the hottest summer since the heyday of the Roman Empire

The summer of 2023 was the hottest summer the Northern Hemisphere has experienced in the past 2,000 years. This is evident from new research.

It was previously announced that 2023 was the warmest summer ever recorded. And since our measurements go back – at best – to 1850, we already knew that 2023 was the warmest summer in 173 years. But in a new study – published in the magazine Nature – scientists are going one step further. Based on tree rings, they conclude that the summer of 2023 – at least in the Northern Hemisphere – was warmer than all summers in the 2000 years before. In other words, the summer of 2023 was the hottest summer since the heyday of the Roman Empire!

“If you look far back in history, you see how dramatic recent global warming is,” says researcher Ulf Büntgen. “2023 was an exceptionally warm year and this trend will continue until we drastically reduce greenhouse gas emissions.”

The numbers
For example, the new research shows that the average temperature measured in the Northern Hemisphere during the summer of 2023 is about 2.07 degrees Celsius above the average temperature measured in the summers from 1850 to 1900. The summer of 2024 also appears to have been no less than 3.93 degrees warmer than the coldest summer in the past 2000 years.

Northern Hemisphere
The researchers focus their study on the Northern Hemisphere. There is a good reason for this; Data on the temperatures that the Southern Hemisphere has experienced over the past two millennia are much scarcer. The Southern Hemisphere also responds differently to climate change, the researchers emphasize. This has everything to do with the fact that a much larger part of the Northern Hemisphere is covered with water.

Annual rings
As mentioned, the researchers base their conclusions on annual rings of trees. It is well known that by counting these annual rings you can get a fairly accurate picture of the age of a tree. But the annual rings can tell us much more, for example about the climate. For example, the annual rings of a tree are narrower in cold periods and wider in warmer periods. And so we can use annual rings to get an idea of ​​what the climate was like years ago. And that’s what the researchers have now done.

Baseline corrected
They first looked at what a large tree ring dataset could tell us about the period 1850-1900. This is an important period because it is often used as a pre-industrial ‘baseline’: when researchers want to express how much the Earth has warmed, they compare the global average temperature with the average temperature between 1850 and 1900. But that baseline is a bit wobbly , according to Büntgen and colleagues. Because temperature measurements from the period between 1850 and 1900 are scarce and inconsistent. Enough reason for scientists to compare those few and inconsistent measurements with what tree rings tell us about that period. And guess what? According to the annual rings, the temperature in the nineteenth century was several tenths of a degree Celsius lower than expected. And based on the new baseline presented by annual rings, the summer of 2023 appears to have been 2.07 degrees Celsius warmer than the average summer between 1850 and 1900, the researchers write.

Climate change is timeless
What the annual rings also tell us is that climate change is timeless. Over the past 2000 years, there have been several periods in which temperatures were temporarily lower or higher. For example, the Northern Hemisphere cooled temporarily in the sixth century and early nineteenth century due to volcanic eruptions. The eruptions released large amounts of aerosols into the stratosphere that blocked sunlight and cooled the surface. It even led to the coldest summer of the past 2000 years in 536. In that summer, the average temperature was no less than 3.94 degrees Celsius lower than in 2023. But there have also been remarkably warm periods in the past 2000 years. Most of them are due to El Niño; a periodic warming of ocean waters along the equator in the eastern Pacific Ocean that affects weather worldwide and boosts global average temperatures. Also 2023 was an El Niño year.

Climate change and El Niño
But against the background of these natural changes in climate – prompted by, for example, El Niño or volcanic eruptions – anthropogenic warming is now also taking place. And there are indications that this El Niño is also strengthening, says researcher Jan Esper. “It is true that the climate is always changing. But the 2023 warming caused by greenhouse gas emissions was further amplified by El Niño, causing us to experience longer and more intense heat waves and longer droughts. When you look at the big picture, it shows that we need to reduce our greenhouse gas emissions immediately.”

The summer of 2024 is now just around the corner. What this will bring remains to be seen. But researchers suspect that it could be another record-breaking warm summer. This again has to do with the mix of climate change and El Niño; Usually, El Niño is in its second year – which in this case is 2024 – has the greatest impact on the global climate.