Human Impacts: Sources, Sinks, Climate Change and Biomagnification

1When people change the cycles

For most of history, the cycles described in the previous lesson were driven by natural processes. Today, human activities have become a major force within them. People burn fossil fuels, clear forests, farm huge areas of land and release manufactured chemicals. Each of these changes how much matter moves between reservoirs, and how fast.

The BC curriculum asks students to understand the human impacts on sources and sinks (for example, climate change, deforestation and agriculture), and the bioaccumulation and biomagnification of substances within ecosystems. This lesson follows each of these in turn, using the evidence that scientific sources provide.

2Sources, sinks and the bathtub model

A helpful picture is a bathtub. The water level depends on the balance between the tap (sources) and the drain (sinks). If inflow equals outflow, the level stays steady. If the tap is opened wider, the level rises. If the drain is blocked, the level rises as well.

The question for each human impact is therefore: does it add to a source, reduce a sink, or both?

3Climate change and the carbon cycle

The greenhouse effect

Sunlight passes through the atmosphere and warms Earth's surface. The warm surface gives off heat as infrared radiation. Greenhouse gases in the atmosphere absorb much of that heat and send some of it back down, which slows heat loss to space. This natural greenhouse effect makes Earth a habitable temperature: water vapour alone has such a strong warming effect that Earth would be uninhabitable for current life without it.

The evidence

Scientists reconstruct past climate from indirect records such as Antarctic ice cores, which trap air bubbles that show past temperature and CO₂. Other records include tree rings, glacier lengths and ocean sediments.

• Over hundreds of thousands of years, atmospheric CO₂ cycled between roughly 180 and 300 parts per million (ppm). It has since passed that maximum, reaching about 382 ppm in 2006 and 392 ppm in 2011.
• NASA reports that industrial activity has raised atmospheric CO₂ by nearly 50% since 1750, and that the carbon in the air has a signature that shows its origin in fossil fuels.
• The Sun is not the cause: total solar energy reaching Earth has followed its normal 11-year cycle with no net increase since 1880, while temperatures rose markedly. Also, the upper atmosphere has cooled while the lower atmosphere has warmed, a pattern consistent with greenhouse gases and not with a more active Sun.

Effects on ecosystems

• Ice. Glacier National Park had about 150 glaciers in 1850 and about 24 larger than 25 acres by 2010. Arctic sea ice is decreasing in size and thickness. Greenland lost roughly 150 to 250 cubic kilometres of ice per year from 2002 to 2006.
• Sea level. It rose an average of 1.8 mm per year overall, and 2.9 to 3.4 mm per year between 1993 and 2010.
• Timing and range. Species shift where they live and when events such as flowering occur. In Great Britain, 385 plant species flowered about 4.5 days earlier than in the previous 40 years. Insect-pollinated plants flowered earlier than wind-pollinated ones, which risks a mismatch with pollinators.
• Biodiversity. Climate change, combined with habitat loss and disease, is considered a major threat. Estimates of the share of species facing extinction by 2050 range from 15% to 40%.

4Deforestation: removing a sink

Forests are sinks for carbon: trees remove carbon dioxide from the air by photosynthesis and store carbon in wood. When forest is cleared, two things happen. First, a sink is lost, because fewer plants remain to absorb CO₂. Second, if the wood is burned or decays, the stored carbon is released as CO₂, so the forest becomes a source. NASA notes that clearing land contributes to higher greenhouse gas levels, though to a smaller extent than burning fossil fuels.

Deforestation harms ecosystems in other ways. Removing a species' habitat can drive it to extinction. Half of the forest in Sumatra is gone, and Borneo has lost a similar area, largely cleared for timber and palm oil plantations. Global forest cover fell by 3.1% in 2000 to 2005.

5Agriculture: changing the nitrogen and phosphorus cycles

Farming alters several cycles at once. It uses large areas of land, adds fertilizer, raises livestock and grows rice.

• Greenhouse gases. Livestock digestion and manure, and rice paddies, release methane. Fertilizer production and use is linked to nitrous oxide. Both are greenhouse gases.
• Fertilizer runoff. Rain washes extra nitrogen and phosphorus from fields and from sewage into rivers, lakes and coasts.

Eutrophication and dead zones

Nitrogen and phosphorus often limit plant and algae growth. When they are added in excess, algae and microbes multiply. When the algae die, microbes break them down and use up the oxygen in the water. Fish and shellfish suffocate. This process, called eutrophication, creates dead zones.

Nitrogen oxides from burning fossil fuels also contribute to acid rain (nitric acid) and to greenhouse gases (nitrous oxide).

6Bioaccumulation and biomagnification

Another kind of human impact is the release of substances that living things cannot easily break down or excrete. Two terms describe what happens to them in ecosystems.

• Bioaccumulation is the build-up of a substance in an organism's body over time, when the organism takes it in faster than it can get rid of it.
• Biomagnification is the increase in the concentration of the substance as it moves up a food chain, because predators eat many prey that each carry it.

Substances that biomagnify tend to share two features: they are persistent (hard to break down or excrete) and often fat-soluble (they dissolve in fat and not in water, so they collect in fatty tissue). Mercury, DDT and PCBs are standard examples.

Case study: DDT and birds of prey

DDT is a synthetic insecticide that was widely used until the 1970s, when many countries banned or heavily restricted it. It is very poorly soluble in water and accumulates in fat. As predators ate contaminated prey, DDT became concentrated in birds such as bald eagles and peregrine falcons. In the 1950s, this caused thinning of eggshells and a steep decline in their populations in North America. After DDT was banned for agricultural use, the populations recovered.

Case study: mercury in fish

Seawater contains little mercury, but algae absorb it, usually as methylmercury, a very harmful form. Organisms take it up efficiently but excrete it very slowly, so it builds up. Herring contain roughly 0.01 ppm, while sharks can exceed 1 ppm. Top predators such as swordfish and sharks therefore carry far more than direct exposure would explain. Humans at the top of a food chain can also be exposed.

Putting the impacts side by side

Practice: a biomagnification data set

A student is given the following invented data for a contaminant measured in a lake food chain, and is asked to describe the pattern.

A good scientific answer also considers limits: the data come from one lake, and other lakes could differ. More samples, and several seasons, would make the conclusion stronger.

Evaluating an impact: a reasoning pattern

7Summary

• The balance of sources and sinks determines how much of a substance a reservoir holds.
• Burning fossil fuels, deforestation and agriculture add greenhouse gases and/or remove sinks, strengthening the greenhouse effect and changing climate.
• Excess nitrogen and phosphorus from fertilizer and sewage cause eutrophication and dead zones.
• Persistent, fat-soluble substances bioaccumulate in organisms and biomagnify up food chains, harming top predators.

?Quick check

Try each question first, then reveal the answer.