Why is the greenhouse bad for the earth

Greenhouse gases are not bad for the earth, they are what keeps the earth warm through the process known as the greenhouse effect.

The greenhouse effect is basically, it allows visible light to pass through a barrier (glass or gases) and reflects a portion of any heat (infrared light) that tries to leave the barrier.

Now, the reason for the "bad" has to do with the amount of greenhouse gases. From a simplistic view, having more greenhouse gases increases the barrier and thus keeps in more heat. More heat will result in everything getting warmer or having a slightly higher temperature. Granted heat storage has a role and certain molecules are capable of retaining more heat than other, but for the sake of simplicity, more greenhouse gases, more heat trapped, warmer planet.

now, a warmer planet means absolutely nothing in terms of the earth. However, it is a huge potential issue in terms of weather patterns, ocean current, sea level, which will in turn, affect crop (food), coastal/island countries (think refugees/displaced people), etc.

Long story short, Earth will be totally fine, the human race/society will start having some problems in the future.