P

Parts per million

 A unit of measurement that can be used to describe the concentration of a particular substance within air, water, soil, or some other medium. For example, the concentration of carbon dioxide in the Earth’s atmosphere is almost 400 parts per million, which means 1 million liters of air would contain about 400 liters of carbon dioxide.

Product life cycle

Product life cycle: The many steps that go into creating, using, and disposing of a product. A product life cycle typically starts by removing raw materials from the Earth (for example, cutting down trees, mining metals, or pumping oil). These raw materials are then transported, processed, and manufactured into usable products. Next, the product is packaged and transported to a place where people can buy it. The final steps occur when people use up, throw away, or recycle the product.

Positive feedback loop

Positive feedback loop: A process in which one change leads to another, which then causes even more of the original change. In climate change, a positive feedback loop occurs when warming causes changes that lead to even more warming. For example, as the Earth gets warmer, the amount of ice that covers the Arctic Ocean is shrinking, which leaves more open water. Ice reflects a lot of sunlight back into space, while the open ocean is dark and absorbs more of the sun’s energy, making the Earth warmer. Thus, melting ice causes the Earth to absorb more energy from the sun and become even warmer.

Plate tectonics

Plate tectonics: A scientific theory that describes how large sections of the Earth’s crust called plates move over time. The Earth has seven or eight major plates, including the North American plate, plus many smaller plates. As these plates collide, spread apart, or grind alongside one another, they cause earthquakes and volcanic activity.