We have long since surpassed the Earth's sustainable carrying capacity. This means that we have too many people living high impact lives to keep the Earth's ecology in balance. And when it's off balance it has to change in an effort to balance itself again. This means changes in temperature and climate among other things. Deforestation means less free oxygen and more carbon dioxide.
More carbon dioxide means enhanced greenhouse effect and that means warmer climate.
Warmer climate means polar ice caps melt and that means a higher sea level which means less land available for people and their agriculture which means more deforestation. It's a series of cycles that fuel each other and spiral down towards disaster. Soil depletion means farmers have to add harmful chemicals to the soil to make anything grow and they end up inside our bodies and poison us. In all of these areas, ecosystems that took millions of years to evolve were irreversibly altered for agriculture. In the U.S. alone more than 260,000,000 acres of forest have been clearcut to provide land for the production of America's animal-based diet.