Holocene climate and oceanography of the coastal Western United States and California Current System

Abstract

Abstract. To understand and contextualize modern climate change, we must improve our understanding of climatic and oceanographic changes in the Holocene (11.75 ka–present). Climate records of the Holocene can be utilized as a “baseline” from which to compare modern climate and can also provide insights into how environments and ecosystems experience and recover from environmental change. However, individual studies on Holocene climate in the literature tend to focus on a distinct geographic location, a specific proxy record, or a certain aspect of climate (e.g., upwelling or precipitation), resulting in localized, record-specific trends rather than a comprehensive view of climate variability through the Holocene. Here we synthesize the major oceanographic and terrestrial changes that have occurred in the Western United States (bounded by 30° N to 52° N and 115° W to 130° W) through the most recent 11.75 ka and explore the impacts of these changes on marine and terrestrial ecosystems and human populations. This three-tiered systematic review combines interpretations from over 100 published studies, codes and geospatially analyzes temperature, hydroclimate, and fire history from over 50 published studies, and interprets nine representative time series through the Holocene. We find that the early Holocene is characterized by warming relative to pre-Holocene conditions, including warm sea surface conditions, a warm and dry Pacific Northwest, a warm and wet Southwest, and overall spatial and temporal stability. In the mid Holocene, these patterns reverse; this interval is characterized by cool sea surface temperatures, a cool and wet Pacific Northwest and warm and dry Southwest. The late Holocene is the most variable interval, both spatially and temporally, and a novel spatial trend appears in terrestrial climate with warmer coastal areas and cooler inland areas. Human communities interacted with the environment throughout the entire Holocene, as evidenced in archeological and paleoenvironmental records, yet the recent era of colonization (1850–present) represents an unprecedented environmental interval in many records. Overall, our analysis shows linkages between terrestrial and oceanographic conditions, distinct environmental phases through time, and emphasizes the importance of local factors in controlling climate through the dynamic Holocene.

Linked fire activity and climate whiplash in California during the early Holocene

“…In addition, we analysed portions of the speleothem younger than 6900 years BP, and these showed higher levoglucosan concentrations up to 4.7 ± 0.3 ng • g −1 (Supplementary Table 1). This increase towards the present may be indicative of higher levoglucosan input into the soil through increased fire activity in the mid to late Holocene, consistent with regional lake records 33,35,36 . However, we do not have precise age constraints on this part of the stalagmite and can only estimate their ages to be between 6900 and ~240 years BP.…”

“…The WMC1 δ 13 C record resolves precipitation changes internal to the 8.2 kyr event and suggests that the interval was characterised by heightened interannual precipitation volatility even though the mean shift in precipitation was small. Similar to links between modern PDSI variability and summer temperatures 43 , we propose that this Article https://doi.org/10.1038/s41467-022-34950-x heightened volatility coupled with the enhanced early Holocene summer temperatures 36 drove the shift to more woody vegetation and elevated fire activity.…”

“…These proxy comparisons describe a link between hydroclimate volatility, vegetation change, and fire occurrence during the 8.2 kyr event similar to that observed in modern tree-ring records that are of higher temporal resolution than the stalagmite, and in Holocene lake sediments that are of lower resolution 31,33,43,44 . Seasonality was enhanced in the early Holocene including during the 8.2 kyr event, with increased summer insolation leading to higher summer temperatures and vice versa 36,45 . Although freshwater forcing led to cooling in the North Atlantic region, modelling indicates only a small (<0.5 °C) reduction in average annual temperatures associated with the 8.2 kyr event in western North America 48 .…”

“…The vegetation change noted in WMC1 is broadly consistent with regional changes in vegetation as recorded by pollen from the marine sediment core ODP 1018 off the coast of Santa Cruz which indicates increased redwood and decreased herbs and chaparral species just prior to 8000 years BP 32 . Lake sediment records from California and southern Oregon suggest increasing fire activity under progressively more intense aridity from the early to middle Holocene [33][34][35][36] , with lakes in the Klamath Mountains suggesting enhanced fire activity at 8400 years BP and the growth of a chaparral understory through the mid-Holocene 37 . However, challenging chronologies and low sediment accumulation rates preclude investigation of fire-vegetation relationships at these sites at temporal resolution comparable to the WMC speleothem.…”