Water and carbon cycles

Living things keep swapping matter with the air, water, soil and rocks around them. The endless movement of materials such as carbon, nitrogen, oxygen and water between the non-living (abiotic) and living (biotic) parts of the Earth is called a biogeochemical cycle (bio = life, geo = earth, chemical = substances).

The water cycle: water evaporates from seas, rivers and lakes (and plants give off water by transpiration) → it condenses into clouds → it falls as rain, hail or snow → it runs off into rivers and the sea, or soaks into the ground as groundwater. On the way, water dissolves minerals and carries nutrients to the oceans.

Climate change is changing the water cycle:

• warmer air holds more moisture, so some places get heavier rain (stronger monsoons) while others get drought;
• melting glaciers swell rivers and, in the long run, raise sea levels, threatening coastal cities like Mumbai and Chennai;
• sudden heavy downpours wash away soil and let less water soak into the ground, so groundwater runs low and farming in dry months gets harder.

The carbon cycle has a fast lane and a slow lane:

• Fast (days to years): plants take in carbon dioxide and make glucose by photosynthesis; breathing (respiration) and rotting (decomposition) put carbon dioxide back.
• Slow (millions of years): buried plants and animals slowly become coal, oil and gas. When we burn them, that carbon goes back into the air in a very short time.

The ocean also swaps carbon dioxide with the air. Dissolved, it forms carbonate and bicarbonate, used by tiny ocean plants (phytoplankton) and by sea creatures to build shells.

The Keeling curve is a graph of carbon dioxide in the air. It rose from 315 ppm in 1960 to 420 ppm today (ppm = parts per million): about 35% more, mainly from burning fossil fuels and cutting forests. Its zig-zag shape shows a dip every year, when plants in the Northern Hemisphere grow and take in carbon dioxide.

Carbon facts:

• carbon is about 49% of the dry weight of living things;
• 71% of the world's carbon is in the oceans;
• the atmosphere holds only about 1%.

So a small change in the air's carbon makes a big difference to the climate.