TAMPA, Fla. (WFLA) — In today’s climate classroom we discuss climate proxies, fossils of the distant past, used to reconstruct historical climate records hundreds and thousands of years ago.

Reliable thermometer records only date back to the 1800s, yet sometimes you’ll see headlines such as one this week, showing Greenland temperatures are warmer now than they have been in over 1000 years.

So, how can we know the climate 1000s of years in the past and is the knowledge reliable? WFLA Chief Meteorologist and Climate Specialist Jeff Berardelli spoke to Dr. Kim Cobb, Paleoclimatologist and Director of the Institute at Brown University for Environment and Society.

Along the Equator, about 1200 miles south of Hawaii, on a place called Christmas Island, is where Cobb peers into the distant past.

Christmas Island in the middle Pacific Ocean

“I’m proud to say that the corals that I work with in the middle of the Pacific Ocean are as good, if not better than the temperature records from satellites,” Cobb exclaims proudly.

In order to understand the climate before modern instruments, scientists have to be crafty, uncovering clues laid down over time. Cobb’s climate proxy of choice is found in layers of marine limestone.

Split view (half-underwater) photograph featuring tropical sunset at heron Island and corals with marine life

Cobb explains that corals grow very quickly, about 1 centimeter per year – about a finger width – and they can give an extremely detailed record of ocean temperatures, “Corals build a calcium carbonate skeleton and the oxygen in that calcium carbonate skeleton is laid down as a function of temperature.”

By analyzing the makeup of oxygen in layers of coral, Cobb can reconstruct a week-to-week record of ocean temperatures hundreds of years back.

“How can we be so sure that these records that we are getting from proxy data are accurate?,” asks Berardelli.

Cobb answers, “We don’t have to make a leap of faith to decide whether to trust a coral or not, we actually use the most recent portion of coral skeleton that has been laid down over the past several decades – we compare the record in the coral skeleton chemistry against the temperature record from satellites.”

Cross section of coral from Kim Cobb

This comparison allows Cobb to develop a calibration, using the specific chemistry of the coral to match the corresponding climate at the time in question.

It similar process for tree rings, where the width of each ring reveals how dry or wet a given year was. That’s how we know last year’s drought in the Western U.S. was the worst in 1200 years.

Old wooden cut surface with cracks and annual rings. Rough and detailed texture of a felled tree trunk or stump.

And in the case of ice cores, scientists can peer back over a million years by drilling into glaciers and analyzing ancient bubbles of air trapped in ice to see how atmospheric composition has changed.

That’s how we know carbon dioxide is 40% higher today than at any time in the last 800,000 years.

“It’s not just one proxy that gets a vote into how the climate has changed, it’s hundreds if not 1000s of proxy records that are brought together to ask what their shared signals are,” explains Cobb.

And those shared signals reveal the toll – human-caused climate pollution – is taking on our planet.

“The proxies are pretty clear in helping us to understand that we are under continued warming today, this warming has accelerated in recent decades, and we know that it is going to get much warmer going forward,” Cobb warns.

It’s proxy data that enabled scientists to reconstruct this temperature record over the past 2000 years, showing abrupt warming only in the last 100 years.

So next time you see a climate record that dates back before 1850, you’ll know the magic behind it.