HMN 2026: How Ancient Spanish trees reveal Mediterranean storms are intensifying

Ancient pine trees growing in the Iberian mountains of eastern Spain have quietly recorded more than five centuries of Mediterranean weather. Now, by reading the annual growth rings preserved in their wood, scientists have uncovered a striking message: today’s storms and droughts are becoming more intense and more frequent than almost anything the region has experienced since the early 1500s.

New research, published in Climate of the Past, reconstructs 520 years of rainfall variability in the western Mediterranean using tree-ring data from long-lived Spanish pines (Pinus sylvestris and Pinus nigra). The findings show that recent decades stand out sharply in the historical record, marked by an escalation of both extreme rainfall events and prolonged dry spells as the climate warms.

Trees as living climate archives

Each year, trees add a new growth ring, forming a natural archive of environmental conditions. In wet years, trees typically produce wider rings as water is readily available for growth, while dry years leave behind narrower rings as growth slows.

By measuring these rings and comparing patterns across many trees, scientists can piece together past climate conditions long before weather instruments existed, a practice known as dendroclimatology.

For this study, researchers focused on pine trees from high-elevation sites in eastern Spain, where growth is especially sensitive to changes in rainfall. Some of the sampled trees are several centuries old, allowing the team to build a continuous record stretching back to 1503.

The result is one of the longest and most detailed reconstructions of precipitation for the western Mediterranean, offering a rare window into how rainfall has varied over generations.

Because instrumental rainfall records typically span little more than a century, tree rings provide a crucial way to place recent extremes in a much longer historical context, extending climate histories back hundreds of years.

To strengthen this perspective, the researchers also compared their tree-ring results with historical documents describing religious “rogation” ceremonies, public appeals for rain during droughts or for relief from excessive rainfall and flooding, which were common in pre-Industrial Mediterranean societies.

Echoes of past extremes

The record reveals that Mediterranean rainfall has never been stable. Over the past five centuries, the region has shifted repeatedly between wetter and drier periods, sometimes lasting decades. These fluctuations shaped landscapes, water availability, and farming long before modern climate change became a factor.

Historical accounts show that severe climate extremes have often coincided with periods of agricultural stress and social disruption, underscoring how closely water availability has been tied to human stability in the region.

What distinguishes the recent period is not simply that extremes exist, but how often and how strongly they now occur. Compared with most earlier centuries, the late 20th and early 21st century show a clustering of unusually intense events.

Severe droughts and heavy rainfall episodes appear more frequently and reach levels that were rare, or absent, in much of the historical record. Rather than moving steadily toward wetter or drier conditions, the Mediterranean climate appears to be becoming more volatile, with sharper swings between opposing extremes.

Storm signals

One of the clearest signals in the tree-ring data is the intensification of extreme rainfall. Heavy downpours, often associated with slow-moving storms that draw moisture from the Mediterranean Sea, have become more pronounced in recent decades. When these systems stall over land, they can deliver large amounts of rain in a short time, increasing the risk of flooding and landslides.

At the same time, dry periods are becoming more severe. Rising temperatures increase evaporation from soils and vegetation, worsening drought conditions even in years when total rainfall does not fall dramatically. This combination—heavier bursts of rain separated by longer, hotter dry spells—places a growing strain on ecosystems and water resources.

Importantly, the study shows that this pattern is highly unusual in the context of the last 520 years. The scale and persistence of recent extremes align closely with modern observations and climate model projections, pointing to a strong influence from human-driven warming.

Why extremes matter

Extreme weather events tend to have far greater consequences than gradual changes in average conditions. Sudden downpours can overwhelm drainage systems and damage infrastructure, while extended droughts reduce crop yields, deplete reservoirs, and stress forests already coping with heat.

The Mediterranean is especially exposed to these risks because it sits at the boundary between temperate and arid climate zones. Small changes in atmospheric circulation or sea temperature can trigger large shifts in rainfall patterns. As the climate warms, this sensitivity appears to be increasing, amplifying the impacts of both wet and dry extremes.

Lessons for the future

The authors argue that the growing instability revealed by the tree-ring record has serious implications for water management, agriculture, and disaster preparedness across the Mediterranean. Systems designed around historical patterns may struggle to cope as extremes intensify.

Ancient Spanish pines cannot forecast the future, but they do provide a powerful warning. Their rings show that the Mediterranean climate is now moving beyond the bounds that shaped societies for centuries and that adapting to this new reality will require preparing for a world of stronger storms, deeper droughts, and greater uncertainty.