Renewable Energy Basics

The United States currently relies heavily on coal, oil, and natural gas for its energy. Fossil fuels are nonrenewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. In contrast, renewable energy resources—such as wind and solar energy—are constantly replenished and will never run out.


Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy, can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses.


The sun's heat also drives the winds, whose energy is captured with wind turbines. Then, the winds and the sun's heat cause water to evaporate. When this water vapor turns into rain or snow and flows downhill into rivers or streams, its energy can be captured using hydropower.


Along with the rain and snow, sunlight causes plants to grow. The organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is calledbiomass energy.


Hydrogen also can be found in many organic compounds, as well as water. It's the most abundant element on the Earth. But it doesn't occur naturally as a gas. It's always combined with other elements, such as with oxygen to make water. Once separated from another element, hydrogen can be burned as a fuel or converted into electricity.


Not all renewable energy resources come from the sun. Geothermal energy taps the Earth's internal heat for a variety of uses, including electric power production, and the heating and cooling of buildings. And the energy of the ocean's tides comes from the gravitational pull of the moon and the sun upon the Earth.


The ocean can produce thermal energy from the sun's heat and mechanical energy from the tides and waves. NREL does not conduct research in ocean thermal energy or ocean mechanical energy. See the U.S. Department of Energy's Consumer Guide Web site for basic informationocean energy.


Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower. NREL doesn't perform any research in hydroelectric power technologies. For more information on hydroelectric power, see the Hydropower Basics from the U.S. Department of Energy's Wind and Hydropower Technologies Program.

Frequently Asked Questions?


* Q: What is climate?
* A: In the Glossary of Terms used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, the definition of climate is:
Climate in a narrow sense is usually defined as the “average weather,” or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

* Q: What is natural climate variability?
* A: Natural climate variability refers to variations, owing to natural internal processes in the climate system or natural external forcing, in the mean state and other statistics of the climate on all spatial and temporal scales beyond that of individual weather events (IPCC 2007).

* Q: What mechanisms cause climate to change naturally?
* A: Climate and climate variability are determined by the amount of incoming solar radiation, the chemical composition and dynamics of the atmosphere, and the surface characteristics of the Earth. The circulation of the atmosphere and oceans influences the transfer of heat and moisture around the planet and thus strongly influences climate patterns and their variability in space and time (Chapin et al. 2002).

* Q: How do human activities affect climate?
* A: Human activities such as fossil fuel burning, industrial activities, land-use change, animal husbandry, and fertilized and irrigated agriculture lead to increases in greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), which contribute to the greenhouse effect and cause the surface temperature of the Earth to increase. Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values (IPCC 2007).

* Q: What is the “greenhouse effect” and what are the major greenhouse gases?
* A: About 30 percent of solar radiation that reaches Earth is reflected back into space by clouds, air molecules, dust, haze, and the Earth’s surface. Another 20 percent of incoming solar radiation is absorbed by the atmosphere. The remaining solar radiation reaches the Earth’s surface and is absorbed. The Earth’s surface radiates this energy back to the atmosphere in the form of infrared radiation. Most (90 percent) of this infrared radiation is trapped in the atmosphere by greenhouse gases. The energy absorbed by the greenhouse gases is reradiated in all directions. The energy that is directed back toward the Earth’s surface contributes to the warming of the planet. This phenomenon is called the greenhouse effect.
Gases that absorb infrared radiation and contribute to the greenhouse effect include carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and nitrous oxide (N2O).

* Q: What is “albedo” and “feedback,” and how does land use affect climate?
* A: Albedo is the reflectance of a surface. Absorbed radiation warms the surface, whereas, reflected radiation does not. Feedback is a general term that encompasses all of the different forms of energy exchange between the land surface and the atmosphere. Positive feedbacks enhance land surface warming; negative feedbacks enhance land surface cooling. Albedo is one component of this energy feedback. Different land covers have varied albedo. Thus, land use change can influence albedo and whether a land surface has a warming or cooling effect. For example, snow has a very high albedo and thus has a cooling effect (negative feedback). Melting of snow or coverage of snow with vegetation or black carbon (from air pollution) results in a higher surface albedo and has a warming effect (positive feedback) (IPCC 2007).

* Q: If climate varied naturally in the past, how do we know that humans are disrupting climate now?
* A: General Circulation Models (GCM) of the atmosphere are now being coupled with those of the oceans (AOGCM), ice, and the Earth’s terrestrial biosphere. These models have been under development for many decades. They spontaneously exhibit interannual and interdecadal oscillations not unlike those observed in the real Earth System. They are run under different starting conditions and using different amounts of solar, volcanic, and greenhouse gas "forcing" of the atmospheric dynamics. Using this "ensemble" approach, various AOGCMs have successfully simulated the Earth’s climate over the past 1,000 years. However, they cannot capture the rapid increase in global temperature of the past half century without including greenhouse gas forcing (IPCC 2007). Similarly, the models are able to simulate the warming of the upper 700 meters of all the major oceans of the world over the past 40 years, but only if they include the greenhouse gas emissions of the industrial age (IPCC 2007).

* Q: Is the rate of climate change greater now than the rate of natural climate change in the past?
* A: According to the Intergovernmental Panel on Climate Change (IPCC), the linear warming trend of 0.13 °C per decade over the 50 years 1956-2005 is nearly twice that for the 100 years 1906-2005. Eleven of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).
Long-term changes in other aspects of climate have also been observed. From 1900 to 2005, precipitation increased significantly in eastern parts of North and South America, northern Europe and northern and central Asia, whereas precipitation declined in the Sahel, the Mediterranean, southern Africa and parts of southern Asia (IPCC 2007).

* Q: What are westwide examples of climate change that occurred over the past century?
* A: From 1916 to 2003, there have been increases in both cool season and warm season temperatures over almost the entire Western United States (Hamlet et al. 2006; Mote et al. 2005). Although the rate of change differs with location and the period examined, the warming has been on the order of 1 ºC per century from 1916 to 2003 (Hamlet et al. 2006). The rate of increase from 1947 to 2003 is roughly double that of the longer period from 1916 to 2003; much of the observed warming has occurred from about 1975 to present.

* Q: What do westwide simulations suggest for climate changes in the 21st century?
* A: Simulated temperatures across the West by the end of the 21st century range from increases of 2 to 3 ºC at the low end of the uncertainty range, and as high as 5 to 6 ºC at the upper end of the uncertainty range (IPCC 2007). Precipitation changes in the West over the next century are more complex and uncertain, however, and differ within subregions. As both the Subtropical Jetstream and the Bermuda High intensify, the summer rains in the Southwest may intensify and shift to the north. Winter rains might decrease in the Southwest but increase in the northern half of the West (Salathé 2006). Interannual and interdecadal variability via El Niño-La Niña cycles may intensify (Timmermann et al. 1999), producing extreme winter events in both the Southwest and the Northwest.

Ecosystem Effects

* Q: How do changing climates affect the water cycle (snow, rain, runoff)?
* A: Changing climates influence the water cycle by influencing factors such as surface temperatures and evapotranspiration rates, precipitation patterns, proportions of precipitation received as rain versus snow, and amount and timing of runoff. Increased temperatures lead to more precipitation falling as rain rather than snow, earlier snowmelt and snowmelt-driven streamflow, and reduced spring snowpack. For the mountainous regions of the Western United States, snowmelt provides approximately 70 percent of annual streamflow. Thus, reduced spring snowpack leads to reduced summer streamflow in these regions. (Mote 2003, Mote et al. 2005, Mote et al. 2008, Stewart et al. 2005).

* Q: Does climate change affect flood and drought severity and frequency?
* A: Warmer temperatures and higher rates of evapotranspiration with climate change in some areas, such as the Southwest United States, will likely lead to increased drought frequency and severity. Overall, drought-affected areas are projected to increase in extent. Although increased temperatures will likely lead to decreased runoff in some areas, increased frequency of heavy precipitation events will likely lead to increased flood risk in many regions. Earlier snowmelt and runoff due to increased temperatures could also lead to increased winter and spring flooding. (IPCC 2007).

* Q: What is the relationship of climate change to fire intensity and frequency?
* A: Widespread fire years and fire extent are associated with warmer and drier spring and summer conditions in the Western United States Warmer spring and summer conditions lead to relatively early snowmelt, and lower summer soil and fuel moisture, and thus longer fire seasons. Increased temperatures and drought occurrence in some locations owing to global warming will likely lead to increased fire frequency and extent. Intensity of fires may also increase in some areas if higher temperatures interact with fuel characteristics to increase fire intensity. (Heyerdahl et al. 2008, McKenzie et al. 2004, Taylor et al. 2008, Westerling et al. 2006).

* Q: What are primary ecological effects of climate change?
* A: Climate controls ecosystem structure and processes such as species distribution and abundance, regeneration, vegetation productivity and growth, and disturbance, including insects and fire. Increasing temperatures and changes in precipitation with climate change will impact both ecosystem structure and ecosystem processes. For example, regeneration of tree species will be influenced by changes in snowpack, length of growing season, and moisture availability. Forest productivity may decrease in lower elevation forest ecosystems because of water limitations. Insects and disease outbreaks may become more frequent and widespread because warmer temperatures may accelerate their life cycles. Fire extent and frequency will likely increase with increased temperatures and longer fire seasons. Species distributions are likely to shift, and invasive species may become more of a problem owing to positive response to increased carbon dioxide levels. Increased disturbances may also create opportunities for invasive species to become established. (Logan and Powell 2001, McKenzie et al. 2004, Westerling et al. 2006).

* Q: How did plants and animals adapt to past natural variability?
* A: Plants and animal species responded to historical climate variability through changes in genetic diversity (evolution) and shifts in geographic range (migration). Genetic diversity is affected as a result of climate-driven changes in natural selection, gene flow and breeding patterns, genetic drift (that is, random effects resulting from changes in population size, isolation, and fragmentation), and changes in mutation rates. Evolutionary change in the past commonly increased adaptation in species, but in other situations resulted in decreased adaptation such as loss of fitness, population extirpation (loss of a population but not entire species), and species extinction.
Plants and animal species also responded to past climate change by migrating to favorable environments. For animals, this happens through individuals moving to new locations for breeding and home ranges. For plants, geographic shifts (migration) usually take place through seed and pollen transport. Mortality and population extirpation in parts of a species’ former range often occur. Over time, extirpation and colonization events cumulatively result in shifts of the species’ distribution range. (Davis and Shaw 2001, Delcourt and Delcourt 1991)

* Q: How do changes in land use affect the ability of plants and animals to adapt to climate change now?
* A: Land-use changes, including fragmentation, urban development, altered fire regimes, vegetation management, and introduction of invasive species, often impede the ability of species to respond to climate change adaptively. For instance, many land-use changes impose barriers to species’ migration to favorable new environments; small population sizes and isolation of populations as a result of land-use impede gene flow; invasive species disrupt opportunities for migration; and altered fire regimes destroy corridors for movement and habitat for breeding. (Joyce et al., in press.)

* Q: What are examples of responses that western plant and animal species have had to 20th-century climate change?
* A: Climate change has altered phenology of some western species. For example, a study in central California showed that 70 percent of 23 species of butterfly have advanced their first flight dates by an average of 24 days over a 31-year period. Climate variables explained 85 percent of the variation in this study, with warmer, drier winters leading to early flight. Honeysuckle (Lonicera tatarica & L. korolkowii) in the Western United States has shown earlier mean flowering dates by 3.8 days per decade. In response to a 1.4 ºC rise in local temperatures at the Rocky Mountain Biological Laboratory in Colorado between 1975 and 1999, yellow-bellied marmots (Marmota flaviventris) emerged from hibernation 23 days earlier. However, the flowering plant phenology did not shift in that time period. Thus, the change in marmot behavior shifted the relative phenology of marmots and their food plants.
Species distributions have changed in response to climate change in the Western United States. For example, the northern boundary of the sachem skipper butterfly (Atalopedes campestris Boisduval) has moved from California to Washington State (420 miles) over a 35-year period. Studies showed that winter cold extremes determine the northern range limit.
Climate change in the Western United States has also resulted in population extinctions. In the Great Basin, since being recorded in the 1930s, 7 out of 25 re-censused populations of the pika (Ochotona princeps) were extinct. There is little human disturbance in the high-elevation pika habitat. It was observed that extinct populations were at significantly lower elevations than populations still present. Other experiments showed that adult pikas were killed within 30 minutes at more than 31 ºC. (Beever et al. 2003; Cayan et al. 2001; Crozier 2003, 2004; Forister and Shapiro 2003; Inouye et al. 2000; Parmesan and Galbraith 2004; Smith 1974).

Clean, Renewable, Sustainable, GREEN ENERGY INDEPENDENCE. ORG!