Economic Risks of Climate Change: An American Prospectus

One study found that a temperature rise of 3.2°F by midcentury would produce a 54 percent increase in annual area burned in the western United States relative to the present day (Spracklen et al. 2009). As burn area is ecosystem dependent, the study found that forests of the Pacific Northwest and Rocky Mountains will likely experience the greatest increases (78 and 175 percent, respectively). A study looking at fire risk in California and Nevada predicts a 10 to 35 percent increase by midcentury, depending on the greenhouse-gas emissions scenario and global climate model used (Westerling & Bryant 2008). More dramatic increases in temperature (such as those expected under RCP 8.5) when accompanied by drought are likely to produce a response in fire regimes that are beyond those observed during the past 3,000 years (Marlon et al. 2012).

In recent decades, warming temperatures, intense droughts, and insect outbreaks have contributed to decreasing tree growth and increasing mortality in many forest types throughout the United States, affecting 20 million hectares and many tree species since 1997 from Alaska to the Mexico border (Bentz et al. 2010). Average annual mortality rates have increased by a factor of 2 to 3, from less than 0.5 percent of trees per year in the 1960s to 1.0 to 1.5 percent today (van Mantgem et al. 2009).

It is difficult to isolate individual causes of tree death, as factors such as drought, higher temperatures, and pests and pathogens are often interrelated (Allen et al. 2010; Joyce et al. 2013). However, according to the 2014 National Climate Assessment, rates of tree mortality have increased with higher temperatures in the western United States (van Mantgem et al. 2009; Williams et al. 2010; Joyce et al. 2013). This effect is less direct in eastern forests, where forest composition and structure appear to drive impacts in recent decades. As the National Climate Assessment notes, although the extent to which recent forest disturbances can be directly attributed to climate change is uncertain, recent research provides a clear indication that climatic variables will affect ecosystems and alter the risks U.S. forests face today.

Tree mortality and forest die-off triggered by dry and hot conditions have been documented in most bioregions of the United States over the past two decades, with increases in wildfires and bark beetle outbreaks in the 2000s likely related to extreme drought and high temperatures in many western regions (Williams et al. 2010). Coniferous tree species have seen widespread and historically unprecedented die-off in recent years, mainly as a result of drought and pests such as bark beetles (Adams et al. 2009). Forests within the southwestern United States have been particularly sensitive to drought and warmth; from 1984 to 2008, as much as 18 percent of southwestern forest area experienced mortality due to bark beetles or drought (Joyce et al. 2013). In Alaska, more than 1 million hectares of several spruce species experienced die-off. Such die-off events can create significant additional risk to surrounding forests and local communities, as tree death and the accompanying increase of dead wood will influence the fire risk of forests (Anderegg, Kane, & Anderegg 2013).

Regional Wildfire Impacts

Yellowstone

Large fires have increased in the northern Rockies in recent decades in association with warmer temperatures, earlier snowmelt, and a longer fire season (Westerling et al. 2011). Although human activity—through fire suppression, forest thinning, and fuel treatment—plays a role, climatic variables were found to be of primary importance in most forests, especially at higher elevations where human activity is less prevalent. Recent studies indicate that the greater Yellowstone ecosystem, a large conifer forest ecosystem characterized by infrequent, high-severity fire, is approaching a temperature and moisture-level tipping point that could be exceeded by the mid-twenty-first century. Westerling et al. estimate that with climate-related increases in fire occurrence, area burned, and reduced fire rotation (down to 30 years from the historical 100 to 300 years), there is a real likelihood of Yellowstone’s forests being converted to nonforest vegetation during the mid-twenty-first century (Westerling et al. 2011).

California

Wildfire in California comes at a very high price. Seven of the ten costliest U.S. wildfires in history before 2011 occurred in California. Wildfire risks and their associated costs pose significant challenges to state and local governments, with state fire-suppression costs of more than $1 billion each year. The risk to private property has also increased over recent years as development along the wildland-urban interface has increased, with now more than 5 million homes in more than 1,200 communities at risk. The largest changes in property damage occurred in areas close to major metropolitan areas in coastal southern California, the Bay Area, and in the Sierra foothills northeast of Sacramento. In 2003, more than 4,200 homes were destroyed by wildland fires in southern California, resulting in more than $2 billion in damage (Radeloff et al. 2005).

Changes in temperature, precipitation, pest and pathogen dynamics, and more extreme climate events such as drought are expected to lead to increased instances of widespread forest die-off in the future (Anderegg, Kane, & Anderegg 2013). Western forests have experienced the greatest impacts, even more severe than recent estimates, and with projected increases in temperature and aridity out to 2100, substantial reduction in tree growth and increased mortality is expected, in particular in the Southwest (Dale et al. 2001; Allen et al. 2010; Scholes et al. 2014). As temperatures increase to levels projected for midcentury and beyond, eastern forests may be at risk of die-off or decline similar to recent die-offs experienced in the western United States.