Complex sentence: In the very recent geologic history, the atmospheric carbon dioxide concentration has climbed to over 390 parts per million and continues to increase, causing the phenomenon of global warming which is mostly attributed to human carbon dioxide emissions.
Split into simpler sentences: In the very recent geologic history, the atmospheric carbon dioxide concentration has climbed to over 390 parts per million. This increase has caused the phenomenon of global warming which is mostly attributed to human carbon dioxide emissions.