Tree rings prove 2023 was the hottest summer for 2000 years

We know the last few summers have been hot. But how hot? Ulf Büntgen’s team at the University of Cambridge have developed a way to extract climate data from the growth rings of trees, and even ancient fossilised wood buried in the Fens, that are allowing them to look back over thousands of years, and - critically - well before thermometers were invented...

Ulf - The main message of this paper is that the last summer, of the year 2023, is the warmest summer of the last 2000 years. We know that not from instrumental measurements, we are using tree ring data. So a tree is recording for us the temperature over the past 2000 years. You may now ask how do you get 2000 year old trees?

Chris - How do you get 2000 year old trees? No, I was just going to ask exactly that. So where did you get the trees from and how do the trees record temperature? I know you're saying you're looking at the rings, but what about the rings records the temperature?

Ulf - There are basically no trees, almost no trees, that are 2000 years old. So we take living ones from maybe the past 3, 400 years and then we extend the information of these living trees with relic wood, with that material. So we, for instance, would go into a college here in Cambridge, in the roofs, and take samples and collect material from historical construction timber. Or, we work together with archaeologists that have excavations where we get wood remains out, or for instance, just last week I was in the fenlands, North of Peterborough, where we collected subfossils, relic wood that comes out of the peat box. All these different wooden sources help us to build a continuous time series, we call that a chronology, that covers 2000 years for different parts of the world. And how do we do that? Or, in fact, how are the trees doing that for us? These trees are functioning like a thermometer. Small changes in summer temperature are reflected in wide or narrower rings. So the tree is suffering if it's getting too cold, and the tree is thriving and producing lots of wood under favourable conditions. Then we have an overlap where we have both the instrumental measurements. Whenever we have an overlap of what the tree rings are recording and what we measured with instruments, we can build a model. We are calibrating, we are trying to understand what does it mean if a ring is wide or narrow and how can we transfer that into temperature.

Chris - Could anything else though affect those tree rings? Is it just temperature and is that discreet. Or could, say, a very wet year make a difference, a very dry year make a difference and therefore other things could tweak the formula?

Ulf - You will be surprised about the answer. Everything affects the tree. So when a tree grows, it's completely affected by a lot of things in its environment and that is all recorded. But we are not relying on a single tree. We average the information for hundreds or even thousands of trees and then we cancel out the noise. Then what we emphasise is a common signal. If we see that a tree shows a narrow ring, let's say, in the European Alps, and at the same time in the same year in the same summer, also in Northern Scandinavia and so on, over a larger geographical region, then we know it can't be a local factor. It must be a larger factor and that is then temperature.

Chris - And how much bigger are the effects now? So the tree rings are saying it's warmer now than 2000 years ago. How much warmer?

Ulf - So trees are only recording something that happens during a growing period, right? That is the summer, let's say, from spring to autumn. So the northern hemisphere summer temperature of 2023 was roughly two degrees warmer than the pre-industrial period. Before our study, people just had information from 1850 to 1900, which we called the period of early instrumental measurements. That is where, in some places on our planet, certainly not everywhere, we had the start of early instrumental measurements. These measurements were also biased. They were not absolutely accurate. They still had to learn how to build up meteorological stations to fine tune it. Based on this period, before industry starts significantly in the 20th century, we would say we have this two degree warming trend. Now with the tree rings, we are able to provide and set the most recent anthropogenic warming in a much longer context. That is the main achievement of this study. It makes a huge difference if you say the last summer was the warmest summer of the past 150 years, or if you say the last summer was the warmest summer of the past 2000 years.