have increased by 31 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.
(have increased by 31 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; said; the report)
have increased by 31 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
said
the report