The correct answer is - greenhouse gases.
The humans have been having a very big impact on the composition of the atmosphere in the past century, and that has started to result into changes in the climate patters all over the globe.
The main problem appears with the huge amounts of greenhouse gases released into the atmosphere, mostly carbon dioxide, but also methane. These two gases have properties that enable the greenhouse effect, which means that they are able to trap bigger amount of heat in the Earth's atmosphere, thus making it warmer.
If the ongoing trend continuous with the release of the greenhouse gases there will for sure be devastating effects on everyone and everything on the planet not so far in the future.