Over the past 60 years, the levels of carbon dioxide in the atmosphere has been continually increasing. MAny scientists hypothesized that the increase has led to carbon cycle return to normal?

Explanation:

Once that there's more carbon dioxide available in the atmosphere, more of this gas will be diluted in the water. This gas in the water can result in a acidic environment which helps to grow organisms which requires amounts of carbonic acid and, by consequence, using this CO2 can decrease the levels of the same. By the other hand, the levels of carbonic acid can modify the natural environment of the sea, affecting all organisms there.

The same occurs in the land, where plants takes the extra carbon dioxide and store inside it as biomass, the excess of CO2 increase the amount of plant growing, which, decrease the ppm of CO2 as well. But by the other hand, the CO2 in the atmosphere increase the temperature of the planet, affecting all organisms in the nature.

the correct answer is the first one.