What are the two reasons carbon dioxide is building up in our atmosphere?

Answer 1

The two reasons are that human activities are adding CO2 and also preventing it from being removed.

1. Manufacturing, energy generation, and transportation all add carbon dioxide, mainly through the burning of fossil fuels.
2. At the same time, activities such as land use, deforestation, and pollution can prevent the reabsorption of carbon dioxide which is primarily done by plants.

Answer 2

1. Burning fossil fuels like coal and oil adds extra carbon dioxide to the atmosphere.
2. Deforestation. Cutting down the great forests of the world means that the trees no longer take carbon dioxide out of the atmosphere, and they also no longer release oxygen.

Answer 3

There is a big seasonal variation caused by the death of northern hemisphere foliage which releases billions of tons of CO2 into the air. This release is offset only partially by an increase in Southern Hemisphere foliage, as there is much less seasonally affected land area in the southern hemisphere.

This change, however, is cyclic, as that CO2 is taken up again in the spring and summer. Sort of like earth breathing the CO2 in and out.

The second major contributor is mankind. We pump roughly 30 billion tons of CO2 into the atmosphere each year, primarily from fossil sources such as coal and oil burning. Limestone is calcium carbonate, and CO2 from the use of it in cement is also released, as is some from clear cutting jungle forest for agriculture. Wood from temperate forests is largely sequestered in the manufacture of lumber for home construction, furniture, paper, and so on. But jungle forest tends to be slashed and burned.

A third source of CO2 is volcanic activity. Volcanic activity is not particularly greater now than it was at any time over the past 800,000 years, per data gathered from ice core samples, suggesting the small fraction of CO2 liberated by volcanic emissions were readily absorbed by the earth. According to the USGS, humans emit more than 100 times all earth's volcanoes combined.