Earth's energy flows: sun, wind, water, heat and life

By the end you will be able to

• Explain how the solar constant of about 1,361 W/m2 becomes about 340 W/m2 on average, and where that energy goes
• Compare the sizes of the main natural flows, from sunlight and evaporation to wind, geothermal heat and tides, with human energy use
• Describe how Earth's energy imbalance and greenhouse gases are connected, and why our waste heat is not the main cause of warming
• Calculate the efficiency and power density of photosynthesis and use them to judge biomass energy claims
• Apply Kleiber's rule and the 10% food-chain rule to animal and human energy needs
• Relate the Energy table's natural source tiles to the planetary flows that feed them

Contents

1. Part 1 Sunlight and the planet's energy budget

   Nearly every energy flow on Earth's surface starts as sunlight. Wind, rain, rivers, waves, crops, forests and the fossil fuels made from ancient plants are all sunlight that has been caught and changed. Before looking at any of them, it helps to know how much sunlight arrives, what share is reflected, what share is absorbed, and how the planet keeps its books balanced by sending the same amount back to space.

   • The Sun delivers about 1,361 W/m2, but a spinning globe averages it down to about 340
   • About 30% is reflected; the rest must eventually be radiated back to space
   • Greenhouse gases keep Earth habitable, and extra ones now tilt the balance by about 1 W/m2
2. Part 2 Air and water in motion

   Sunlight does not heat the planet evenly. The tropics take in more than they radiate and the poles radiate more than they take in. Air and water carry the surplus towards the poles, and in doing so they create winds, rain, rivers, ocean currents and waves. These are the flows that the Energy table lists as Wind, Water cycle, Ocean currents and Ocean waves, and they are very large next to human demand.

   • Uneven heating turns the atmosphere into a heat engine, and wind is its small mechanical output
   • The water cycle moves more energy than anything except radiation
   • Oceans store most of the heat, keep it moving, and add waves and tides
3. Part 3 The restless Earth: heat from below

   Not every natural energy flow comes from the Sun. A steady trickle of heat escapes from Earth's interior, left over from the planet's formation and topped up by radioactive decay. It is tiny next to sunlight, yet over millions of years it builds mountains, opens oceans and decides where coal, oil and ores are found. Where it reaches the surface in concentrated form, people can turn it into electricity and heat.

   • Earth's interior releases about 45 TW, a dilute flow of under a tenth of a watt per square metre
   • Plate tectonics is that heat at work, and it sets the stage for everything else
   • Geothermal energy works where heat is concentrated, and shallow ground heat is mostly solar
4. Part 4 Photosynthesis and the productivity of life

   Photosynthesis is the conversion on which nearly all life, all food and all fossil fuels depend. Plants, algae and cyanobacteria use sunlight to make sugars from carbon dioxide and water. The process is ancient and robust, but it is also strikingly inefficient, and that inefficiency explains why biomass energy needs so much land. This part looks at the reaction, its variants and the total amount of life it supports.

   • Photosynthesis stores sunlight in sugar, but rarely more than a few percent of it
   • C3, C4 and CAM plants run the same reaction three ways, and respiration takes a large cut
   • How much life there is, and how fast it is made
5. Part 5 How animals use energy, and the biosphere as one system

   Animals cannot capture sunlight. They live by eating what plants make, or by eating animals that did. Their energy use follows strikingly regular rules: metabolism scales with body size, warm blood costs dearly, and each step up a food chain loses most of the energy. Stepping back, the whole biosphere works as a single system of linked energy and material cycles, which humans now shape.

   • Metabolism scales with size: Kleiber's three-quarter rule
   • Moving, working and eating up the food chain
   • The biosphere works as one linked system, and it is thin, old and now shaped by people
6. Wrap-up Key terms, try it in the table, final quiz, sources