Causes and effects of the increasing concentration of CO2 in the atmosphere

Causes:

Fossil fuel combustion/use

The largest human source of carbon dioxide emissions is from the combustion of fossil fuels. This produces 87% of human carbon dioxide emissions.
Electricity/Heat sector
Electricity and heat generation is the economic sector that produces the largest amount of man-made carbon dioxide emissions. This sector produced 41% of fossil fuel related carbon dioxide emissions in 2010
Transportation sector
The transportation sector is the second largest source of anthropogenic carbon dioxide emissions. Transporting goods and people around the world produced 22% of fossil fuel related carbon dioxide emissions in 2010
The industrial sector is the third largest source of man-made carbon dioxide emissions. This sector produced 20% of fossil fuel related carbon dioxide emissions in 2010.

http://whatsyourimpact.org/greenhouse-gases/carbon-dioxide-sources

Effects:

Ocean Acidification

The oceans are the Earth's largest carbon storage medium, so if the atmospheric CO2increase were "natural", it would likely be coming from the oceans.

Oceanic CO2 Rising Fastest at the Surface

If CO2 were being driven into the ocean from the air, the oceanic concentration would rise fastest at the surface.

http://www.skepticalscience.com/co2-increase-is-natural-not-human-caused.htm

Sea level rise - densely settled coastal plains would become uninhabitable with just a small rise in sea level, which would result from melting of the ice caps.

Impacts on agriculture - Global warming could have major effects on agricultural productivity.

Reduction of the ozone layer - Warming would result in increase high cloud cover in winter, giving chemical reactions a platform in the atmosphere, which could result in depletion of the ozone layer.

Increased extreme weather - A warmer climate could change the weather systems of the earth, meaning there would be more droughts and floods, and more frequent and stronger storms.

Spread of diseases - Diseases would be able to spread to areas which were previously too cold for them to survive in.

Ecosystem change - As with the diseases, the range of plants and animals would change, with the net effect of most organisms moving towards the North and South Poles.