Since the 18th century, humans have been extracting fossil fuels from their safe stores deep underground and burning them to generate electricity or power machinery. We have now turned coal, oil and gas into more than two trillion tons of carbon dioxide and other heat-trapping greenhouse gases and added them to the atmosphere. Current result? The average temperature on the surface of the planet is about 1.2 ℃ higher than in the pre-industrial era. This is because adding new carbon to the world's natural carbon cycle has caused an imbalance in the amount of energy entering and leaving the Earth system. It takes an extraordinary amount of extra energy to heat the entire planet. Recent research shows that in the last 50 years alone, we have added the energy of 25 billion nuclear bombs to the Earth system. Billions of nuclear bombs to produce 1.2℃ of heat - so what? This seems small considering how much the temperature varies on a daily basis. (The world's average surface temperature in the 20th century was 13.9°C.) But almost all of that energy has so far been absorbed by the oceans. No wonder we are seeing rapid warming in our oceans. Goldilocks Zone: Mercury is the planet closest to the Sun. It is heated at an average temperature of 167 °C. But it has no atmosphere. This is why the second planet, Venus, is the warmest in the solar system, with an average temperature of 464 °C. This is due to an atmosphere that is much denser than Earth's, dense in carbon dioxide. Venus may once have had liquid oceans. But then there was the greenhouse effect, which trapped a really huge amount of heat. One of the reasons we live is because our planet orbits in the Goldilocks zone, at exactly the right distance from the Sun so that it is neither too hot nor too cold. Little of the Earth's internal heat penetrates the cold crust where we live. This makes us dependent on another source of heat - the Sun. (by Andrew King and Steven Sherwood, The Conversation)