Tree-Ring Talk

The climate state of the tropical Pacific strongly influences regional rainfall through changes in the
Intertropical Convergence Zone (ITCZ), essentially a band of heavy rainfall just north of the equator. While
we understand seasonal and year-to-year rainfall changes well, longer patterns of wet and dry periods
(spanning decades) remain mysterious, mainly because reliable instrumental measurements only go
back a few decades. To address this information gap, I use natural marine archives such as corals that
lived for centuries. Corals grow in layers, much like tree rings. As they grow, they record chemical clues
about the surrounding seawater. Specifically, the oxygen isotope chemistry (8®O) in coral skeletons tells
us about seawater salinity, which is directly linked to ITCZ rainfall in the tropical Pacific. By studying
seven overlapping coral records from the Marshall Islands, we pieced together nearly 260 years of rainfall
history (1765-2024). About two-thirds of the oxygen isotopic signal in Marshall Islands corals reflects
changes in seawater salinity driven by rainfall shifts connected to the ITCZ. The record reveals distinct dry
and wet periods on timescales from seasonal to multi-decade. Interestingly, until the mid-1900s, decadal-
to-multidecadal cycles dominated, after which year-to-year climate variability from the El Niño Southern
Oscillation (ENSO) increasingly controlled rainfall variability. During an El Niño, the warm phase of ENSO,
the ITCZ shifts southward, leaving the Marshall Islands in severe drought. If El Niño intensifies or
becomes more frequent as climate change continues, the tropical Pacific rain belt may shift southward,
potentially leaving islands north of the equator uninhabitable in the future.