The temperature inside a greenhouse is usually higher than the temperature outside. This is because light from the Sun freely penetrates the greenhouse glass and is absorbed by the plants. The sunlight is then re emitted but at longer wavelengths which do not pass through the glass nearly so easily. The radiation is trapped and serves to heat the interior of the greenhouse.
The Earth's atmosphere is full of gases which behave in the same way as the glass of the greenhouse- water vapour, methane, and of course carbon dioxide, which is produced by the combustion of fossil fuels.
The greenhouse effect is a natural phenomenon, vital for life on Earth. Life would be very different if there were no greenhouse effect, but the effect is believed to be accelerating because of the burning of fossil fuels and the cutting down of the Earth's forests, which would otherwise absorb much of the emitted carbon dioxide. This is causing droughts, famines, heatwaves and extinctions which are a real threat to the future of life on Earth.