Sierra Nevada Network

Stressor: Climate Change

Climate Change

Giant Sequoias and understory of White Fir, Sequoia National Park.

Average global temperatures have been rising, and the earth’s atmosphere is warmer than at any point during the last several centuries. There is broad international consensus among climatologists and atmospheric scientists that “most of theobserved warming over the last 50 years is likely [attributable to a human-induced] increase in greenhouse gas [e.g., CO2, from the burning of fossil fuel] concentrations”. Global temperatures (and globally-averaged surface temperatures) are projected to increase another 1.4 to 5.8ºC over the next century—a rate probably unprecedented over the last 10,000 years. This is expected to have profound effects on weather and climate.

The last several decades in the Sierra Nevada were among the warmest of the last millennium. Recent simulations of climate change models suggest that by the years 2050 to 2100, average annual temperature in the Sierra Nevada could increase by as much as 3.8º C—the equivalent of about an 800 m upward displacement in climatic zones. Average temperatures in May could increase by 9º C.

Paleoecological records show the early and middle Holocene (ca. 10,000 to 4,500 years ago) was a period of generally higher global summer temperatures (perhaps by 2º C) and prolonged summer drought in California. During this period, fire regimes and plant community composition of Sierra Nevada forests differed from those of today (including some species combinations that no longer exist). For example, early Holocene forests (growing on sites that presently support giant sequoia groves) were much more heavily dominated by pines, including lodgepole pine, which no longer occurs in sequoia groves. Overall, firs were less abundant than today, and giant sequoias were quite rare. Mortality could increase among adult trees as a result of drought stress, which would make them more vulnerable to insects, pathogens, and air pollution. Although the past is an imperfect analog of the future, these and other paleoecological records indicate climatic change smaller than, or comparable to, those projected for the next century could profoundly alter Sierra Nevada ecosystems.

Phenological studies indicated that in much of the West, lilacs and honeysuckles are responding to the warming trend by blooming and leafing out earlier. Human-influenced temperature patterns are significantly associated with discernible changes in plant and animal (invertebrate, bird, amphibian, tree, shrub) phenological traits.

Researchers predict that even a relatively modest mean temperature increase (2.5 °C) would significantly alter precipitation, snow pack, surface water dynamics (e.g., flow), and hydrologic processes. The most pronounced changes would probably be earlier snowmelt runoff and reduced summer base flows and soil moisture (Dettinger et al. 2004, Dettinger 2005), a lower snowpack volume at mid-elevations, and increased winter and spring flooding. Two climate models predict significant reductions in Sierra Nevada snowpack by the year 2100: one model predicts 30 –70% reduction, the other a 73 – 90% reduction.

Flows in many western streams begin a week to almost three weeks earlier than they did in the mid 20th century. There is also a trend towards slightly later precipitation. Observed stream flow timing and winterspring warming trends are consistent with current projections of how greenhouse effects may influence western climates and hydrology. Changes in precipitation type and timing may result in longer and drier summers, i.e., less water available during the months when it is most needed. Glacial extent in the Sierra Nevada has declined markedly in the past several decades.

Changes in Sierra Nevada climate related to precipitation quantity (e.g., snowpack) are less certain. If current trends continue, researchers predict that natural reservoirs provided by snowpack will become progressively less useful for water resources management. In addition, flood risk may change in unpredictable ways and Sierra Nevada ecosystems may experience increasingly severe summer-drought conditions. Prolonged summer drought alters natural fire regime and would increase the potential for high-severity wildfires and further threaten water quality.

Global warming is likely to shift habitats to higher elevations. Some organisms with limited mobility or specific habitat needs (e.g., amphibians) may not be able to move or survive such habitat shifts and could be locally extirpated. Consequently, species diversity may decline. Some habitats (e.g., high alpine) may shrink dramatically or disappear entirely, leading to irreversible loss of some species (e.g., Clark’s Nutcracker) Two climate models predict significant reductions in Sierra Nevada alpine/subalpine forest by the year 2100: one model predicts 50–75% reduction, the other a 75–90% reduction.

The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years; the current rate of increase is unprecedented during at least the last 20,000 years. About three-quarters of anthropogenic emission of CO2 to the atmosphere is due to fossil fuel burning; the rest is predominantly due to land-use change, especially deforestation.

It has been argued that the earth’s biosphere (primarily, terrestrial biosphere) may have the capacity to sequester much of the increased carbon dioxide (CO2) in the atmosphere associated with fossil fuel burning. This effect is termed “CO2 fertilization” because, in the envisioned scenario, higher ambient CO2 levels in the atmosphere literally fertilize plant growth. Further, because photosynthesis by plants converts CO2 into oxygen, it has been argued that “CO2 fertilization” could potentially provide a strong negative feedback on changing CO2 levels.

However, climatologists contend that as CO2 concentration of the atmosphere increases, ocean and land will take up a decreasing fraction of anthropogenic CO2 emissions. The net effect of land and ocean climate feedbacks as indicated by models will further increase projected atmospheric CO2 concentrations, by reducing both the ocean and land uptake of CO2.

Global climate change is also likely to exacerbate three other systemic stressors: altered fire regime, air pollution, and non-native invasive species. Some models predict future climate change will be accompanied by increased lightning strikes at latitudes spanned by the Sierra Nevada. Compounding the increase in wildfire ignitions, extreme weather conditions such as drought are likely to result in fires burning larger areas, being more severe, and escaping containment more frequently. Warm temperatures create the perfect conditions for the production of smog and ground-level ozone. Global warming is therefore likely to make air pollution problems worse. A warmer climate would create conditions that would allow the expansion of species better adapted to such conditions.

This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).