Solar Radiation: The Energy Behind Life, Weather and Water

1The Sun: the engine behind almost everything

Every morning, an enormous amount of energy arrives at Earth from the Sun. That energy lights the planet, warms it, grows its plants, stirs its winds, moves its oceans and lifts water into the sky. Remove it, and nearly all of this would stop.

The BC curriculum expresses this in a single content item: solar radiation provides the energy required for most life on Earth, drives wind and ocean currents, distributes energy and nutrients, and powers the water cycle. This lesson takes each part of that statement in turn.

The Big Idea behind this lesson and the next ones is that the biosphere (living things), geosphere (rock and soil), hydrosphere (water) and atmosphere (air) are interconnected, as matter cycles and energy flows through them.

2Uneven heating: why latitude matters

The Sun does not warm the whole planet equally. Near the equator, sunlight strikes the surface nearly head-on. At high latitudes (near the poles), the same sunlight strikes at a slant and is spread over a larger area. Averaged over time, the tropics gain more energy than they lose to space, and the poles lose more than they gain.

Latitude also affects how much the amount of sunlight changes through the year. Equatorial regions get steady, direct sunlight, while higher latitudes have strong seasonal swings in daylight and warmth. These differences, together with rainfall, help to decide what kind of ecosystem (biome) can exist in each region.

3Capturing sunlight: photosynthesis

Only a few kinds of organisms can capture the energy of sunlight directly. Plants, algae and cyanobacteria are photoautotrophs (“self-feeders using light”). They carry out photosynthesis, which is the only biological process that captures sunlight and stores its energy in the chemical bonds of sugars. Animals, fungi and most bacteria are heterotrophs: they get energy by eating other organisms or their remains.

In words, the process is:

carbon dioxide + water + light energy → sugar + oxygen

In chemical shorthand, six molecules of carbon dioxide and six of water yield one sugar molecule and six oxygen molecules: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

• Where. In plants, it takes place mostly in the leaf. Gases enter and leave through tiny pores called stomata. Within the cells, photosynthesis happens in chloroplasts, which hold the green pigment chlorophyll.
• Two stages. The light-dependent reactions capture light energy and store it in energy-carrying molecules. The light-independent reactions (the Calvin cycle) use those molecules to build sugars from carbon dioxide.
• Why it matters. Photosynthesis supplies the food energy that moves through food chains, and it releases oxygen that many organisms need. For a longer explanation, see How photosynthesis works.

Life without sunlight

The statement says solar radiation provides energy for most life, not all. Where sunlight does not reach, such as around deep-sea hydrothermal vents, chemoautotrophic bacteria use chemical energy from substances such as hydrogen sulfide released by the vents. They make organic molecules that support a local community of animals such as shrimp, crabs and mussels. These exceptions show that the definition of “energy source” depends on the ecosystem, but for the great majority of ecosystems it is the Sun.

4Energy flows through food chains

Energy enters most ecosystems through producers (autotrophs). From there, it moves along a food chain: producers are eaten by primary consumers (herbivores), which are eaten by secondary consumers, and so on up to apex consumers. Each feeding step is a trophic level. Decomposers break down dead material and recycle nutrients. Many connected food chains form a food web (see What is an ecosystem?).

Energy is lost at each step

At every transfer, a large share of energy is lost, mainly as heat released by the organisms' life processes (consistent with the idea that energy cannot be recycled). Only what remains in the body of the organism is available to the next level.

Because so much energy is lost at each step, only about four to six transfers can occur before too little energy remains to support another level. This is why food chains are short, and why there are fewer top predators than herbivores in most ecosystems. An energy pyramid, with a wide base for producers and narrower layers above, is always upright.

5Wind and ocean currents: moving energy around

Because the tropics gain more energy than the poles, the atmosphere and oceans move heat toward higher latitudes. The result is the pattern of global winds and currents.

Wind

Warm air near the equator rises, flows toward the pole high in the atmosphere, and sinks again at around 30 degrees latitude. Some of the sinking air returns toward the equator along the surface; these are the trade winds. The rotation of Earth deflects moving air (the Coriolis effect), so surface winds blow at an angle and form the pattern of trade winds and westerlies, rather than moving in a straight line.

Ocean currents

Ocean currents are movements of water from one place to another. Three forces drive them: tides (strongest near shore), wind (which pushes water at or near the surface and drives currents across whole ocean basins) and differences in water density (caused by temperature and salinity) that set up a slow circulation. Currents carry warm water from the equator toward higher latitudes and cold water from polar regions toward the equator.

Distributing nutrients

Currents and winds also move nutrients. Wind near coasts can produce upwelling, which is local rising of water. Rain and runoff carry carbon, nitrogen, phosphorus and sulfur from land into water. In this way, solar-driven movements of air and water spread energy and materials around the biosphere.

6The water cycle: powered by the Sun

Energy from the Sun drives the water cycle by turning liquid water into vapour. Solar energy warms the oceans and other surface waters, which makes water evaporate (and ice sublimate, turning directly into vapour). Plants also return water to the air through their leaves. Together, evaporation and the water released by plants are called evapotranspiration, the main way water enters the air.

1. Evaporation and transpiration. Solar energy turns liquid water into vapour.
2. Condensation. Rising vapour cools and condenses into clouds.
3. Precipitation. Gravity pulls water back to Earth as rain, snow or hail.
4. Runoff and infiltration. On land, water flows across the surface as runoff and streamflow, or soaks into the ground and recharges groundwater, which can later reappear in springs and rivers or reach the ocean.

Practice: following energy up a chain

Energy calculations are a good way to test understanding of why chains are short. The example uses the measured transfer efficiency of about 14.5% between the first two levels as an illustration. Real efficiencies vary from one ecosystem and one step to another, so the numbers here are for practice only.

The same logic also explains a practical fact. An ecosystem can support many more individual herbivores than carnivores, because the carnivores depend on the small share of energy that passes up from the herbivores. Pyramids of numbers and biomass can sometimes appear inverted (a few large trees supporting many insects, or tiny fast-growing plankton outweighed by the animals eating them), but a pyramid of energy is always upright.

Reading a climate pattern

A second skill is to connect a biome to its energy input. A reasoning pattern is useful: describe the sunlight, link it to the climate, and then to the organisms.

7Putting the Sun's work together

• For life. Photosynthesis captures sunlight and feeds most food chains; energy is lost as heat at each step and must be resupplied by the Sun.
• For climate. Uneven heating sets up winds and ocean currents that move heat from the tropics toward the poles.
• For matter. The Sun powers the water cycle, which carries materials from land to sea and supports the cycles of other substances covered in the next lesson.

Energy flows in one direction and leaves as heat. Matter cycles. How atoms of carbon, nitrogen and phosphorus move through living and non-living parts of Earth is the topic of the next lesson.

?Quick check

Try each question first, then reveal the answer.