1 How coastlines change

Coastlines are always changing. Waves claw at the sand, dragging beaches slowly back into the water. Cliffs crumble and collapse. Dry land falls into the sea. Dunes shift in the wind. Mangrove and coral trap sand, building up beaches where there were none before. Rivers deposit sediment, extending deltas out into the water. Arable land turns into marsh. Marsh dries into desert. The processes may be slow, taking centuries, or they may happen overnight.

Urbanization

It is natural for coastlines to change, but human presence accelerates those changes, damaging the ecosystems that we need to survive.Thirty-seven percent of the world's population lives within 100 kilometers of the sea, and most of the world's large cities are on coasts. In traditionally agricultural countries of Latin America, Asia and Africa, people are moving to coastal cities in large numbers as farming becomes more difficult.

The growth of urban settlements affects coastal ecosystems in many ways:

• Housing, roads and other types of infrastructure destroy natural environments and fragment ecosystems, by preventing the free movement of animals, water and nutrients from one location to another.
• Local resources such as mangrove trees, sand and gravel are extracted for use in construction.
• Coastal wetlands are drained and rivers diverted to make way for settlement.
• Higher water extraction for human consumption, manufacturing and agriculture reduces river flows and depletes aquifers.

Salt Water Intrusion

Rivers supply fresh water to coastal wetlands, reducing the salt content. When river flows are reduced, wetlands become too salty from the sea water, killing mangrove and other fauna. Something similar happens to aquifers when too much water is extracted from the ground. Sea water seeps into the aquifer, replacing the freshwater that has been removed. Salt water intrusion can make groundwater unuseable for humans.

Eutrophication

Garbage can kill local wildlife, while human and animal waste fills waters with excess nitrogen and phosphorus. This can lead to eutrophication, an explosive growth of algae, which feed off these chemicals. The algae extracts oxygen from the water, suffocating other plants and animals. The algae may be poisonous, even to humans. Algae also cling to coral, blocking sunlight, which inhibits the coral's growth. When coastal settlements grow, new settlers may try to make a living by fishing, which can deplete fish stocks.

The greenhouse effect

Perhaps the most serious human threat to coasts comes from climate change. Industry, transportation, large scale agriculture and electrical power plants all pour millions of tons of carbon dioxide and methane into the air each year. These "greenhouse gases" trap energy in the atmosphere by permitting the passage of short wave radiation from the sun while reflecting back long wave radiation that rises from the earth. This is similar to how a glass roof traps heat in a greenhouse, and is often called the a greenhouse effect. The result is that the earth is heating up.

Since 1880, the earth's temperature has risen about 1°C. This may not sound like much but that extra energy causes many significant changes to the climate all over the world. Extreme weather events are now more common, leading to both frequent drought and stronger storms. In 2022, Pakistan was hit by floods that covered a third of the country, while parts of the United States faced extreme drought. Ice in the Arctic and Antarctic is disappearing rapidly, and scientists predict that in a decade, summers in the Arctic will be entirely ice free.

Climate change today

It is common to talk about climate change as if it was a problem for the future, but there are already many signs of it today.

• Warming temperatures are causing some plants to produce more pollen, extending allergy seasons, a major health threat for asthma sufferers.
• Some researchers argue that the bloody civil war in Syria was caused partly by an unusual drought that led to social unrest.
• Heatwaves and drought have forced up the price of food around the world.
• The world's largest lakes and rivers are drying up; today the Colorado River, that formed the Grand Canyon in the United States, doesn't even reach the sea.

Climate change is affecting the ocean in three important ways:

Ocean acidification

The carbon dioxide produced by humans doesn't just affect the atmosphere. Some of it is absorbed into the oceans. There it mixes with the water to form a kind of acid in a process called ocean acidification. Through a series of chemical reactions, the acid reduces the amount of calcium carbonate in the ocean which is essential for the formation of shells and skeletons. This inhibits the growth of sea life, especially shellfish and coral. In the early 2000s, ocean acidification almost wiped out the $110 million dollar oyster farming industry in the US.

Other species are affected by ocean acidification in different ways. For example, tropical fish depend on coral as a feeding ground and for protection from predators. There is also evidence that ocean acidification makes fish sluggish, reacting more slowly to predators. If weak and unprotected species disappear, an entire food chain could collapse, destroying important economic fish stocks.

Ocean warming

The oceans not only absorb carbon, they also absorb 90% of the heat created by global warming. The average temperature of the ocean has been steadily increasing. The last decade was the warmest on record for oceans. Ocean warming has several serious effects on the planet.

• The extra energy fuels stronger storms
• Algae that live inside coral structures, feeding the coral polyps and giving the coral its distinctive colors, are expelled. Sometimes coral can survive coral bleaching, but other times it dies.
• Warm water expands, similar to metal on a hot day, contributing to sea level

Sea level rise

As global temperatures rise, snow and ice in different parts of the world melt, adding more water to the ocean. Combined with the expansion of warm water, these two factors cause the level of the world's oceans to rise. Observations show that on average the sea has risen by about 24 centimeters since 1880, and the rate of increase has doubled since 1993. Some researchers say a sea level rise of 1.5 meters is not impossible.

While sea levels rise, many coastal communities are actually sinking. Dams, dykes and levees built to protect coastal communities cause the land to dry out. Extracting groundwater has a similar effect. Drier ground shinks, like clothes after you squeeze water out of them, and this causes subsidence of the land. The city of New Orleans, in the United States, is a dramatic example. In 1895 only 5% of the city was below sea level. Today, that number is 50%.

As with temperature, the rates of sea level rise and land subsidence may sound small, but they have an outsized effect on coastal regions. This is because of the horizontal slope of land: a small increase in height translates into a large distance inland. And while the sea may not creep inland very far on a calm day, the extra height makes it easier for waves to breach the tops of barriers during a storm, a phenomenon known as overtopping. It has been estimated that a sea level rise between 1 and 10 centimeters doubles the probability of coastal flooding during storms.

The impacts of climate change that we have discussed are a problem for all countries with coastlines, but islands like Haiti are especially vulnerable. In Haiti, nobody lives more than 100 kilometers from the sea, and there are few places to move if a coastal community becomes unliveable. In 2015, world leaders gathered for the Paris Climate Conference to set a target for limiting global warming. Leaders of many so-called small island countries warned that if global warming was not kept to 1.5° Celsius, they were at risk of disappearing all together. "We cannot be expected to sign off on a small island death warrant here in Paris," Tony de Brum, the foreign minister for the Marshall Islands told a reporter. Soon after, the Intergovernmental Panel on Climate Change (IPCC), the organization of scientists that investigates climate change for the UN, published a report which said that it was technically possible for countries, especially rich developing countries, to reduce their greenhouse gas emissions enough to achieve that target.

Sadly, those warnings were not heeded. In 2022, the UN stated it does not think the world will meet this target, effectively condemning islands to catastrophic change. It is now up to islands like Haiti to prepare for that change.

Flooding and erosion on islands

Alexandre Magnan studies the effects of sea level rise on coastal communities in the Pacific. Listen to him discuss how climate change is leading to erosion and flooding on small islands today.

Extreme weather and coastal agriculture

Sebastian Weissenberger is an environmental scientist who has studied the effects of climate change on coastal communities in Africa, Canada and Haiti. Here he talks about the effects of extreme weather and water flows on coastal livlihoods.

• Document how your coastline has changed.
• How much does climate change (coastal change) affect social unrest, other problems in your community?