There has been a rise of Carbon Dioxide (CO2) concentration in the earth's atmosphere of around 20% since the beginning of global industrialization.

 

The rise in concentration is directly related to the rate of CO2 emissions.  Anything you can do to reduce CO2 emissions will directly effect the concentration of this gas, albeit in a small way.

 

CO2 is a naturally occuring gas.  Plants need it to live and grow.  But over billions of years, plants have used and trapped a large portion of the CO2 in the earth's atmosphere.  As the plants turned into oil and coal, the CO2 was trapped underground. 

 

Humans evolved in the last few million years, in an environment with reduced CO2 concentration.  Modern use of oil, coal, natural gas, etc. is releasing this trapped CO2 back into the atmosphere, undoing millions of years of nature's work every year. 

 

The main effect of CO2 in the atmosphere is that it acts as a "greenhouse gas," trapping the heat of the sun inside the atmosphere and making the earth warm up.  The more CO2, the warmer the earth will become.  Global average temperature increases have already been recorded over the past 100 years.

 

No one knows what the result of continued increase of CO2 will be, but it is likely to result in the earth's climate (weather) becoming more and more like the climate the dinosaurs would have known.  This would mean warmer temperatures.  And warmer temperatures will mean the icecaps and glaciers of the south pole and other areas will melt, raising the sea level. 

 

If this buildup of CO2 continues, the results will be disastrous.  Cities will be flooded and farmland will be destroyed.  Millions of people may die of starvation and the natural world as we know it will be changed drastically.