A new approach to Holocene climate in Alaska

The Holocene represents the most recent warm period on Earth. It is currently being critically examined with high-resolution, multi-proxy records across the globe as it contains important implications for future climate scenarios. However, Alaska (USA) represents an important missing link in this global Holocene reconstruction. There are currently no high- resolution, multi-proxy records that span the entire Holocene in Alaska. Because of this, it is largely unclear how this region will respond to future warming. Here, we propose incorporating speleothems (which are abundant in Southeast Alaska) to reconstruct a high-resolution, multi- proxy record to better constrain future climate scenarios in this sensitive region.

Although sourced in the Tropical Pacific, El Nino southern Oscillation (ENSO) is one of the most important climate phenomena globally, akin to Earth's climate engine. It's impacts have been connected to almost every aspect of human life; ranging from disease outbreaks, agricultural yields, salmon productivity, natural disasters, availability of water resources, and energy demand. However, it is still unknown what drives this naturally occurring oscillation in the past. This information is necessary to understand how ENSO will change in the future, especially with humans now rapidly altering the environment. With this FWF research project, we used cave deposits (speleothems) from Alaska to piece-together a 13,500 year ENSO record, vastly improving our understanding of this important climate phenomena. We find that the sun is the principle controller of ENSO on century timescales until the 1970s. After this date, humans overprint the natural forcing of the sun. Importantly, this is the first robust example of ENSO entering a new mean state with a clear connection to human forcing. Additionally, our extraordinary dataset was essential in developing a new mechanism termed the Walker Switch. The Walker switch mechanism is forced by the sun and results in rapid zonal sea surface temperature changes in the Tropical Pacific. We argue that the Walker switch mechanism is an important driver of millennial-scale climate change and potentially represents a paradigm shift in climate dynamics.