If at least 95% of CO2 is emitted by nature, why is man's contribution important?

Each year nature's emissions vary by a greater amount then man produces. How can nature distinguish between higher CO2 output from natural sources in a given year and man made contributions? If higher emissions are caused by industrialization or volcanic eruptions or dying trees, the net result is the same. The... show more Each year nature's emissions vary by a greater amount then man produces. How can nature distinguish between higher CO2 output from natural sources in a given year and man made contributions? If higher emissions are caused by industrialization or volcanic eruptions or dying trees, the net result is the same. The earth's systems have absorbed varying levels of CO2 input before industrialization.
Update: Consumption of vegetation by animals & microbes accounts for about 220 gigatonnes of CO2 per year. Respiration by vegetation emits around 220 Gt. The ocean releases about 330 Gt. In contrast, human emissions are only around 26.4 Gt per year.
Update 2: Direct measurements of atmospheric CO2 taken during the 1800s show little change from the present.
Why should we take a proxy (which is what is used to argue lower CO2 levels) over direct measurements.
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