The average temperature of the Earth is a fundamental part of what allows Earth to be habitable. Of all of the planets that have been found, Earth is the only one known to support life. This "perfect" temperature exists because Earth lies in a "Goldilocks zone" in terms of temperature, being neither too hot nor too cold to support life. In contrast, if Earth was even slightly warmer or colder, life as we know it would not exist on this planet. Venus and Mars provide examples of what different temperatures on the surface of a planet similar in composition to the Earth could range between.
NASA has reported that the average temperature of the earth is 15°C. However, extreme temperatures are still possible on Earth. The hottest temperature ever recorded on Earth was measured to be 70.7°C in the Lut Desert of Iran in 2005, and the coldest temperature was -89.2°C in Vostok, Antarctica. In addition, temperatures around the planet vary based on where you are. Generally speaking, the hottest areas are near the equator where the energy incident from the Sun hits the Earth at nearly a right angle. Conversely, the coldest areas on the Earth are around the poles where the energy incident from the Sun hits the Earth at a more shallow angle.
The greenhouse effect is the phenomenon that allows Earth to be at relatively comfortable temperatures. If there was no greenhouse effect, the temperature of the Earth would be below zero Celsius, causing water to freeze.
The average temperature of the Earth has changed throughout Earth's history, however the increasing temperature of the Earth and the associated climate change due to the increasing amount of greenhouse gases in the atmosphere is a cause for concern. Since the 1880s, the average surface temperature has risen by 0.8°C with the highest increases occurring in the past few decades. One of the biggest concerns associated with this temperature increase in melting polar ice and rising sea levels.