Opinion
Climate change is real: The world is hotter than it’s been for two millennia
Summary
- Global warming is upon us. Long-term global temperature trends revealed by tree rings in a new scientific study confirms tree-life long heat conditions in the northern hemisphere. The Paris Agreement cap sadly lacks a global consensus.
It’s one thing to say the northern hemisphere summer of 2023 was the hottest in 150 years of mercury measurement. This claim is often dismissed by sceptics of global warming who point out that the Earth has a long history of temperature fluctuations. That’s why it’s important that a new paper shows last summer was actually the hottest in the last 2,000 years— and that our current temperatures are even more of an outlier than we realized.
[The question is] whether our recent warming is a major shift or a blip. Thankfully, tree rings hold records that go back thousands of years. In a paper published in Nature, scientists used tree rings to plot summer heat in the northern hemisphere over the past two millennia. 2023 was the hottest of them all. The next hottest 25 have all occurred since 1996. The next runner-up was way back in 246 CE.
What we know of global warming has been changed dramatically by long-term trends revealed by tree rings, ice cores, sediment layers and other such monitors. In 1998, scientists published a ‘hockey stick graph’ of the last 600 years. It showed that global temperatures rose and fell like gently rolling hills until the mid-20th century, when they suddenly soared.
That gave people a graphic image of how unusual things are today. Since then, there have been dozens of detailed reconstructions of our climate history. This latest one covers only the part of the globe that has the most trees—the mid-latitude northern hemisphere—but it goes back 2,000 years and highlights climate change today in the context of centuries of natural variability.
Even seemingly small fluctuations can have a big impact on human life. Take 536 CE, dubbed “the worst year to be alive" by historian Michael McCormick. An Icelandic volcano erupted, spewing particles into the air and veiling much of Europe and Asia in a dark fog. That caused cold, famines and a wave of plague that coincided with the collapse of the Eastern Roman Empire. That year’s summer was just 1.9° Celsius below the long-term average, shows the Nature paper, and 3.9° Celsius colder than the summer of 2023.
Also read: Temperatures of 50°C will become much more common around the Mediterranean
Volcanoes have been to blame for cooler years. But the cause of past warm spells is not as well understood.
The year 246 CE was also unusually warm. More recently, the medieval-era warm period between 800 CE and 1400 CE allowed orchards and pastures to spread into north Europe, Iceland and Greenland, and also triggered mega-droughts, famine and the collapse of civilizations in the American southwest.
Tree rings are helping scientists decipher the role of climate in that period and others throughout history. The oldest trees, bristlecone pines, can live nearly 5,000 years; scientists can extract a pencil-thin core to study the rings without harming the trees. But researchers don’t have to use such ancient trees to explore the distant past because they can also read information from rings in wood that’s been incorporated in old buildings and ships or preserved in bogs.
Trees growing in cold conditions can reveal temperature history because it is the main factor limiting their growth. The new study depended on such trees in nine different sites analyzed by 15 teams, said its lead author Jan Esper of Johannes Gutenberg University.
Esper said he was interested in better understanding what the Earth’s temperature was like in the pre-industrial area, before human-generated emissions started warming the planet. The Intergovernmental Panel on Climate Change defines pre-industrial temperature as the average measured from 1850 to 1900. The Paris Agreement makes it a goal to keep global temperatures within 1.5° C of that pre-industrial period—a level we’re about to exceed.
Also read: How to know when the world has passed 1.5°C of global warming
But measurements before 1900 were sparse, and Esper says the tree rings suggest the actual pre-industrial era was a bit cooler. From 1850 to 1900, temperatures were already about a quarter of a degree warmer than the average over the previous 2,000 years. That means our current temperatures might be more abnormally warm than we realized.
“You often hear politicians—ignorant politicians—saying climate varies and it’s been warmer in the past so don’t get too excited about all this greenhouse gas we’re putting into the atmosphere," said Ray Bradley, a climatologist of the University of Massachusetts, Amherst, who was an author of the hockey-stick paper. But the natural records suggest it hasn’t been this warm in 2,000 years, maybe longer, “so that’s a pretty exceptional situation."
Exceptional, but not hopeless. Today’s climatologists say it’s not too late to keep global warming within a manageable range. If we listen to what nature is telling us, we can keep 536 CE as the worst year to be alive. bloomberg
