Overview: Impacts on Biological Systems

As with global observations, biological impacts consistent with warming have been found in California: shifts in elevational or latitudinal range; changes in the timing of key plant and animal life cycle events; and changes in species abundance and in community composition.

Aug 24, 2018

Climate change impacts on terrestrial, marine and freshwater ecosystems have been observed in California. As with global observations, species responses include those consistent with warming: elevational or latitudinal shifts in range; changes in the timing of key plant and animal life cycle events (known as “phenology”); and changes in the abundance of species and in community composition. With continued climate change, many species may be unable to adapt or to migrate to suitable climates, particularly given the influence of other factors such as land use, habitat alteration, and emissions of pollutants.

Humans

Humans are better able to adapt to a changing climate than plants and animals in natural ecosystems. Nevertheless, climate change poses a threat to public health. While it is difficult to track its influence using indicators, climate change can impact human well-being in many ways. Examples include injuries and fatalities from extreme events and respiratory stress from poor air quality. Indicators of the impacts of climate change on human health show that:

Warming temperatures and changes in precipitation can affect vector-borne pathogen transmission and disease patterns in California. West Nile Virus currently poses the greatest mosquito-borne disease threat.
Heat-related deaths and illnesses, which are severely underreported, vary from year to year. In 2006, they were much higher than any other year because of a prolonged heat wave.

Vegetation

Warming temperatures, declining snowpack, and earlier spring snowmelt runoff can create stresses on vegetation. A measure of plant stress, climatic water deficit, reflects the demand plants have for water relative to the availability of water in the soil. Increases in climatic water deficit are associated with a warming climate.

Since 1950, the area burned by wildfires each year has been increasing, as spring and summer temperatures have warmed and spring snowmelt has occurred earlier. During the recent “hotter” drought, unusually warm temperatures intensified the effects of very low precipitation and snowpack and created conditions for extreme, high severity wildfires that spread rapidly. Five of the largest fire years have occurred since 2006. The largest recorded wildfire in the state (Thomas Fire) occurred in December 2017.

Evidence of how the state’s forests and woodlands are responding to climate change has been found in studies that compared historical and current conditions. Historical data are from a 1930s survey of California’s vegetation.

The structure and composition of the state’s forests and woodlands are changing. Compared to the 1930s, today’s forests have more small trees and fewer large trees. Pines occupy less area statewide and, in certain parts of the state, oaks cover larger areas. The decline in large trees and increased abundance of oaks are associated with statewide increases in climatic water deficit.

On the western side of the northern Sierra Nevada Mountains, the lower edge of the Ponderosa pine forest has moved upslope. Since the 1930s, the forest has retreated from elevations that no longer experience freezing winter temperatures at night. The loss of conifers in this elevation was accompanied by an expansion of forests dominated by broadleaf trees.

Other indicators of the impacts of climate change on vegetation show that:

Tree deaths have increased dramatically since the 2012-2016 drought. Approximately 129 million trees died between 2012 and December 2017. Higher temperatures and decreased water availability made the trees more vulnerable to insects and pathogen attacks.
Vegetation distribution has shifted across the north slope of Deep Canyon in the Santa Rosa Mountains in Southern California. Dominant plant species have moved upward by an average of about 65 meters (213 feet) in the past 30 years.
Compared to the 1930s, today’s subalpine forests (forests at elevations above 7,500 feet) in the Sierra Nevada are denser, as small tree densities increased by 62 percent while large tree densities decreased by 21 percent.
In parts of the Central Valley, certain fruits and nuts (prunes and one walnut variety) are maturing more quickly with warming temperatures, leading to earlier harvests. Shorter maturation times generally lead to smaller fruits and nuts, potentially causing a significant loss of revenue for growers and suppliers.

Wildlife

Changes in temperature, precipitation, food sources, competition for prey, and other physical or biological features of the habitat may force changes in the timing of key life cycle events for plants and animals and shift the ranges where these plants and animals live. These factors, along with the inherent sensitivity of the species, interact in ways that can affect species responses differently.

Certain birds and mammals are found at different elevations in three study regions of the Sierra Nevada Mountains today compared to a century ago. Range shifts have been observed in almost 75 percent of the small mammal species and over 80 percent of the bird species surveyed. High-elevation mammals tended to move upslope; birds and low-elevation mammals moved downslope as frequently as upslope. Across the three study regions, species did not show uniform shifts in elevation. The varied responses reflect the influence of intrinsic sensitivity to temperature, precipitation or other physical factors. They may also be due to changes in food sources, vegetation and interactions with competitors.

Marine species respond to changing ocean conditions, especially during periods of unusually warm sea surface temperatures. A nudibranch sea slug, Phidiana hiltoni, has expanded its range northward by 210 kilometers (130 miles) — from the Monterey Peninsula to Bodega Bay — since the mid-1970s in response to warming ocean conditions. This nudibranch was found for the first time in Bodega Bay in 2015. Unlike other nudibranch species, P. hiltoni has persisted at this northernmost location after warm water conditions ended.

Other indicators of the impacts of climate change on wildlife show that:

Over the past 45 years, Central Valley butterfly species have been appearing earlier in the spring. Their earlier emergence is linked with hotter and drier regional winter conditions.
Since 1980, the timing of spring and fall migratory bird arrivals at a coastal site in northern California have shown a diversity of changes.
Across the state, wintering bird species have collectively shifted their range northward and closer to the coast over the past 48 years. In both cases, species’ responses have not been uniform: some species have shifted to higher elevations or latitudes, and the shifts have occurred to varying degrees.
The effects of ocean acidification on marine organisms involve a wide range of biological processes. The most widely observed effect is interference with shell-formation in mollusks. (Since there are no trend data tracking these effects, this is a “Type III” indicator.)
Ocean conditions strongly influence marine organisms in the California Current, as seen with copepod populations. At the base of the food chain, the abundance and types of copepod species have been correlated with the abundance of many fish species.
The number of adult Chinook salmon returning from the ocean to the Sacramento River has become more variable over the last two decades. This number is impacted by extreme mortality events among juvenile salmon. As residents of both marine and freshwater environments, salmon are at risk from the impacts of climate change on these habitats.
Over a 45-year period, the breeding success of Cassin’s auklets on Southeast Farallon Island near San Francisco has become increasingly variable. It is associated with the abundance of prey species that are influenced by ocean conditions such as warming.
During years when sea surface temperatures are unusually warm in their breeding area, there have been fewer California sea lion pup births, higher pup mortality, and poor pup conditions at San Miguel Island off Santa Barbara. Sea lions are vulnerable to fluctuations in the abundance and distribution of their primary prey, which are directly influenced by ocean conditions.