Question

How have humans impacted the carbon cycle?

Answer 1

The major impact on any "carbon cycle" has been the rapid release of "bound" carbon ("fossil" fuels) into atmospheric carbon.

Explanation:

What exactly the "carbon cycle" should look like might be debatable. Wasn't is much different BEFORE the formation of our current "fossil fuels"? Didn't their formation "impact" the carbon cycle of the time?

In any case, the impact of human existence over time - and especially in recent centuries - has been to release much of that solid and liquid-contained carbon into the atmosphere (and then back into the land and hydrosphere) in the forms of carbon dioxide, carbon monoxide and particulate material.

Whether that "impact" is significant, good, bad, or indifferent is another question. A change will always happen, whether we like it (or cause it) or not.