If the greenhouse effect is natural, then why is today's climate changing a bad thing?

A. A small increase in greenhouse gas concentration can have a large effect of increased warming

B. Abrupt changes to the climate system pose challenges for societies, weather and spread of the diseases

C. Humans have altered a natural process and exaggerated changes that might normally occur over millions of years

D. Once releases into the atmosphere, greenhouse gases remain potent for many years, making it difficult to reverse the process.​