What causes the counter-intuitive presence of CO2 in the atmosphere during spring and summer months?
What are the ranges of natural (or pre-industrial era) levels of CO2 in the atmosphere in ppm and tons?
Are small reductions in CO2 emissions better than no reductions, in terms of mitigating climate change?
If we burn all carbon in the Earth's crust, by how much would the atmospheric concentration of $CO_2$ increase?
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