Dynamic equilibrium: balanced but always changing

0:00If something stays steady, is nothing happening?

0:04Picture a bathtub with the tap on and the drain open.

0:08Water flows in at 10 litres per minute and out at 10 litres per minute, so the level stays steady.

0:16But the water never stops moving: new water pours in as old water drains away.

0:22This is dynamic equilibrium: dynamic means always changing, and equilibrium means balanced.

0:30Equilibrium does not mean nothing is happening; it means the flows in and out balance.

0:37Now turn the tap up to 15 litres per minute.

0:41More flows in than out, so the level rises.

0:46Deeper water drains faster, so the level settles higher, where 15 litres per minute flow out.

0:53The tub is a system with flows in and out: a source adds, and a sink removes.

0:59Lakes, animal populations and carbon dioxide in the air all work the same way.

1:06For carbon dioxide, sources include respiration and decay; sinks include plants and the ocean.

1:13Before industry, they balanced, keeping carbon dioxide near 280 parts per million for thousands of years.

1:22That is 280 molecules of carbon dioxide in every million molecules of air.

1:29Burning fossil fuels added a new source, so now more flows in than out.

1:35It has climbed to over 420 parts per million, and is still rising.

1:41Dynamic equilibrium: balanced flows hold a level steady. Change one flow, and the level shifts.