percent over the past 250 years, reaching a concentration unseen on the planet in 420,000 years and perhaps as far back as 20 million years, the report said.
(percent over the past 250 years; reaching; a concentration unseen on the planet; T:in 420,000 years and perhaps as far back as 20 million years) (the report; said; )
percent over the past 250 years the report
reaching said
a concentration unseen on the planet T:in 420,000 years and perhaps as far back as 20 million years