Ngama-70s nama-80s ekhulu lama-20, umkhuba wokuthuthukiswa kwezobuchwepheshe wokunciphisa isivinini emhlabeni unomkhuba omkhulu wokuthuthuka, futhi ukuthambekela kokuthuthuka kwenguquko yesayensi nezobuchwepheshe kuxhumene kakhulu.
