[Arnt]

It's not a new thing, in principle, just something that's changing.

Some materials transmit heat better than others. If you change the chemical composition of the atmosphere, you change its heat transmission characteristics.

What we're doing to the atmosphere has several effects, one of them is trapping the heat from sunlight a little more efficiently. Of course that means that the temperature outside drops. If you insulate your house better it'll grow warmer inside and the outside walls will cool down.

https://en.wikipedia.org/wiki/Greenhouse_effect BTW.