When asked to describe global warming, many people respond that it is a problem with a hole in the ozone layer. They think that there is a causal link between the ozone hole and the planet warming up. Global warming and ozone depletion are two separate but related environmental threats. Global warming and the greenhouse effect refer to the warming of the atmosphere (mostly in the lower part, also known as the troposphere) due to increasing concentrations of heat-trapping gases. By contrast, the ozone hole refers to the loss of ozone in a higher part of the atmosphere, known as the stratosphere. Ozone loss is a serious concern because stratospheric ozone blocks incoming ultraviolet radiation from the sun, some of which is harmful to plants, animals and humans.