What is one way we *know* that the extra (added) CO2 that is pushing our climate system out of balance and retaining just a small amount of extra heat each year is related to human activities?
a.
because China is producing more fossil fuel emissions than any other country in history
b.
because CO2 concentrations are rising
c.
because Al Gore says
d.
the ratio of stable carbon isotopes in atmospheric CO2 is shifting toward the ratio in fossil fuels
