Scientists fear that too much more disturbance of the status quo will lead to extreme weather, polar ice caps melting and the ensuing changes will directly affect the way we live now. Agriculture under water or in drought conditions; population centres moving to higher ground etcetera.
Cover a grapefruit in clingfilm. If the grapefruit is Earth, (12,742 kilometres in diameter), the clingfilm is the thickness of our habitable atmosphere. Humans cannot live comfortably at heights over 3000 metres (3km). Passenger jets fly at 10,000 metres. 3 km divided by 12742 = 0.000235; not a readily absorbed number but recognizably quite a thin slice.
Sea makes up 71% of Earth's surface area; ice 10-16%. Of the remaining 13% we must consider inhospitable mountains, deserts and hot humid jungles so, while we appear spread out over the globe, we don't take up a lot of room. Look out of the window next time you fly or go on a train. Go to the Science Museum in London and look at the globe presentation covering electric lights as seen from space.
Our atmosphere (dry): 78% Nitrogen, + 21% Oxygen (=99%) + Argon 0.93% = 99.93%
Not forgetting water vapour - read this <<>> and this <<>> One of the articles suggests that CO2 makes up 20% of the green house gases + Methane, Ozone and Water Vapour but may be responsible for 60% of the anthropogenic (literally poisoning by humans) cause of GW.
Of the remaining 0.07% 0.039% is CO2 (or 390 parts per million). Ice core samples indicate 275 ppm 250 years ago. CO2 levels are up nearly 40%.
Trees, plants and algae use photosynthesis in daylight to build and grow. Photosynthesis seizes carbon from the air in a process called CARBON FIXING. Oxygen is a waste product of their daytime activity. Some CO2 is given off as a natural part of respiration. At night plants revert to normal respiration and give off CO2 as we do. I read that the Amazon rainforest can have 400 ppm CO2 levels by night at the height of the canopy.
Of the 21% Oxygen we animals breathe in, we breathe out 16%. Humans breathe out 8.5% of all CO2 emissions per year.