The significant difference at this point in our history is the speed with which the changes are expected to occur. Human activities, not natural cycles, are behind the build-up of greenhouse gases, and the impacts will be measured over decades, not centuries or millennia. Human populations have flourished since the end of the last ice age. As societies have grown and become more sophisticated, they have also become more dependent on the planet's natural resources. Human activities have literally changed the face of the world as we have cleared wilderness to build cities and create agricultural land.
Deforestation ultimately results in the release of carbon dioxide into the atmosphere.
Trees and other vegetation act as natural storehouses for carbon dioxide. More land has been cleared for human use in the last 100 years than in all the rest of human history. The conversion of forests to agricultural land over this period is estimated to have released more than 100 billion tonnes of carbon dioxide into the atmosphere. Agricultural practices, especially rice production and raising domestic animals, result in increased emissions of methane. Modern fertilization techniques result in increased emissions of nitrous oxide.