Moving from microorganisms to slightly larger beings, There are 7.5 billion people on the planet. If 1 billion of us, from the industrialised world reduced by just one tonne, this would add up to 1 Gt. One by one, tonne by tonne.
The average American generates a massive 16 tons of CO2 per year (source: Wikipedia/EDGAR), and the average European around half that. If us heavy emitters shift our behaviour enough to reduce their CO2 footprint by a tonne a year, this would have a considerable impact. Most of the changes needed are not onerous, and some are positively pleasurable; recycling rubbish, turning off lights and air conditioners, unplugging chargers, buying energy-efficient equipment, and food shopping from local farm shops. They all add up, and ‘one by one, tonne by tonne’ we collectively add up to a gigaton of savings every year.