Hot history: Tree rings show that last northern summer was the warmest since year 1 (2024)

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The broiling summer of 2023 was the hottest in the Northern Hemisphere in more than 2,000 years, a new study found.

When the temperatures spiked last year, numerous weather agencies said it was the hottest month, summer and year on record. But those records only go back to 1850 at best because it’s based on thermometers. Now scientists can go back to the modern western calendar’s year 1, when the Bible says Jesus of Nazareth walked the Earth, but have found no hotter northern summer than last year’s.

A study Tuesday in the journal Nature uses a well-established method and record of more than 10,000 tree rings to calculate summertime temperatures for each year since the year 1. No year came even close to last summer’s high heat, said lead author Jan Esper, a climate geographer at the Gutenberg Research College in Germany.

Before humans started pumping heat-trapping gases into the atmosphere by burning coal, oil and natural gas, the hottest year was the year 246, Esper said. That was the beginning of the medieval period of history, when Roman Emperor Philip the Arab fought Germans along the Danube River.

Esper’s paper showed that in the Northern Hemisphere, the summer of 2023 was as much as 2.1 degrees Fahrenheit (1.2 degrees Celsius) warmer than the summer of 246. In fact 25 of the last 28 years have been hotter than that early medieval summer, said study co-author Max Torbenson.

“That gives us a good idea of how extreme 2023 is,” Esper told The Associated Press.

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The team used thousands of trees in 15 different sites in the Northern Hemisphere, north of the tropics, where there was enough data to get a good figure going back to year 1, Esper said. There was not quite enough tree data in the Southern Hemisphere to publish, but the sparse data showed something similar, he said.

Scientists look at the rings of annual tree growth and “we can match them almost like a puzzle back in time so we can assign annual dates to every ring,” Torbenson said.

Why stop the look back at year 1, when other temperature reconstructions go back more than 20,000 years, asked University of Pennsylvannia climate scientist Michael Mann, who wasn’t part of the study but more than a quarter century ago published the famous hockey stick graph showing rising temperatures since the Industrial Age. He said just relying on tree rings is “considerably less reliable” than looking at all sorts of proxy data, including ice cores, corals and more.

Esper said his new study only uses tree data because it is precise enough to give summer-by-summer temperature estimates, which can’t be done with corals, ice cores and other proxies. Tree rings are higher resolution, he said.

“The global temperature records set last summer were so gobsmacking — shattering the prior record by 0.5C in September and 0.4C in October — that it’s not surprising they would be clearly be the warmest in the past 2,000 years,” said Berkeley Earth climate scientist Zeke Hausfather, who wasn’t part of the study. “It’s likely the warmest summer in 120,000 years, though we cannot be absolutely sure,” he said, because data precise to a year doesn’t go back that far.

Because high-resolution annual data doesn’t go back that far, Esper said it’s wrong for scientists and the media to call it the hottest in 120,000 years. Two thousand years is enough, he said.

Esper also said the pre-industrial period of 1850 to 1900 that scientists — especially the Intergovernmental Panel on Climate Change — use for the base period before warming may be a bit cooler than the instrumental records show. The instruments back then were more often in the hot sun instead of shielded like they are now, and tree rings continue to show that it was about 0.4 degrees (0.2 degrees Celsius) cooler than thermometers show.

That means there’s been a bit more warming from human-caused climate change than most scientists calculate, an issue being hashed out by researchers over the last few years.

Looking at the temperature records, especially the last 150 years, Esper noticed that while they are generally increasing, they tend to do so with slow rises and then giant steps, like what happened last year. He said those steps are often associated with a natural El Nino, a warming of the central Pacific that changes weather worldwide and adds even more heat to a changing climate.

“I don’t know when the next step will be taken, but I will not be surprised by another huge step in the next 10 to 15 years, that’s for sure,” Esper said in a news briefing. “And it’s very worrying.”


This story has been corrected to refer to Jesus of Nazareth, rather than Jesus Christ.


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Hot history: Tree rings show that last northern summer was the warmest since year 1 (2024)


Hot history: Tree rings show that last northern summer was the warmest since year 1? ›

Hot history: Tree rings show that last northern summer was the warmest since year 1. The broiling summer of 2023 was the hottest in the Northern Hemisphere in more than 2,000 years, a new study found.

What is the tree ring climate record? ›

Each ring represents one year. Rings generally grow wider in warm, wet years and thinner in cold, dry years. Tree ring records can go back hundreds to thousands of years, depending on when the tree lived and how old it was. Scientists examine tree rings to learn about past climate conditions.

What is the history of tree rings? ›

What do tree rings tell us. The underlying patterns of wide or narrow rings record the year-to-year fluctuations in the growth of trees. The patterns, therefore, often contain a weather history at the location the tree grew, in addition to its age.

Can tree rings predict climate change? ›

By understanding the past climate using tree rings and other paleoclimate proxy data sources, scientists can more accurately predict future changes in the climate system.

What do tree rings tell you? ›

The Short Answer: The characteristics of the rings inside a tree can tell scientists how old a tree is and what the weather conditions were like during each year of that tree's life. Very old trees can offer clues about what the climate in an area was like long before measurements were recorded.

What is the most tree rings ever found? ›

Counting the rings later revealed that Prometheus contained 4,862 growth rings. Due to the harsh conditions these trees grow in, it is likely that a growth ring did not form every year. Therefore, Prometheus was estimated to be 4,900 years old, the oldest known tree of its time.

How accurate are tree rings? ›

Trees contain some of nature's most accurate evidence of the past. Their growth layers, appearing as rings in the cross section of the tree trunk, record evidence of disastrous floods, insect attacks, lightning strikes, and even earthquakes that occurred during the lifespan of the tree.

What is the oldest tree in the world? ›

In eastern California, a Great Basin bristlecone pine (Pinus longaeva) known as Methuselah has long been considered Earth's oldest living thing. According to tree-ring data, Methuselah is 4,853 years old — meaning it was well established by the time ancient Egyptians built the pyramids at Giza.

Do Milankovitch cycles affect climate? ›

The small changes set in motion by Milankovitch cycles operate separately and together to influence Earth's climate over very long timespans, leading to larger changes in our climate over tens of thousands to hundreds of thousands of years.

Do tree rings show age? ›

Tree ring counting alone is not a perfect aging method. A tree might appear to produce more than one ring in a year that starts dry and becomes very wet. In very dry years, a tree can also skip growth rings altogether. So tallying the rings of just one tree can yield a misleading age.

What do trees mean in one dark window? ›

Trees are a recurrent symbol in One Dark Window, representing Blunder's magical origins. They surround the kingdom, serving as the god-like Spirit of the Wood's misty domain. In an homage to the Spirit, Blunder's citizens choose a tree to serve as both family name and house insignia—and with them, social power.

What do dark rings in a tree mean? ›

Because the layers of wood a tree forms in the spring grow fast and consist of large cells, the rings are lighter. The slower summer growth has denser cells so those rings are darker.

How far back does tree ring data go? ›

Each ring marks a complete cycle of seasons, or one year, in the tree's life. As of 2020, securely dated tree-ring data for some regions in the Northern Hemisphere are available going back 13,910 years.

What is the tree line climate? ›

The tree line follows the line where the seasonal mean temperature is approximately 6 °C or 43 °F. The seasonal mean temperature is taken over all days whose mean temperature is above 0.9 °C (33.6 °F). A growing season of 94 days above that temperature is required for tree growth.

What climate information can be found in tree rings quizlet? ›

Describe what climate information can be found in tree rings. The width of a tree ring tells how much the tree grew in one year. Knowing the amount of growth can help in determining the amount of precipitation for that year.

How do you count tree rings? ›

Count the dark rings to calculate the age of the tree. Start in the middle of the stump or cross-section of wood and count the first dark ring you see. Continue counting outwards from the middle ring until you reach the last dark ring. The total number of dark rings represents the age of the tree in years.

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