I’m ready for summer, but if this year is anything like last year, it will be a challenge. In fact, the summer of 2023 in the Northern Hemisphere was the hottest in more than two thousand years, according to a new study released by Nature in early May.
If you’ve been following the headlines, you probably already know that last year was a hot one. But I was surprised by the title of this study when I first saw it. “The hottest in two thousand years”, how do we know that?
There were no thermometers in Year 1, so scientists need to get creative when comparing our current climate to that of centuries, or even millennia, ago. Here’s how our world compares to the climate of the past, how we know it, and why it matters for our future.
Today, there are thousands and thousands of weather stations around the world, monitoring temperatures from Death Valley to Mount Everest. So there’s plenty of data to show that 2023 was, in a word, scorching hot.
Daily global ocean temperatures have been the highest on record for more than a year in a row. Sea ice levels have reached new lows. And, of course, the year recorded the highest average global temperatures since records began in 1850.
But scientists decided to look even further into the past for a year that could compare to our current temperatures.
To do this, they turned to trees, which can function as low-tech weather stations.
Concentric rings within a tree are evidence of the plant’s annual growth cycles. Lighter colors correspond to rapid growth during spring and summer, while darker rings correspond to fall and winter. Count the pairs of light and dark rings and you can tell how many years a tree has lived.
Trees tend to grow faster during hot, humid years, and slower during colder years. So scientists can not only count the rings, but measure their thickness and use that as an indicator of how hot a given year was. They also look at factors like density and track different chemical signatures found within the wood. You don’t have to cut down a tree to get help with climate studies—you can just drill a small cylinder from the center of the tree, called the core, and study the patterns.
The oldest living trees allow us to peek a few centuries into the past. Furthermore, it is a matter of crossing the patterns of dead trees with living ones, extending the record in time, like putting together a puzzle.
It took several decades of work and hundreds of scientists to develop the records that researchers used for this new study, Max Torbenson, one of the study’s authors, said at a press conference. There are more than 10,000 trees from nine regions across the Northern Hemisphere represented, allowing researchers to draw conclusions about individual years over the past two millennia. The year 246 AD once held the record for the hottest summer in the Northern Hemisphere in the last two thousand years. But 25 of the last 28 years have surpassed that record, Torbenson says, and the summer of 2023 surpasses them all.
These conclusions are limited to the Northern Hemisphere, as there are only a few records of tree rings in the Southern Hemisphere, says Jan Esper, lead author of the new study. And using tree rings doesn’t work very well for the tropics, because the seasons are different here, he adds.
Because there is no winter, there is generally no such reliable alternating pattern in tropical tree rings, although some trees have annual rings that track wet and dry periods of the year.
Paleoclimatologists, who study ancient climates, can use other methods to get a general idea of what the climate was like — tens of thousands to millions of years ago.
The biggest difference between the new study using tree rings and methods looking further into the past is accuracy. Scientists can, with reasonable certainty, use tree rings to draw conclusions about individual years in the Northern Hemisphere (AD 536 was the coldest, for example, probably due to volcanic activity). Any information from before the last two thousand years will be more of a general trend than a specific data point representing a single year. But these records can still be very useful.
The oldest glaciers on the planet are at least a million years old, and scientists can drill through the ice to look for samples. By examining the proportion of gases such as oxygen, carbon dioxide and nitrogen within these ice cores, researchers can determine the temperature at the time corresponding to the glacier layers. The oldest continuous ice core record, which was collected in Antarctica, goes back about 800,000 years.
Researchers can use fossils to look even further back into Earth’s temperature record. For a 2020 study, researchers drilled into the seafloor and looked at the sediment and preserved shells of ancient organisms.
From the chemical signatures in these samples, they discovered that the temperatures we are about to record could be hotter than anything the planet has experienced on a global scale in tens of millions of years.
It’s a little scary knowing that we are changing the planet in such a dramatic way.
The good news is that we know what we need to do to reverse this situation: reduce emissions of planet-warming gases, such as carbon dioxide and methane. The longer we wait, the more expensive and difficult it will be to stop warming and reverse it, as Esper said at the press conference: “we must do as much as possible, as quickly as possible.”
( fonte: MIT Techonology Review )
