Human Impact on the Environment

3. Acid rain

Acid rain is rain with a lower pH than normal (typically below pH 5). It is caused mainly by sulfur dioxide and nitrogen oxides released by burning fossil fuels.

How acid rain forms

1. Power stations and vehicles release sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere.
2. In the atmosphere, these gases react with water vapour in clouds.
3. Sulfur dioxide reacts to form sulfuric acid (H₂SO₄). Nitrogen oxides react to form nitric acid (HNO₃).
4. These dissolved acids fall back to earth in rain, snow, fog or dew. The result is acid precipitation (acid rain).

Effects on living things

On trees and other plants:

• Damages the waxy cuticle on leaves, making them more vulnerable to disease and to drying out
• Leaches important mineral ions (especially magnesium, calcium and potassium) out of the soil
• Releases toxic ions (especially aluminium) that were previously locked into the soil
• Result: trees grow more slowly, lose leaves, and eventually die. Vast areas of forest in Europe, North America and China have been damaged

On aquatic ecosystems (lakes and streams):

• Lowers the pH of the water
• Fish eggs cannot hatch below pH 5
• Adult fish struggle to breathe; many species cannot survive below pH 4.5
• Invertebrates (mayflies, snails) die off too
• Eventually the lake becomes a "dead zone" with almost no aquatic life
• Famous example: many lakes in Scandinavia and eastern Canada were left almost fishless in the 1970s and 80s

On buildings and statues:

• Sulfuric and nitric acid react with limestone, marble and concrete, dissolving them slowly
• Historic buildings (e.g. medieval cathedrals, statues) gradually lose their detail

What has been done

Modern regulation has helped. Most coal-fired power stations now have flue-gas desulfurisation units that remove SO₂ from exhaust before it reaches the atmosphere. Vehicle exhausts have catalytic converters that reduce NOₓ emissions. Switching to renewable energy (wind, solar, nuclear) removes the source entirely. Acid rain is much less of a problem in Europe than it was in 1980, but it remains a serious issue in fast-industrialising countries.

4. The greenhouse effect and climate change

The biggest environmental challenge of the 21st century is climate change caused by rising levels of greenhouse gases in the atmosphere.

The natural greenhouse effect

The Earth would be far too cold to support life without the natural greenhouse effect. Here is how it works:

1. The Sun emits high-energy radiation (mostly visible light) towards the Earth.
2. Most of this passes through the atmosphere and reaches the surface.
3. The Earth's surface absorbs the energy and warms up.
4. The warm surface re-emits the energy as lower-energy infrared radiation (heat).
5. Some of the infrared radiation escapes back to space.
6. Some is absorbed by greenhouse gases in the atmosphere (carbon dioxide, water vapour, methane and others). The trapped energy keeps the lower atmosphere warm.

The natural greenhouse effect keeps the Earth's average surface temperature at about 15 °C instead of the -18 °C it would otherwise be. Without it, the oceans would be ice and there would be no life.

The enhanced greenhouse effect

The problem is not the natural greenhouse effect itself, but the enhanced version of it that humans have created by adding extra greenhouse gases to the atmosphere. The most important human-emitted greenhouse gases are:

The result of adding all these extra greenhouse gases is that more infrared radiation is trapped in the atmosphere, and the Earth's average temperature is rising. Atmospheric CO₂ has risen from about 280 ppm in 1750 to over 420 ppm today, and global average temperature is about 1.2 °C above pre-industrial levels.

Consequences of climate change

A warming planet has many cascading effects:

• Melting polar ice and glaciers, raising sea levels and threatening coastal cities and low-lying countries
• Warming oceans, which expand and add further to sea-level rise
• Coral bleaching: corals expel the algae they depend on when water is too warm, turning white and often dying
• More frequent and severe extreme weather: heatwaves, droughts, wildfires, storms, flooding
• Shifts in habitats and species ranges: some species move towards the poles or to higher altitudes; those that can't, decline or die out
• Disruption to food chains and loss of biodiversity as ecosystems struggle to adapt at the speed change is happening
• Spread of disease: warmer climates allow tropical diseases (malaria, dengue) to spread to new areas
• Crop failures in regions hit by drought; some agricultural land becomes unproductive

What is being done

Reducing emissions is the only long-term solution. International agreements (Kyoto Protocol, Paris Agreement) have set targets for emission reduction. National measures include:

• Renewable energy (wind, solar, hydro, nuclear) replacing fossil-fuel power stations
• Electric vehicles replacing petrol and diesel cars
• Insulation of buildings to reduce heating energy
• Reforestation to absorb CO₂ from the atmosphere
• Carbon capture (still experimental) to remove CO₂ from power station exhaust

5. Eutrophication

Eutrophication is the over-enrichment of a body of water with nutrients, leading to the collapse of its ecosystem. It is the leading cause of dead zones in lakes, rivers and coastal seas worldwide.

The five-step sequence

1. Excess nutrients (nitrate and phosphate) enter a body of water. Most commonly from agricultural fertilisers washed off fields by rain, or from sewage containing detergents.
2. The extra nitrates and phosphates cause explosive growth of algae at the water surface (an algal bloom).
3. The thick algal layer blocks sunlight from reaching the deeper-water plants. Those plants cannot photosynthesise and die.
4. The dead plants and (later) the algae are broken down by decomposer bacteria. The decomposers multiply rapidly and use up the dissolved oxygen in the water as they respire.
5. Fish, invertebrates and other aquatic animals suffocate because there is no longer enough oxygen in the water. The water body becomes a "dead zone".

Why fertilisers are the main culprit

Modern agriculture uses huge amounts of nitrogen-based fertiliser to maximise crop yields. Plants only absorb a fraction of the applied nitrate; the rest stays in the soil and is leached out by rainfall into rivers and groundwater. Even small fields can release enough nitrate to disturb a downstream lake.

Phosphates from older washing detergents used to make the problem worse; phosphate-free detergents have largely solved this part of the problem in the UK, but agricultural runoff remains the dominant source.

Where eutrophication happens

• Lakes and slow rivers in farming areas
• Coastal seas at the mouths of large rivers draining intensive agricultural land. The Gulf of Mexico has a dead zone the size of New Jersey every summer, fed by nitrate runoff from the Mississippi River
• The Baltic Sea, surrounded by farming countries, has experienced widespread eutrophication

Solutions

• Reduce fertiliser use by applying only as much as the crop will absorb, at the right time of year
• Buffer strips: leave strips of grass and trees along rivers and ditches to absorb runoff before it reaches the water
• Improve sewage treatment to remove nitrate and phosphate before discharging treated water
• Use phosphate-free detergents (now standard in the UK)
• Restore wetlands that naturally filter excess nutrients

6. Sewage pollution and bioindicators

Sewage is the waste water from homes, businesses and industry, containing human waste, food scraps, detergents, and sometimes industrial chemicals. When raw or poorly-treated sewage enters a water body, it causes a similar oxygen-depletion problem to fertiliser-driven eutrophication.

How sewage damages aquatic ecosystems

1. Sewage contains lots of organic matter that bacteria can digest.
2. Aerobic bacteria multiply rapidly, breaking down the organic matter.
3. As they respire, they use up the dissolved oxygen in the water.
4. Oxygen-sensitive species (mayflies, stoneflies, trout) die or move away.
5. Only species that can tolerate low oxygen survive (sludge worms, blood worms, rat-tailed maggots, certain bacteria).
6. Biodiversity collapses.

Sewage also contains pathogenic bacteria and viruses that can cause disease in humans who drink the water, swim in it, or eat shellfish from it.

Bioindicators: using species to measure pollution

You can tell how polluted a water body is by looking at which species are living in it. Different species have different tolerances for oxygen and pollution:

Sampling a stream with a kick net and identifying what is there is a standard way to assess water quality. Bioindicator species are useful because they reflect conditions over weeks or months, not just a single moment (which a chemical test would).

Lichens as air-quality bioindicators

The same idea works for air. Different lichens (small symbiotic organisms growing on rocks and tree bark) have different tolerances for sulfur dioxide:

A walk through a city centre and a walk through unpolluted countryside will quickly show this pattern.

7. Deforestation

Deforestation is the large-scale clearing of forests, usually to make room for agriculture (especially cattle ranching, palm oil and soy plantations), logging for timber, road and city building, or mining.

The world has lost about a third of its forests since the start of the agricultural revolution. Tropical rainforests are being lost at a rate of around 10 million hectares per year. Each lost hectare typically contained hundreds of species of plants, insects, and animals.

Five major consequences of deforestation

1. Loss of biodiversity

Forests, especially tropical rainforests, are home to a huge fraction of Earth's species. Cutting down a rainforest destroys habitats and pushes many species to extinction. Some are lost before they have even been discovered or studied.

2. Soil erosion

Tree roots hold soil together. Without trees, heavy rain washes the topsoil away, leaving infertile subsoil. The lost soil ends up in rivers, where it can clog spawning beds and reduce water quality. Many tropical regions now have impoverished farmland that was tropical forest 50 years ago.

3. Leaching of soil minerals

Trees absorb minerals (especially nitrate) from the soil through their roots, holding them in plant tissue. When the trees are gone, those minerals are washed out of the soil by rainfall (leaching) into waterways. The soil becomes nutrient-poor and unable to support agriculture for long.

4. Disruption of the water cycle

Forests release enormous amounts of water vapour through transpiration. The Amazon rainforest, for example, transpires enough water vapour to generate much of its own rainfall. When the forest is cleared:

• Less transpiration → less water vapour in the air → fewer clouds → less rainfall
• The area becomes drier
• Streams and rivers can dry up
• Other vegetation cannot grow back as easily

5. Disruption of the carbon cycle and accelerated climate change

This effect is the biggest at global scale:

• Forests are huge carbon sinks: trees hold enormous amounts of carbon in their wood (and in the soil beneath them). When forests are cut down and burned, that stored carbon is released as CO₂ to the atmosphere.
• Removing the trees also reduces future photosynthesis, so less CO₂ is being absorbed from the atmosphere in years to come.
• Both effects push atmospheric CO₂ higher, accelerating climate change.
• Deforestation is responsible for around 10–15% of human-caused CO₂ emissions.

What is being done

• Reforestation and afforestation: planting new forests and replanting cleared areas
• Protecting old-growth forests through laws and national parks
• Sustainable forestry: managed harvesting where trees are replaced as they are taken
• Reducing demand: choosing products certified as not linked to deforestation (e.g. FSC-certified timber, RSPO palm oil)
• Paying tropical countries to protect their forests, through schemes like REDD+

8. Biological control of pests

A pest is any organism (insect, weed, fungus) that damages crops, livestock or people. The traditional way to manage pests is to spray chemical pesticides on them, but this has several drawbacks:

• Pesticides kill non-target species (bees, ladybirds, songbirds) as well as the pests
• Pests evolve resistance (see topic 14), making the pesticide useless over time
• Pesticides can biomagnify up food chains (see topic 15)
• Chemical pesticides contaminate soil, water and the food we eat

What biological control is

Biological control is the use of a living organism to control a pest, instead of a chemical. The control organism is typically a natural predator or parasite of the pest:

• A predator that eats the pest
• A parasite or parasitoid that lives on or inside the pest, weakening or killing it
• A disease-causing microbe that infects the pest

Advantages of biological control

• No chemical pollution of soil, water or food
• Targets only the pest species if the control organism is well-chosen
• Self-sustaining once established; the predator population maintains itself
• No resistance develops in the same way as with chemicals
• Cheaper in the long run than repeated chemical sprays

Disadvantages and risks

• The control organism can become an invasive species if it has no natural predators in its new habitat
• It can attack non-target species, sometimes more so than the pest it was meant to control
• The cane toad in Australia is a famous example: introduced to control beetles, the toads ignored the beetles, multiplied uncontrollably, and now devastate native wildlife by being poisonous to anything that tries to eat them
• It can take a long time to work, so cannot deal with sudden pest outbreaks
• It does not eliminate the pest entirely; it just keeps the population low

Integrated pest management (IPM)

Modern farming usually combines several approaches in integrated pest management:

• Biological control as the foundation
• Targeted pesticide use only when pest numbers are very high
• Crop rotation to break pest life cycles
• Pest-resistant crop varieties (sometimes from selective breeding or genetic modification)
• Physical barriers (netting, traps)

Integrated pest management uses far less chemical pesticide than traditional farming, with similar or higher yields.

9. Fish farming and sustainable food production

Why fish farming is needed

Wild fish populations have been severely overfished. Many fisheries that fed coastal communities for centuries have collapsed in the last 50 years; the Atlantic cod off Newfoundland is the most famous example. The UN estimates that around a third of all wild fish stocks are overfished and another half are at the limit of sustainability.

At the same time, demand for fish keeps rising as the world's population grows and as fish (especially oily fish) are seen as healthier than red meat. Fish farming (aquaculture) is supplying more and more of the world's seafood, now over half of it.

How fish farming works

Fish farms keep fish in enclosed cages or ponds, allowing them to be raised in controlled conditions:

• Marine cages: salmon, sea bass, sea bream raised in coastal sea cages
• Freshwater ponds: trout, carp, tilapia raised in inland ponds
• Recirculating tank systems: enclosed tanks where water is filtered and reused

Fish farmers control:

• Stocking density: how many fish per cage
• Feed: usually a pellet mix containing fishmeal, soy, vegetable oils and vitamins
• Water quality and oxygen levels
• Disease: vaccinations and (sometimes) antibiotics
• Temperature and lighting to maximise growth rate

Advantages of fish farming

• Reduces pressure on wild fish stocks
• Provides a reliable source of protein for a growing population
• Fish convert feed into body mass more efficiently than cattle or pigs, so they need less feed per kg of meat
• Creates jobs in coastal and rural areas

Disadvantages and concerns

• Pollution: uneaten food, fish faeces and urine pollute the surrounding water with nutrients, sometimes causing local eutrophication
• Disease and parasites (especially sea lice in salmon farms) can spread to wild fish populations nearby
• Escape of farmed fish that breed with wild populations, potentially reducing the wild population's genetic fitness
• The feed contains wild fish: small fish are caught from the sea and ground up into fishmeal, so fish farming does not entirely remove pressure on wild stocks
• Antibiotic use can drive resistance (see topic 14)
• Habitat destruction: mangrove forests have been cleared in many tropical countries to make space for shrimp farms

Making fish farming more sustainable

Modern aquaculture is trying to reduce these problems:

• Use of closed-containment systems that filter out waste
• Plant-based fish feed to reduce reliance on wild fish
• Vaccinations instead of antibiotics
• Integrated multi-trophic aquaculture: farming fish, shellfish and seaweed together so the shellfish and seaweed take up the fish waste

Sustainable wild fishing

For wild fish stocks, sustainable practices include:

• Catch quotas that limit how much can be caught each year
• Mesh size regulations so small juvenile fish escape and grow to breeding age
• Marine protected areas where no fishing is allowed
• Seasonal closures to protect breeding times
• Better fishing gear that reduces bycatch (accidental catch of non-target species)

When properly managed, wild fish stocks can recover. The North Sea cod population has slowly recovered from overfishing thanks to strict EU quotas.

10. Conserving biodiversity

The biodiversity loss caused by all the above pressures is alarming. Scientists estimate that current extinction rates are 100 to 1000 times the natural background rate. Many efforts are underway to slow or reverse the loss.

Methods of conservation

Habitat protection:

• National parks and nature reserves set aside large areas where wildlife is protected
• Marine protected areas for ocean wildlife
• Wildlife corridors linking fragmented habitats so animals can move between them
• Site of Special Scientific Interest (SSSI) designations in the UK to protect small but ecologically valuable sites

Species-specific programmes:

• Captive breeding in zoos for endangered species. The Arabian oryx was extinct in the wild by 1972, bred in zoos, and reintroduced. There are now over 1000 in the wild.
• Seed banks preserve plant genetic diversity. The Svalbard Global Seed Vault in Norway stores millions of seed samples as a backup against agricultural catastrophe.
• Frozen zoos preserve animal cells (sperm, eggs, tissue) for possible future use, including potential cloning of extinct or endangered species.

Legal protection:

• CITES (Convention on International Trade in Endangered Species) bans trade in many endangered species
• National laws protect specific species (e.g. all UK bats and their roosts are protected)
• Anti-poaching enforcement in countries with iconic but threatened species (elephants, rhinos, tigers)

Sustainable use:

• Sustainable forestry allows forest products to be harvested without destroying the forest
• Sustainable fishing quotas allow long-term harvesting
• Ecotourism lets people pay to visit wildlife without harming it, providing economic incentives for local communities to protect their wildlife

Habitat restoration:

• Reforestation of cleared land
• Wetland restoration to recover lost biodiversity hotspots
• Reintroducing species to areas where they had been lost (e.g. wolves reintroduced to Yellowstone National Park, beavers reintroduced to Britain)

Reducing demand for wildlife products:

• Education campaigns to reduce the market for rhino horn, tiger bone and other illegal wildlife products
• Banning trade in ivory and other endangered-species materials

Why conservation matters

Beyond the moral question of whether humans have the right to drive other species to extinction, there are practical reasons to protect biodiversity:

• Ecosystem services: insects pollinate crops, soil organisms recycle nutrients, wetlands filter water. Lost species mean lost services.
• Medicines: many drugs come from wild species; future discoveries are impossible if the species is extinct first.
• Food security: crop wild relatives carry genes that may be vital for future agriculture.
• Climate regulation: forests, peat bogs and oceans help regulate the global climate.
• Cultural and recreational value: people value wildlife for its own sake.

Conservation success stories

It is easy to be pessimistic, but conservation does work when given resources and time:

• Bald eagle (USA): nearly extinct in 1970 after DDT; now over 70,000 pairs
• Humpback whale: hunted to near-extinction by 1960; protected since 1966; now over 80,000 worldwide
• Mountain gorilla: down to 250 individuals in the 1980s; now over 1000 thanks to anti-poaching and ecotourism
• California condor: down to 27 birds in 1987; captive-bred and reintroduced; now over 500
• European bison: extinct in the wild by 1927; reintroduced from zoos; now several thousand in Poland and Belarus

These examples show that with concerted effort, declines can be reversed. But the underlying pressures (habitat loss, climate change, pollution) continue to grow, and many more species are at risk than there are conservation programmes to save them.