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Author ORCID Identifier

N/A

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Geosciences

Year Degree Awarded

2019

Month Degree Awarded

February

First Advisor

Raymond S. Bradley

Second Advisor

Isla S. Castañeda

Subject Categories

Biogeochemistry | Climate | Environmental Indicators and Impact Assessment | Fresh Water Studies | Sedimentology

Abstract

Climate change is one of the most complex and challenging issues facing the world today. A changing climate will affect humankind in many ways and alter our physical environment, presenting ethical challenges in how we respond. The impact of climate change will likely be exacerbated in heavily populated regions of the planet, such as the Northeastern United States (NEUS). The NEUS is comprised of complex, sprawling urban centers and rural regions, both of which are vital to the economic and cultural character of the region. Furthermore, both urban and rural areas in the NEUS contain communities that have been historically susceptible to climate change (Horton et al. 2014). Over the past 120 years, average temperatures have increased by 2°F, precipitation has increased by 10%, and sea levels have also risen (Kunkel 2013).

One poorly understood consequence of climate change is its effects on extreme events such as wildfires. Robust associations between wildfire frequency and climatic variability have been shown to exist (Scholze et al. 2006; Westerling et al. 2006), indicating that future climate change may continue to have a significant effect on wildfire activity. The NEUS has been home to some of the most infamous and largest historic “megafires” in North America, such as the Miramichi Fire of 1825 and the fires of 1947 (Irland 2013). Although return intervals in most areas of the NEUS are high (hundreds of years), wildfires have played a critical role in ecosystem development and forest structure in the region (Carlson 2013). Therefore, predicting fire occurrence and vulnerability to large wildfires in the NEUS is economically and culturally relevant. However, predicting fire occurrence is not a simple task due to the nature of wildfire activity in the NEUS. While in most regions of the world, natural factors such as lightning are the driving cause of wildfires, it has been estimated that the vast majority (>99%) of wildfires in the NEUS are caused by anthropogenic activity and not natural causes (Pyne 1982). Consequently, only studying data associated with fire occurrence (i.e. area burned, number of fires) is likely inadequate for the investigation the region’s fire risk over time. Furthermore, little is known about how the direct (temperature & precipitation trends) and indirect (ecosystem-wide species distribution) effects of climate change will impact fire risk in the NEUS under future climate scenarios. Fully understanding the natural mechanisms that control fire risk and occurrence requires continuous records of past fires and climatic variability on centennial to millennial timescales. However, historical fire records in the NEUS are temporally limited, and do not provide an adequate analysis of the impacts of regional wildfire regimes, prior to human disturbance and anthropogenic climate change.

We find that regional climatic fire risk for the NEUS can be estimated most accurately using the Keetch Byram Drought Index (KBDI) from 20th century historical meteorological records from various stations located throughout the region. Regional fire risk is then estimated through 2100 AD, using the KBDI and dynamically downscaled regional climate models from CMIP5 climate models. Under RCP 8.5, average KBDI and max yearly KBDI is shown to increase by 300% and 500%, respectively, in an exponential trend. Under RCP 4.5, KBDI is also expected to increase through 2100 AD to a lesser extent. Interestingly, these increases in regional fire risk are present regardless of increases in precipitation, indicating that future fire risk in the NEUS is driven largely by changes in temperature as opposed to precipitation.

In order to investigate long-term regional wildfire activity over the past millennium, we examine PAHs and macrocharcoal from a varved sedimentary record from Basin Pond, Fayette, Maine (USA). We find elevated concentrations of the PAH retene were found to be highly correlated with known large-scale regional wildfire events that occurred in 1761-1762, 1825, and 1947 (A.D.). To distinguish between biomass burning and anthropogenic combustion, we examined the ratio of the PAHs retene and chrysene. The new Basin Pond PAH records, along with a local signal of fire occurrence from charcoal analysis, offers the prospect of using this multi-proxy approach as a method for examining wildfire frequency at the local and regional scale in the NEUS.

Finally, we report seasonally resolved measurements of brGDGT production in the water column, in catchment soils, and in a sediment core from Basin Pond. We utilize these observations to help interpret a Basin Pond brGDGT-based temperature reconstruction spanning the past 900 years. This record exbibits similar trends to a pollen record from the same site and also to regional and global syntheses of terrestrial temperatures over the last millennium. However, the Basin Pond temperature record shows higher-frequency variability than has previously been captured by such an archive in the NEUS, potentially attributed to large scale atmospheric patterns. These new records of temperature variability and wildfire activity, when compared to regional hydroclimate records, shed insight into pre-historic wildfire risk in the NEUS.

Available for download on Saturday, February 01, 2020

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