2 . The last time CO₂ levels were as high as today, ocean waters drowned the lands where big cities like Houston, Miami, and New York City now exist.

It’s a time called the Pliocene (上新世) or mid-Pliocene, some 3 million years ago, when sea levels were around 30 feet higher (but possibly much more) and giant camels dwelled in a forested high Arctic. The Pliocene was a significantly warmer world, likely at some 5 degrees Fahrenheit (around 3 degrees Celsius) warmer than pre-Industrial temperatures of the late 1800s. Much of the Arctic, which today is largely clad in ice, had melted. Heat-trapping carbon dioxide levels, a major temperature lever, hung around 400 parts per million, or ppm. Today, these levels are similar but relentlessly rising, at some 418 ppm.

Humanity is currently on track to warm Earth to Pliocene-like temperatures by this century’s end—unless nations ambitiously slash carbon emissions in the coming decades. Sea levels, of course, won’t instantly rise by tens of feet: Miles-thick ice sheets take many centuries to thousands of years to melt. However, critically, humanity is already setting the stage for a relatively quick return to Pliocene climes, or climates at least significantly warmer than now. It’s happening fast. When CO₂ naturally increases in the atmosphere, pockets of ancient air preserved in ice show this CO₂ rise happens gradually, over thousands of years. But today, carbon dioxide levels are skyrocketing as humans burn long-buried fossil fuels.

“CO₂ in the atmosphere has gone up 100 ppm in my lifetime,” said Kathleen Benison. a geologist at West Virginia University who researches past climates. “That’s incredibly fast geologically.”

“You don’t have to be a scientist to realize something totally unusual is going on, and that unusual thing is humans,” noted Dan Lunt, a climate scientist at the University of Bristol who has researched the Pliocene.

1. What was the world like in the time of the Pliocene?

A．Its climate was much colder than what it is today.

B．The land where cities like Houston lie was under water.

C．Much of the Artic was covered in thick snow.

D．Heat-trapping carbon dioxide levels are lower than today.

2. What does the underlined word in the third paragraph mean?

A．Accelerate.	B．Accumulate.
C．Allocate.	D．Reduce.

3. How does scientist learn the CO₂ change over thousands of years?

A．By measuring the CO2 in the air.

B．By researching the long-buried fossil fuels.

C．By researching the ancient air locked in ice.

D．By measuring the CO2 in the Pliocene.

4. What might Kathleen Benison and Dan Lunt mean?

A．Measures should be taken to stop global warming.

B．The change of CO2 in the atmosphere is geological.

C．Climate changes is normal compared to past climates.

D．Humans are to blame for the unusual rise of CO2.