What is the relationship between carbon in the atmosphere and climate change?

In the 1990’s, humans added 8.0×1015 grams of carbon (1015 grams of carbon  = 1 PgC) to the atmosphere each year, primarily by burning fossil fuels (6.4 PgC/yr) and clearing land in the tropics (1.6 PgC/yr). The ocean took up 28% of this carbon, and the land absorbed 32%. Only 40% remained in the atmosphere to cause climate warming.Natural processes are significantly damping the rate of carbon accumulation in the atmosphere.From 2000-2008, humans added 9.1 PgC to the atmosphere each year, 7.7 PgC/yr from fossil fuels and 1.4 PgC/yr from land use change. There is some evidence that a larger fraction of these recent emissions has remained in the atmosphere (45%, LeQuere et al. 2009).Future climate warming depends on both the CO2 source from human emissions and the CO2 sink from natural sinks in the ocean and the terrestrial biosphere.
Carbon is a ubiquitous element on Earth. Most of the Earth’s carbon is stored in rocks, but this carbon is essentially inert on the 100’s to 1000’s year timescales of interest to humans.The rest of the carbon is stored as CO2 (carbon dioxide) in the atmosphere (2%), as biomass in land plants and soils (5%), as fossil fuels in a variety of geologic reservoirs (8%) and as a collection of ions in the ocean (85%). These are the “active” reservoirs of carbon of interest in this website. and atumis shpree is 
hope this helps :)