How do scientists know what the climate used to be like?

“So how can anyone possibly know that?” Mia asked. “Maybe they are just guessing.”

That sounded like a question for José, a researcher who spends a pretty amount of time digging through very old weather measurements.

“That is exactly the right question,” José said. “We ask ourselves the same thing all the time. And the answer depends on how far back you want to go.”

“Let’s start with the easy part. People have been measuring the weather for a long time. Not only scientists, but also farmers, teachers and lighthouse keepers. Every morning they read their thermometer and wrote the number in a notebook. Sailors did it too: on every ship, someone wrote down the wind, the storms and the ice in the logbook. Many of those notebooks still exist, and right now scientists are typing the numbers in so that they are not lost.”

“So you can just read the weather of the past?” Mika asked.

“For some places, yes. But there are gaps. Nobody was standing in the middle of the Pacific Ocean with a thermometer every single morning, even today. So, we let a computer fill in the gaps between the measurements we do have. It is like a jigsaw puzzle with missing pieces: you cannot see the whole picture, but you know how it should continue between the two pieces you are holding.”

“And before thermometers?” Mia insisted. “There is nothing then.”

“There are no numbers,” José agreed. “But people were still writing things down. In Japan, for more than a thousand years, they have recorded the day the cherry trees bloomed: early in warm springs, late in cold ones. In the Netherlands, they wrote down every winter the canals froze hard enough to skate on. Farmers noted when the harvest came in, and boatmen complained in writing when the river was too low.”

“They were not doing science,” Mia said slowly. “They were just living their lives.”

“Exactly. And they left us a diary anyway.”

“And the ice age?” said Mika. “Nobody was writing anything then.”

José smiled. “No. But nature was.”

Trees, he explained, grow a new ring every year: a fat ring in a good year, a thin one in a cold or dry year. A tree is a diary with one page per year, and some trees have thousands of pages. And in Antarctica, snow falls year after year and never melts. The old snow is squeezed into ice, layer under layer, and inside that ice there are tiny bubbles with real air, from the atmosphere of that year, locked in and waiting. Scientists drill out a long stick of ice, open the bubbles, and breathe a little piece of an ancient atmosphere. The deepest ice we have is more than 800,000 years old.

“But how do you know it is really true?” Mia asked.

“That is the best question of all. For the last hundred and fifty years we have both things at once: the tree rings and the thermometers. So we check. Does the tree say ‘cold’ in exactly the years the thermometer says cold? When they agree, year after year, we can trust the tree for the years before any thermometer existed. And remember: trees can live hundreds or thousands of years. And we never rely on one diary alone. We compare the trees with the ice, and the ice with the old notebooks. When very different diaries tell the same story, it is not a guess anymore.”

That night, Mia opened her new diary and wrote:

Note to self: the Earth keeps a diary too. It just writes with wood and ice instead of a pencil.

Do you keep a diary? In fifty years, someone might read it to find out what your winters were like.