; this is quantified by its “aragonite saturation state”—a measure of how easily the main type of calcium carbonate created by reef-building corals can form, which depends on the concentration of calcium and carbonate ions in seawater. Despite their long geological history, coral reefs are fragile ecosystems that occur within a narrow and specific range of environmental conditions. As with many tropical plants and animals, this makes them more vulnerable to relatively small changes in their environment compared to species living at higher latitudes which are accustomed, for example, to a much wider temperature range during the course of a year. Many human activities are now altering the make-up of coral reef communities and taking coral reefs outside of their environmental comfort zone.

What it means for us. Healthy coral reefs provide many direct goods and services to about 500 million people worldwide including food, income from fishing and tourism, and coastal protection. About 30 million people are entirely dependent on coral reefs because they live on coral islands and atolls (ring-shaped reefs, islands, or island chains made of coral). The high aesthetic value of healthy reefs reflects their high biodiversity and the potential—for the discovery of novel pharmaceutical products, for example. Global estimates of the value of healthy coral reefs range from $30 billion to $375 billion per year.

This social and economic value of healthy coral reefs has been eroded, as reefs worldwide have been declining for decades, if not longer. Scientists have long known the principal causes of what has been termed the “coral reef crisis.” Overfishing, resulting in the removal of large predators or grazing fish, can have far-reaching consequences for the entire ecosystem. Additional sources of reef degradation include destructive fishing practices, such as the use of dynamite or cyanide to harvest fish.

Reefs are also degraded by poor water quality due to increased sediment from land-use changes; nutrient and chemical pollution from agricultural runoff and coastal aquaculture; and coastal development such as dredging, the building of ports, the mining of coral for building materials, and more. These local pressures are most evident and have had most impact on southeast Asian reefs but even what is regarded as the best-protected and managed reef system, the Great Barrier Reef, is not immune. These localized problems are, however, manageable and potentially reversible with strong and effective governance, capacity building (enhancing the ability of local communities to effectively and sustainably manage their natural resources), and monitoring. Although the necessary remedial actions are well-known, there has been limited effective large-scale implementation worldwide.

The crisis has now deepened as global climate change impacts many of the world’s coral reefs. The most immediate threat is higher water temperatures on coral reefs, as the world warms due to increasing levels of atmospheric greenhouse gases. Although adapted to warmer tropical waters, corals live within a narrow thermal range and only 1 to 2degrees Celsius below their upper thermal limit. Unusually warm temperatures during the summer season causes the breakdown of the coral/algal symbiosis and loss of algae and photosynthetic pigments from the coral. The white skeleton becomes visible through the now translucent tissue—hence the term “coral bleaching.”

The phenomenon of coral bleaching has been known for many years, and it is a general stress response of the coral host to, for example, unusually cool or warm water temperatures and high or low salinity. In the past, these events were only seen on a small scale in response to local environmental stresses such as unusually low tides. That was until the latter decades of the 20th century.

Recent bleaching trends. Most alarming, reports of mass coral bleaching—where whole reefs were affected—emerged as a relatively new phenomenon, in the eastern tropical Pacific in 1983. Since then, large-scale events affecting many of the world’s reefs occurred in 1998, 2010, and currently from 2014 to the present. These recent events are unequivocally linked with unusually warm water temperatures.

Bleaching is not a death sentence, although some corals may die—starving to death as their main food source is lost. Some corals may only partially die, while others may recover completely once the stress is removed and they regain their algal symbionts from the sea water. Not all corals are equally affected; often it is the fast-growing, branching corals that are most susceptible while the slower-growing massive corals are least susceptible. During extreme thermal stress events, however—such as happened in the northern Great Barrier Reef in 2016—bleaching affected all corals, regardless of species. Sometimes, when two corals of the same species are growing side by side, one may bleach and the other not.

The key to such contrasting responses is the different types of algal symbionts—remember that plant-animal symbiosis—hosted by the corals. Recent advances in molecular and genetic techniques revealed that the coral algae are not a single species, as previously thought, but a diverse assemblage of different types. Some kinds allow the host coral to better withstand thermal extremes, though this may be at the expense of slower growth rates. Scientists are still trying to determine whether corals might be able to recruit more thermally tolerant symbionts after bleaching and even whether we can assist evolution of the coral/algal holobiont (coral reef scientists now use this term to describe the intimately linked ecological unit of coral and algae) to one better able to cope with future climate extremes.

Bleaching does not just affect corals; there are follow-on effects to many reef-associated organisms. Fish, for example, typically decrease in abundance—even those species that do not feed directly on coral. Consequently, a coral bleaching event can dramatically change the make-up of a coral reef both in the short and long-term. A structurally complex reef (providing food and habitat) becomes much flatter and less diverse as susceptible species are lost, leaving only heat-tolerant and resilient survivors.

Outlook for the future. More frequent bleaching events on the world’s reefs since the latter part of the 20th century are unequivocally linked with the relatively modest amount of global warming that has already occurred. Tropical water temperatures are now warmer than they used to be, and although the change is relatively small, it is enough to shift these fragile ecosystems beyond their locally adapted thermal limits. The risk of mass coral bleaching is dramatically increased during El Niño years—when large parts of the tropical oceans are warmer than usual due to this natural fluctuation of the Pacific ocean-atmosphere climate—responsible for many climate anomalies around the world such as droughts, heatwaves, and floods. As the world climate warms, the magnitude of tropical ocean warming due to an El Niño has also increased: the 1998, 2010, and current 2014-2016 coral bleaching events are a consequence of an El Niño plus this additional background global warming. Recovery from stresses such as mass bleaching takes time, and also depends on sufficient numbers of surviving corals on the reef (or from unaffected nearby reefs) to provide new coral larvae. As bleaching occurs more often, the time for recovery decreases and the chances of returning to the original diverse coral community diminishes.

As well as being vulnerable to warmer oceans, coral reefs are also threatened by climate change’s “evil twin”: ocean acidification. About a third of the extra carbon dioxide that humans have injected into the atmosphere has been absorbed by the oceans; if this had not happened, the world would have warmed even more than it has. But this extra carbon dioxide is changing the chemistry of water of the world’s seas, making them less alkaline and more acidic (that is, lower pH) with fewer carbonate ions necessary for coral calcification.

A site in Papua New Guinea where carbon dioxide naturally bubbles up from the sea floor has acted as a sort of natural laboratory, allowing us to travel from reefs with normal pH to those with the lower pH levels expected by the end of the 21st century. There are clear and stark differences between these reefs. Most dramatic is the loss of biodiversity, from a structurally complex reef containing many coral and other species to a reef dominated by a single massive coral species. Progressive ocean acidification will also increasingly slow the growth of corals and shell-forming reef organisms.

Humans have hit the world’s tropical coral reefs with a multiple whammy: over-exploitation of reef resources, pollution of reef waters combined with warmer and more acidic oceans. In addition, extreme weather events (exacerbated by global warming) such as freshwater low salinity flood waters and more intense tropical cyclones will further compromise reefs.

There are few bright spots in this sea of change for coral reefs. Some resilient reefs have bounced back after bleaching events, though often with a different mix of coral species. Some reefs have escaped bleaching altogether—sometimes a result of ocean circulation patterns and sometimes due to sheer luck, such as cloudy, windy weather at the right time. Some reefs have not been over-exploited and have escaped local, human-caused degradation.

This lack of local human influence did not, however, protect the northernmost pristine sector of the Great Barrier Reef from hot waters in 2016. Nor did it safeguard reefs in Marine Protected Areas.

There is no evidence that coral reefs can keep up with the current pace of environmental change by evolutionary adaptation—the mismatch of time scales is too great. The issue is not simply one of corals adapting to a new climate, but that for the foreseeable future the climate will be changing, and it could be a long time before a new stable climate is achieved, even if global actions to mitigate are rapidly and forcibly implemented. Within a generation, there may well be a shift from reefs dominated by corals to reef communities dominated by seaweeds and filter feeders such as sponges. There is already some evidence of “last chance tourists” visiting reefs before they change. Local actions such as improving water quality and reducing over- exploitation may buy some time, but the overall prognosis is grim. Loss of these rich, diverse marine ecosystems, as we know them, may be one of the earliest and most profound consequences of global climate change. Everyone will be a loser but the real losers will be the several hundred million people who rely every day of their lives on the social and economic benefits of healthy coral reefs—the same people who have contributed least to the global climate crisis.

The photos show the Lizard Island section of Australia’s Great Barrier Reef, before and after the recent coral bleaching event. The image on the left shows a healthy, colorful, vibrant reef as of October 2014, while the right shows the same reef full of bleached corals overgrown by seaweeds, as of May 2016. All images are courtesy of the XL Catlin Seaview Survey.