1. The carbon-dioxide increases in atmosphere by fossil-fuels, decaying-dead, automobiles etc. and when animals or plant die they get buried and carbon content increases under the ground.
2. Greenhouse gas are those gases which trap heat in the atmosphere and maintain the temperature of the earth.
3. Diamond and graphite are solid forms of carbon.
4. Dry ice is the carbon as liquid.
5. Autotrophs make simple sugar glucose via photosynthesis and this sugar is used by heterotrophs as a substrate for respiration in the presence of oxygen.
Explanation:
When the fuels from automobiles, fossil-fuel, dead decaying organism release carbon in the atmosphere, the increase in carbon dioxide is the cause of global warming.
Greenhouse gases absorb the infrared radiations from the earth's surface and reradiate it back to the atmosphere. In short greenhouse gases allow sun's rays to pass through and maintain the earth's temperature otherwise the earth will become too cold to sustain lives.