I know that even though I'm conscious of my carbon footprint and try to minimize it, I contribute to carbon emissions that cause global warming and climate change. I don't think I've met anyone who doesn't.
Transportation is one way in which people contribute to global warming, and probably the biggest. I ride a bicycle to work and for errands and drive a fuel-efficient car when I do drive, but I'm still burning some fossil fuel. I also contribute to carbon emissions by using other forms of transportation such as flying. Products that I buy must be transported to the retailer, using fossil fuel and creating emissions. Some of the employees of the businesses I frequent arrive at work via fossil fuel transportation. Motor vehicle exhaust is the biggest source of greenhouse gases worldwide.
Electricity is also a source of carbons emissions, as some electricity in the U.S. is produced from burning coal. I live in an area that produces hydroelectricity, but the nature of the electrical grid is such that we all contribute to climate change by using electricity from the grid. In addition, electricity is needed to produce products that we all use.