Why is the greenhouse effect harmful to humans?
It causes global warming.
Except the fact that all these gases can be toxic at high concentration, the main problem they rise is global warming.
The greenhouse effect (originated by greenhouse gases) is an essential process to life on earth.
As you can see it on the picture, the role of greenhouse gases is to trap a part of the solar energy and to re-emit it toward earth. This process allow the Earth to keep a warmer temperature than the one it would have without atmosphere. It is essential to life.
The problem is when the concentration of greenhouse gases is altered. More greenhouse gases in the atmosphere means more solar energy traped and converted into heat. This consequently leads to an increase of the global earth temperature. That is what we call 'global warming'.
The consequences will differ depending on the regions on earth, but among the most problematic for humans population you can find - melting icecaps, sheets, and glaciers
- increase of sea levels causing floods and submersion of some islands and lands at the sea level
- biodiversity changes
- severe heat waves
The Greenhouse Effect itself is not inherently harmful to humans but the levels of greenhouse gases in the atmosphere can trap more heat, causing global warming.
The greenhouse effect itself is not inherently harmful to humans. The greenhouse effect releases some energy from the planet but it also retains energy, warming the planet. Refer to this related question on how greenhouse gases are good and bad.
It is the rapidly rising levels of greenhouse gases in the atmosphere that trap more heat, causing global warming. Refer to this related question on how the greenhouse effect is connected to global warming.