We will see later on that humans are significantly altering the global nitrogen cycle by producing fertilizer that often pollutes aquatic ecosystems. It's quite shocking to learn that in the past few decades, human production of nitrogen fertilizer now is greater than all natural nitrogen fixation on earth. Like nitrogen, it is extremely limited in ecosystems because it weathers slowly from rocks and it gets bound up easily to soils and made unavailable to plants. Compared to carbon and nitrogen, the phosphorus cycle is not as severely impacted by humans. The global cycle of sulfur is characterized by both atmospheric and geological movements.
Sulfur plays two important roles in global change biology.
The emission of SO4 from fossil fuel plants combines with water to form acid rain. Sulfur particles, or aerosols , emitted from volcanoes and from the oceans cause water to condense in the atmosphere and form clouds. The more sulfur aerosols in the atmosphere, the more clouds. Because clouds reflect light from the sun, rising sulfur aerosols may lead to global cooling. Indeed, when Mt. Pinatubo blew up in 1992, it sent volcanic debris into the stratosphere and resulted in a global cooling of 1 degree in 1993.