Answer: Climate Change is a massive product of 21st century humanity, as we as a species pollute more then ever before. There are many ways in which we pollute, however that is not important as your question requests why it is an important issue. The byproducts of internal combustion (Vehicular burning), and factory pollution not only pollutes the atmosphere with extra co2, but also with unnecessary heat. Our planet Earth is based off a delicate balance between man, plant life, animals, and the atmosphere. Ever since the 1750's we have begun polluting more and more. This causes many problems, the co2 makes less oxygen in the atmosphere meaning while plantlife will flourish, Earth's wildlife and mankind will go on the road to extinction. Another byproduct is heat, which can melt the polar ice caps. This will cause heavy damage to manmade infrastructure, and inevitably wipe out 1/5 of earths population (If not evacuated). Finally, Agriculture, while intense co2 might allow agriculture to flourish, the heat will not allow it to cool down to the specific temperature ratio for the crop to successfully provide a bountiful harvest. This is why we have things like "Strawberry Season" which grows a specific type of crop at a specific point in the year.
Hope the above was useful!