By Jan Chaiken and Marcia Chaiken

In May 2015, Huatulco experienced a mar de fondo, a regularly spaced sequence of extremely high waves. Although it may appear to witnesses to be similar to a tsunami, a mar de fondo is not caused by seismic activity but rather by a large storm at some distance from the beach where the waves are landing. A delight to surfers, the large waves can, however, destroy boats and piers in marinas and inundate the lower floor of buildings close to the coastline. Along the Oaxaca coast, the mar de fondo made some highways impassible, and some hotel lobbies and schools were flooded.

This phenomenon illustrates how changes that occur in oceans, or in weather patterns over the oceans, can affect a locality even if the water there is crystal clear and does not seem to have changed much over the past few decades. The Oaxaca Coast is highly dependent on a favorable relationship with the ocean – for attracting tourists, for transportation, for fishing and other occupations, and for survival of buildings that were not constructed to withstand repeated inundations or violent weather. Major port cities, whether or not attractive to tourists, are also dependent for their commerce on a somewhat placid and predictable ocean. The cargo off-loaded at ports is delivered to localities all over the country, so in this sense all of Mexico is dependent on stability of its coasts.

The United Nations sponsors a study group at its Institute for Environment and Development that conducts global research on low elevation coastal zones (LECZs), which are defined as areas that begin at the coastline of a sea or ocean and continue inward up to 10 meters (about 33 feet) above sea level. Portions of Huatulco, of course, fall in this category, as do many other inhabited coastal areas of the state of Oaxaca. Recent advances in satellite photography have enabled researchers to map accurately the land use of coastal zones and thereby to estimate the population in low elevation coastal zones and to study the changes in population there. To those of us who have specifically sought out an ocean resort community, it is no surprise that the research shows a fairly rapid increase in the percentage of population that lives or works in such coastal zones.

Mexico is a country that has a relatively high percentage of its population in coastal zones. Some countries such as Switzerland, of course, have none, while island nations have high percentages of their population there. Mexico, with 5% of its population in low elevation coastal zones, ranks higher in this regard than Russia, Canada, the United States, China, Australia, Brazil, India, and many other countries with extensive coastlines. Think about major cities in the Americas – many of them touch the coast (43% of cities in Latin America with population over 500,000, according to the UN study group). As the oceans change, life in those localities will change.

Why suddenly are scientists concerned about populations living close to the coast? The recognition of changes in the oceans have prompted this concern.

Most notably, the temperature at the surface of the water as well as the temperature of the atmosphere has been increasing gradually since 1900. Temperatures prior to 1900, although not as accurately measured as today’s, indicate at least several centuries of stable ocean heat. Since 1990, the total temperature rise of the oceans has been 0.8ºC. In addition, since 1955, when methods were developed to estimate global sea level, the sea level has risen steadily to a total of 20 cm (8 in).

These apparently small changes in ocean surface temperature have been causing disproportionately large changes in the weather patterns over the oceans. Storms over the ocean are increasing in frequency and growing in size and violence, creating more frequent storm surges on coastal land. While a community can restore itself after a hurricane once every 30 years or so, no coastal community can long withstand 2 major inundations per year. In large cities with a coastline, many kinds of businesses will simply choose to move elsewhere if experiencing repeated flooding. Inundations and violent disruptions of vacations at beach resorts anywhere in the world gradually make all coastal destinations – even those unaffected — less attractive to tourists and investors, as few people seek vacations that may be anxiety-ridden.

Other visible effects of warming are well established – less snow, ice, and glaciers in the Northern Hemisphere, melting of the Greenland ice sheet, less ice in the Arctic Ocean (particularly in the summer and autumn, down 40% just since the year 2000), and ice sheets in Antarctica tottering on the brink of sliding into the ocean. When ice melt declines as a source of river water, a remarkable change occurs where the rivers meet the ocean: the water in those deltas becomes more brackish, and marine animals that previously thrived there are no longer found in the waters.

These kinds of changes in oceans also threaten food production, coastal infrastructure, and marine animals. Since 1970, six species of whales have nearly vanished from the Gulf of Mexico – Bryde whale, blue whale, finback whale, sei whale, humpback whale, and sperm whale.

Sea turtles are remarkably threatened by small changes in their environment. All female turtles come ashore at nesting beaches, dig nests in the sand, lay their eggs, and return to the sea. Erosion of the beaches and flooding of the nests threatens the nests. The maximum temperature that turtle eggs can incubate may be exceeded, in which case none of the eggs in that particular area will hatch.

Even if the eggs hatch, the temperature during incubation influences the sex of the hatchlings, with more females and fewer males at higher temperatures. In some areas that were previously good nesting grounds for turtles, 90% of the newly hatched turtles are females. Since turtles migrate long distances from the place they hatch to areas where they mate, the chance of females meeting up with a suitable sperm donor will be greatly reduced by this skewed sex ratio. Fortunately, female turtles can store sperm for relatively long periods of time, so if they meet up with a suitable mate they can produce eggs for several seasons, and the effects of the changing sex ratio are mitigated.

Commercial fish and shellfish also have temperature limits under which they can reproduce and thrive. Many species can easily adapt by gradually moving to new areas. For example New England lobsters are vanishing in areas south of the Gulf of Maine. Although the fish and shellfish may survive elsewhere, how will the food needs of coastal communities be met as these changes occur?

Coastal erosion can make rapid, dramatic changes in the suitability of beaches for recreation and relaxation. Previously broad sand beaches can become primarily rocky or essentially disappear. Short-term measures such as shipping sand to the beaches create a financial strain on small towns that are primarily tourist destinations.

A distinct and separate change that has been occurring in oceans is the gradual increase in their acidity (lower pH). This is caused by the oceans’ absorption of carbon dioxide from the atmosphere, a chemical process that is well understood. Some marine animals (such as corals, clams, mussels, and some other shellfish) have shells composed of a material that dissolves in acid. As the ocean pH declines, these organisms have greater difficulty surviving and reproducing. In some areas, the soft shells of juvenile sea snails dissolve as soon as they metamorphosize from their larval stage, and coral reefs are readily seen to be deteriorating along the coasts of Mexico. The increasing acidity of the oceans is also killing off very small marine organisms such as plankton, which are an important part of the food chain for larger aquatic life. The magnitude and implications of these changes are not well understood, because the pH levels of the oceans have been stable until quite recently.

Some say that these are just natural cycles that the earth has continuously undergone. In fact there was a period of high ocean acidity, 252 million years ago. During that period, 96% of marine species were extinguished, with the “side effect” that 90% of life on land also perished. Of course, there weren’t any humans around then, to worry about this major extinction of life forms. But, hey folks, should we really just complacently assume that we’ll be among the 10% of species that will survive? Or should we address the underlying causes and continue to enjoy life – especially in Coastal Oaxaca?