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.
Highlights
1.1 Overview of Holocene Climate40 In contrast to past intervals of glacial/interglacial variability, the Holocene has largely been regarded as a climatically stable interval
3.1 Early Holocene (11.75-8.2 ka) Thirty-seven papers encompassing 60 sites fit the criteria for inclusion in the spatial analysis
3.1.1 Regional Synthesis Spatial analysis of coded hydroclimate, temperature, and fire history indicate that the early Holocene was generally characterized by a dry and warm PNW and a wet and warm SW (Fig. 2)
Summary
1.1 Overview of Holocene Climate40 In contrast to past intervals of glacial/interglacial variability, the Holocene has largely been regarded as a climatically stable interval. Meta-analysis of global temperatures and patterns through the Holocene show a globally warm period from 11 to 5 ka, termed the Holocene Climatic Optimum, largely driven by a concurrent peak in solar insolation in the Northern Hemisphere (Mayewski et al, 2004; Wanner et al, 2008; Renssen et al, 2012; Marcott et al, 2013; Bader et al., 45 2020). Following this period of global warmth, competing lines of evidence show divergent global temperature regimes in the last 5ka (some records show global warming and others global cooling); this phenomenon is termed the Holocene Conundrum (Wanner et al, 2008; Marcott et al, 2013; Liu et al, 2014; Marsicek et al, 2018; Bader et al, 2020). Episodes of high fire activity were relatively common in the early Holocene with a globally observed increase in fire activity between 3-2 ka, likely due to a change in climate (Marlon et al, 2013)
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