The Earth's climate has changed throughout history. In the
last 650,000 years there have been seven cycles of glacial
advance and retreat, most of which are attributed to very
small variations in Earth's orbit that change the amount of solar
energy our planet receives. The current warming trend is of
particular significance because most of it is extremely likely to be the
result of recent human activity and is proceeding at a rate that is
unprecedented over decades to millennia. Human (anthropogenic)
impact on the environment includes changes to biophysical
environments and ecosystems, biodiversity, and natural resources
caused directly or indirectly by humans. Which of these human
impacts most likely caused the greatest change in atmospheric CO₂
levels, leading to increased global warming and climate change?