JAYNE BELNAP, RESEARCH ECOLOGIST
U.S. GEOLOGICAL SURVEY
U.S. DEPARTMENT OF THE INTERIOR
SENATE COMMITTEE ENERGY AND NATURAL RESOURCES
SUBCOMMITTEE ON PUBLIC LANDS AND FORESTS
HEARING ON THE MAJOR ENVIRONMENTAL THREATS TO
THE GREAT BASIN IN THE 21ST CENTURY
OCTOBER 11, 2007
Mr. Chairman and Members of the Subcommittee, thank you for the opportunity to appear here today to discuss how climate change models can help us better understand the interaction between climate change and environmental threats in the Great Basin/Colorado Plateau region.
Climate change is perhaps the most complex and multi-faceted challenge facing public land managers. Climate change affects biota, water, ecosystems, cultures, and economies. Although climate change is a natural, continuous Earth process, changes to the Earth's climate are related to human activities as well. Whether the causes are natural or from human influences, the U.S. Geological Survey (USGS) climate change focus is on understanding its impacts and the potential adaptive strategies for managing natural resources and ecosystems in the face of these changes.
Climate Change Modeling
The most recent generation of global climate models are called Atmospheric-Ocean Global Circulation Models (AOGCM) because the predictions from these models are based on data from the atmosphere, oceans, and land masses. Atmospheric data in the models describe transfers of heat, radiation, and water vapor, and the processes of cloud development and precipitation. Oceanic factors include sea surface temperatures, sea ice, and ocean currents. Land factors include vegetative cover, soil types, water storage and the type of water delivery (i.e., rain versus snow). As the name implies, AOGCM combines these factors to create global climate models.
There are many issues that create uncertainty in these models. The most problematic concern how clouds, sea ice cover, and atmospheric greenhouse gas concentrations affect climate. Clouds affect climate in many ways, including increasing or decreasing radiation, creating precipitation, and affecting small-scale circulation patterns. To illustrate the problem, clouds cover approximately 60 percent of the Earth's surface and are responsible for up to two thirds of Earth's albedo (reflectance of light from the surface – which is about 30 percent). A decrease in albedo by only 1 percent can increase temperatures by about 1°C. Secondly, the future extent of sea ice and snow fields, which have a large influence on the outcome of the models, is another unknown. As the concentrations of greenhouse gases rise and warm the Earth, snow and ice begin to melt. As the underlying ground or water is darker than the snow and ice, they absorb more heat from the Sun, causing more melting, which results in additional warming. This creates a feedback loop known as the 'ice-albedo feedback'. Lastly, the level of emissions (carbon dioxide and other greenhouse gases) that can be expected in the future is unknown. Detecting, understanding and accurately quantifying such feedbacks and emissions is extremely difficult, but the valuation of these factors can greatly alter climate predictions.
There are issues associated with downscaling of the AOGCM projections as well. Whereas we are fairly confident in global-scale drivers of climate, the effect of local factors are much less certain. There are two main approaches to downscaling. The first approach constructs an empirical relationship between a local factor (e.g., stream flow) and large scale atmospheric circulation model prediction of that factor. The second approach, dynamical downscaling, basically uses a weather prediction model to downscale AOGCM output to much higher resolutions. Both methods have their advantages and disadvantages. Empirical downscaling requires a long record of high quality data in order to build the required empirical relationships. For many parts of the United States, such records are lacking. For example, there are very few long-term climate station records in the Great Basin/Colorado Plateau region that can be used to create or verify downscaled models. In addition, the paucity of climate stations means that climate information for a specific location can only be modeled (that is, data from a few stations are extrapolated over a larger area that has similar elevation, topography, etc.). Thus, data for the model is often coming from another model, increasing the risk of error.
The primary disadvantage of dynamical downscaling is the high computational cost. Both methods will give erroneous climate projections if the large-scale circulation provided by the AOGCMs is incorrect, as they provide the boundary conditions for the heat, water vapor, and pressure fields. As physical equations are then used to calculate what these fields are in higher resolution, any error in the large scale fields is propagated throughout the downscaled models.
Use of Models in Understanding Future Conditions
It is not valid simply to extrapolate the observed past changes in climate change forward into the future. However, the demonstrated success of current climate models in simulating the global pattern of observed 20th century changes means that those models are credible, though far from perfect, tools for looking into the future. As discussed in more detail below, given the most realistic assumptions about future atmospheric carbon dioxide concentrations and other drivers of climate change, these models project a long-term drying trend in the Southwest, including the Great Basin. The drying trend in the Southwest implies an increasing probability of occurrence of Southwestern drought.
These projections are, at best, a general outline of climate change for the real future. I note, however, that there is much room for improvement. For example:
Modeling and Research Findings
The averaging of 21 climate models predicts that temperatures will increase by up to 6o C (11o F) in the Great Basin/Colorado Plateau region during the next century (Christensen et al., 2007). This is a large increase, and thus, it is likely to have profound effects on water resources and the living systems that depend on those resources. Atmospheric carbon dioxide and nitrogen levels are also likely to increase. There is much more uncertainty in predicting future precipitation than temperature. Precipitation predictions vary widely, depending on how the models are constructed. The Intergovernmental Panel on Climate Change averaged model predicts 5-10 percent increase in winter precipitation, 0-15 percent decline in summer precipitation, and 0-5 percent decline in annual precipitation (Christensen et al., 2007).
In addition, a review of these models shows that extreme events (e.g., drought, wet years, floods, high winds) will increase. These extreme events will cause significant challenges to the biological components of the Earth system in terms of their ability to adapt or mitigate to other areas as a result of abruptly-changing climate (Christensen et al., 2007).
Land use activities (e.g., recreation, clearing for housing, grazing, cropland, military activities) are also increasing rapidly in this region and will further exacerbate the effects of climate change on biological resources. These activities enhance the invasion of exotic plants, reduce or remove vegetative cover, and destroy physical and biological soil crusts, leaving soils unprotected, reducing forage and habitat, and increasing the reflectance, or albedo, of the soil surface (Foley et al., 2005; Notaro et al., 2006).
With climate change and land use invasive plants, especially exotic annual grasses, will likely increase. Soil surface disturbance, elevated carbon dioxide levels, the deposition of atmospheric nitrogen, and increased fire will all contribute to a likely increase in exotic annual grasses such as cheatgrass (D'Antonio and Vitousek, 1992; Brooks et al., 2004). In an area such as the Great Basin/Colorado Plateau region, where exotic annual grasses have been replacing native perennial plant communities, this could have severe consequences, resulting in years where such landscapes will have little or no forage and habitat for wildlife and livestock, resulting in a severe loss of biodiversity. During this time, soils will also be highly vulnerable to erosion. In addition, annual grasses alter soil biota, decomposition rates, and nutrient cycling rates, resulting in lower soil fertility.
Fire frequency and severity will also increase with the invasion of annual plants and future extreme wet/dry conditions. Re-burning of areas facilitates further annual plant invasion, which will lead to increased fire frequency (Brooks et al., 2004). Because most desert shrubs grow slowly and require extended periods without fire to re-establish, more frequent fire is particularly destructive in shrub-dominated desert systems such as those found in the Great Basin/Colorado Plateau region. With the loss of perennial vegetation, important microclimates are lost, including those that enhance the germination and establishment of native plants and habitat for native animals. Fire can also create hydrophobic soils that, when combined with loss of vegetation cover, allow for increase soil erosion, and can deplete the nutrient and carbon stocks in soils. Biota living at, or just beneath, the soil surface are often killed, slowing decomposition cycles and reducing soil nutrient availability.
As temperatures rise, soil moisture will decrease. One study has shown that, by 2050, even if there is no decrease in precipitation, increasing temperatures alone will result in average soil moisture conditions being lower than those experienced during any of the mega-droughts of this century (Dust Bowl years of the 1930s; drought years 1953-1956 and 1999-2004; Andreadis and Lettenmaier, 2006). This will result in reduced plant cover and biomass, and thus, less forage and habitat for livestock and wildlife. Insect outbreaks are also often associated with lower soil moisture, as the resistance of vegetation to infestation is reduced as a result of this stress. The combination of dry soils and insect infestation have been known to kill thousands of square miles of vegetation (e.g., the 2002-2003 Ips beetle infection/infestation of Pinyon Pine in the Southwest United States), leaving the area highly susceptible to fire and subsequent invasion by weeds (Breshears et al., 2005).
Observations during dry periods of above average temperature have also shown that shallowly rooted plants, such as perennial grasses and cactus, will be highly vulnerable to future dry and hot conditions (Ehleringer et al. 1999; Breshears et al. 2005). Many animals at the base of the food chain (e.g., mice, rabbits) depend on grass and cactus for food and shelter; thus, a reduction in these species is expected to reverberate upward, resulting in the loss of predators such as raptors, mountain lions, and bears. Grass is also the main food for cattle and elk. Soil lichens, which add stability, carbon, and nitrogen to soils, also die with increased temperatures. Their loss will further contribute to a reduction in soil stability and fertility (Belnap et al., 2006).
Research by USGS and colleagues shows that increased warming could decrease runoff by up to 30 percent in many streams and rivers in the Great Basin/Colorado Plateau region (Milly et al., 2005). This includes water in the Colorado River, which currently supplies the needs of 25 million people in seven U.S. states, two Mexican states, and 34 Native American tribes (Pulwarty et al. 2005). As population grows, the demand for water will increase at the same time that water availability is decreasing due to climatic conditions (and soil erosion, see below). Small springs and streams may dry up earlier in the season, or completely, placing plants, animals, and humans that depend on surface water at risk.
Research by USGS and others shows that desert soils are mostly stable until disturbed (Marticorena et al., 1997; Belnap, 2003). However, the interaction of lower soil moisture, fire, exotic plant invasions, and surface-disturbing activities will reduce the cover of natural soil stabilizers (plants, physical and biological soil crusts, rocks) and result in greater soil erosion. Restabilization of these soils often depends on heavy precipitation events; thus soils will continue to erode during continued drought. As erosion differentially removes the fine particles in soils to which nutrients are attached and which increase water-holding capacity of the soil, the remaining soils are less fertile and dry more quickly. This will result in less plant biomass and thus less forage and habitat for wildlife and livestock. In addition, reduced soil fertility will likely result in a reduction in the nutritive quality of the plant tissue (Marschner, 1995). Thus, livestock and wildlife will need to eat more to meet their nutritional requirements.
Soils eroded by water increase the sediment load in streams and, ultimately, large rivers. These sediments are often heavily laden with salts and heavy metals, contributing to water-quality problems downstream. Soil deposition into small springs and streams can be especially problematic, as the amount of water present is so low the resource can be completely lost.
One largely overlooked issue regarding soil erosion by wind is that it can produce dust storms that can have profound and lasting effects. Dust obscures visibility on highways and thus endangers travelers. If inhaled, the fine particles found in dust can cause asthma and other respiratory disease. Dust can carry Valley Fever, which can be fatal (Kirkland and Fierer, 1996). Dust storms can cause large economic losses through lost work time and ruined machinery. Blowing sediment can bury plants and eliminate habitat and forage. Dust also affects water storage and delivery. When dark-colored dust is deposited on the snowpack of downwind mountains and absorbs solar radiation, the underlying snowpack melts 30 days or more earlier than normal (Painter et al., 2006). Earlier melting reduces water storage in the snowpack, thereby reducing the amount of water that is available in streams and rivers during late summer. A faster melting rate may also increase spring flooding, reducing the opportunity to store water in those downstream reservoirs (Parker, 2000).
The loss of vegetation turns the Earth's surface from a dark color to a light color. Thus, the energy from the sunlight hitting a lightened surface is reflected upwards, rather than being absorbed by dark vegetative surface. In addition, the surface is smoothed and moisture evaporated from plants is lacking. The resultant rising hot and dry air reduces cloud formation, thus reducing subsequent precipitation. The result can be dramatic. Areas with reduced vegetative cover receive less precipitation than adjacent land covered by vegetation (Charney et al., 1975). Therefore, as land use, drought, fire, or a combination of these factors results in reduced vegetative cover, we can expect a reduction in precipitation as well (Foley et al., 2005; Notaro et al., 2006). This often creates a feedback loop, where drought reduces vegetative cover which increases albedo; this increase, in turn, increases the severity of the drought, which further reduces vegetative cover. This problem is especially severe where native perennial plants have been replaced by annual grasses. Under drought conditions, soils in these areas often completely lack vegetative cover, and thus albedos are greatly increased.
Because plant recovery depends on soil moisture availability, lower soil moisture will slow or prevent the recovery of plants and soils from fire or surface-disturbing activities. The presence of invasive annual grasses will often prevent the re-establishment of native vegetation by facilitating frequent fires, killing the native plants (Brooks et al., 2004). Almost all the research done on restoring drylands has occurred during the past 30 years, which was a relatively wet period. Thus, many of the restoration techniques that have been developed may not work under anticipated future dry conditions. Additional research will be needed to determine restoration techniques under dry conditions.
Natural and human-caused disturbances have interacted over the past several decades to change rangelands and ecosystems across as much as one half of the Great Basin's one hundred million acres (McIver et al., 2004). Protracted drought coupled with invasive species, altered fire regimes, grazing, human settlement and recreation, and energy exploration and development have yielded suites of vegetation that often cannot support wildlife species. Increasing annual temperature and decreasing precipitation regimes have exacerbated these ecological changes, and climate change will continue to interact with plant and animal dynamics on dry lands.
To better understand the interaction between climate change and these environmental threats, and to provide the science needed by resource managers and decision makers, the USGS is working to:
Mr. Chairman, thank you for this opportunity to present testimony. I would be pleased to respond any questions that you or other Members of the Subcommittee may have on this topic.