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Item Added: Climate Change. View Wishlist. The most widely studied offshore wind events, termed Santa Ana winds in southern California, increase in frequency in the fall and peak in winter Abatzoglou et al. Strong offshore winds with very low relative humidity can quickly dry fuels and spread large wildfires when they occur prior to the onset of the winter precipitation season in California's Mediterranean climate Billmire et al. The effects of anthropogenic climate change on California's fire regimes are likely to be diverse and complex, varying by region and season Liang et al.
Climate model projections of warming and increased atmospheric aridity in California are strong and robust across models Pierce et al. By contrast, model projections of precipitation in California are highly uncertain but with a tendency toward increased precipitation annual totals, particularly in northern California during winter Maloney et al. However, many climate models have systematic biases in North Pacific storm tracks and tropical Pacific sea surface temperatures that should lead to strong skepticism regarding model simulations of future precipitation in California Seager et al.
Climate models also project precipitation frequency declines in spring through fall that would partly offset winter increases, resulting in increased precipitation variability AghaKouchak et al. However, concurrent warming and decreased fall precipitation may, to some degree, counteract the effects of reduced offshore winds on fall fire risk in southwestern California Hughes et al.
While much has been published on projected changes in wildfire activity due to climate change e. Here we provide a comprehensive empirical assessment of the observed effects of climate variability and change on California wildfire by season, region, and land cover. We then use climate model simulations to determine whether observed trends in the climate variables most pertinent to regional wildfire activity are consistent with expectations of anthropogenic climate change.
A thorough and nuanced understanding of how, when, and where anthropogenic climate change has or has not affected wildfire in California over the past several decades is critical to guide sustainable societal decisions ranging from where to develop housing to how limited resources can be optimized for landscape management. While humans ignite the majority of wildfires statewide, lightning accounts for the majority of ignitions in North Coast and Sierra Nevada, particularly in summer Balch et al.
Summer is the season when most burned area occurs on average in all for study regions, but large and destructive fires can also occur in fall in all regions, particularly in South Coast where fall marks the peak in mean fire size and suppression cost Jin et al. Regional boundaries were defined by merging Bailey ecoregions at the section level Text S1. We exclude the agricultural Central Valley and desert areas where large wildfires are rare due to lack of fuels.
We compiled a comprehensive record of 39, California wildfires during — within our four regions of interest by merging records from multiple government agencies. We excluded all fires smaller than 0. We did not consider years prior to as this is when most of the multiagency records begin. See Text S2 for additional details about the wildfire data set. To evaluate observed climate trends in the context of anthropogenic climate change, it is desirable to evaluate a period longer than the — wildfire record.
The temporal extent of the climate records varies by variable and data product. Monthly climate grids for — came from the National Oceanic and Atmospheric Administration's Climgrid data set for precipitation and temperature Vose et al. See Text S3 for details.
See Text S4 for more details about these records. See Text S5 for details.
The science and impacts of climate change
The simulated climate response to anthropogenic forcing was assessed using climate model simulations produced as part of the fifth phase of the Coupled Model Intercomparison Project CMIP5; Taylor et al. The forcing scenarios considered were the Historical scenario for — and the 8. The RCP8. We evaluate interannual relationships between climate and the logarithm of seasonal and annual burned area log burned area using linear regression and the Pearson correlation coefficient. We used the logarithms of burned area because burned area has an exponential distribution e. VPD is a measure of the aridity of the atmosphere, calculated as the difference between the saturation vapor pressure dictated by temperature and the actual vapor pressure dictated by specific humidity; Text S3.
Time series of precipitation total were converted to the standardized precipitation index SPI because raw precipitation totals often have a skewed nonnormal distribution. Climate effects on wildfire vary among seasons. To test for an independent relationship between a climate variable Variable B and burned area after relationships with another climate variable Variable A have been accounted for, we performed a partial regression analysis.
Here time series of log burned area and Variable B were each regressed against Variable A. Residuals for log burned area and Variable B were calculated by subtracting away the linear relationships with Variable A. The two residual time series were then regressed against each other to assess the component of the relationship between burned area and Variable B that is independent of Variable A. We did not control for nonclimate effects on wildfire such as human effects on ignitions, fire suppression, or vegetation cover.
In and , the total area burned ranked third and first largest during our study period, respectively. The general lack of significant trends in fall is partly due to high interannual variability, driven by large wildfires in relatively few years. As was the case for summer, increases in the occurrence of large fall wildfires were observed in North Coast and Sierra Nevada but not for Central or South Coast. This result is consistent with the tendency for interannual variability in regional burned area to be more sensitive to variations in fuel aridity in more heavily vegetated zones where fuel abundance is less limiting Abatzoglou et al.
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- Observed Impacts of Anthropogenic Climate Change on Wildfire in California.
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Consistent with previous findings e. In addition, aboveground biomass is generally lower in these regions due to warmer and drier conditions, causing fuel availability to often limit fire spread in grasslands and potentially shrublands with nonnative grasses Keeley, This highlights the likelihood that nonforest wildfire is promoted by large interannual swings in precipitation total, from wet conditions that drive accumulation of grasses to dry conditions that promote desiccation of fuels.
Importantly, nonforested landscapes in California are highly diverse, and the broad patterns described above do not apply everywhere. Finally, in areas where wildfire is promoted by dry conditions, burned area tends to correlate more strongly with atmospheric aridity e. This was observed previously across broader portions of the western United States e.
However, correlative analyses with a single variable may artificially confound or inflate its importance due to covariance with other variables or factors Holden et al. For example, VPD is negatively related to precipitation cloud shade and soil moisture negatively force VPD , so the effect of one variable is entrained in the correlation between burned area and the other variable. Importantly, the positive correlation between summer burned area and March—October VPD remained strong and significant 0. Further, the effect of aridity on burned area which is strongest in forest is exponential as implied by the log scale of the y axes.
The above results strongly suggest that the observed increase in California summer burned area during — which mainly occurred in northern California forests was mainly due to increased VPD and not concurrent changes in nonclimate factors such as forest management, fire suppression practices, or human ignitions.
This is not to say that nonclimate factors were negligible in dictating modern annual burned areas. To the contrary, human ignitions greatly enhance the number of wildfires relative to that expected in their absence Balch et al.
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However, while effects of human activities are evident in multicentury assessments of fire activity e. Interannual correlation between fall total burned area and climate is weak because fall burned areas are dominated by a small number of years. For all regions, mean Pfire on days with at least one large fall wildfire exceeded twice that of days with no large fall wildfire. Importantly, many of the large wildfires that occur under dry conditions during fall are in nonforest landscapes where summer burned area does not appear to be strongly promoted by drought.
In contrast to summer, when many nonforest areas are consistently hot and dry, an aridity limitation on nonforest wildfire may emerge during fall due to lower temperatures and onset of the precipitation during that season. During —, March—October Tmax averaged across the four California study regions increased by 1. Discrepancies between observed and simulated climate model trends in South Coast may be due to a range of factors including natural multidecade climate variability Lehner et al. We would like to ask you for a moment of your time to fill in a short questionnaire, at the end of your visit.
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Free Shipping Free global shipping No minimum order. Environmental scientists, climatologists, energy researchers. Climate Change Through Earth's History 1. Introduction 2. Climate Models 3. Long-Term Climate Trends 4. Early Climate History 5. Phanerozoic Glaciations 6. The Mesozoic — Early Cenozoic Greenhouse 7. Development of the Cenozoic Icehouse 8. Astronomical Modulation of Climate 9.
Quaternary Sub-Milankovitch Cyclicity The Holocene Climate of the Anthropocene Conclusions Part 2. Indicators of Climate Change Chapter 2. Global Surface Temperatures 1. Basic Data Availability 3. Analyses of Land Surface Air Temperatures 4. Analyses of Sea Surface Temperatures 5. Global Changes 6. Uncertainty Quantification 7. Characterization of Extremes and Variability 8. Future Research Directions 9. Conclusions Chapter 3. Arctic Sea Ice 1.
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Classifying and Comprehending the Observed Changes 4. Conclusions Chapter 4. Why Antarctic Ice Is Different 3. Snow on the Ice 4. What Is Happening to the Ice? Response of the Antarctic to Changes Elsewhere Chapter 5. Mass Balance of Glaciers and Ice Sheets 3. Long-term Behaviour 4. Observations of Recent Changes 5. Concluding Remarks Chapter 6. Poleward Expansion of the Atmospheric Circulation 1. The General Circulation of the Atmosphere 3. Evidence for Circulation Change 4. Cause for Circulation Change 5. Emerging Dynamical Mechanisms 6.
Weather Pattern Changes in the Tropics and Mid-latitudes 1. Observed Changes in Sea Level Pressure 3.
follow Observed Changes in Extratropical Patterns 4. Changes in Tropical Patterns 5. Conclusions Chapter 8. Bird Ecology 1.
Indicators of Change 3.