A year ago, I wrote about future climate change scenarios, stating that their historical outcome is now clear, scientifically proven, and unanimously accepted.

Now, all of this is true, and I do not deny it, but let us take a step back and ask ourselves: how did we demonstrate these certainties?

Because, ultimately, it is true that “the climate has always changed”; however, not all changes are the same. The elephant we cannot pretend not to see in the room (yes, the rhetoric is necessary here) is not so much whether the climate has changed in the past, but how much, how quickly, and through which mechanisms.

To answer these questions, some study animals, others study rocks, others glaciers… well, I study plants, and plants (or rather, plant wood) are what I want to talk to you about today.

And the branch of science that studies the relationships between plants and climate has its own name: dendrochronology, that is, the study of tree rings, in simple terms.

Tree rings, in fact, allow researchers not only to simply count the years of a tree (or shrub, to be precise), but also to reconstruct past climate, much like glaciologists do with ice cores [1].

In this respect, however, tree rings have subtle but clear advantages: (1) rings do not deteriorate over time; (2) their analysis requires simpler and less expensive technologies compared to ice cores; and (3) trees are found almost everywhere in the world (yes, even in Greenland, in that case alongside ice cores, of course), which facilitates their availability and increases the number of samples [2].

Almost everywhere, because at certain latitudes trees are indeed present but lack a fundamental component for dendrochronological analysis: tree rings.

Radial wood growth (i.e., tree rings) develops through the intra-annual alternation of seasons: in simple terms, winter produces narrow, dark rings, while summer produces wide, light ones. This, however, poses a fundamental issue: where seasons do not exist—as in the tropics—tree rings cannot exist either.

Recent studies have attempted to address this lack of information by identifying other factors capable of triggering ring formation in tropical environments, such as altitude or microclimatic conditions that can compensate for the absence of seasonal alternation [3] [4].

The technique, however, remains the same: by combining analyses of wood from living trees, wood from dead trees (for example, used in ancient constructions), and wooden remains found intact under particular environmental conditions (such as in peat bogs), it is possible to reconstruct past climate even centuries back, with—of course—annual precision!

Distinct variations and “signatures” in tree rings, indicative of years with climates markedly different from the average, can be traced back through past centuries. By matching these signatures—like a kind of forest barcode—across different wood samples, a timeline emerges that reveals what has happened to the climate over the centuries, as recorded in the tree itself.

If we look at the last five centuries, for example, we can clearly see the succession of colder and warmer phases, driven by shared drivers. The period known as the Little Ice Age (between the 15th and 19th centuries) is the most classic example: not a uniform block of cold, but a sequence of oscillations, with harsher phases alternating with relatively milder periods.

Extreme events, such as major volcanic eruptions (Tambora in 1815 is the textbook case), leave very clear signatures in tree rings: years of reduced growth, widespread stress, and large-scale synchronized anomalies. The point is that these disturbances are, and remain, temporary—transient and limited to the period in which they occur—before giving way once again to the normalcy of the climatic statistical average.

In other words: the climate of the past did change, even markedly, but it did so by oscillating and more or less always returning to a common equilibrium.

And this applies not only to temperatures, but also to variables such as water availability. Dendroclimatic chronologies show alternations between dry periods and wetter ones, even prolonged, but they rarely reveal persistent directional trends on centennial scales.

Figure 1: Photo by Michael Bußmann on Pixabay