10.1: Natural Environments and Nature-Society Interactions

Learning Objectives

• Classify North American biomes based on climate patterns and physical features.
• Analyze greenhouse gas emissions and the impacts of a warming planet.
• Explain how water resources are influenced by natural and human factors.

 

Biomes

North America has a vast range of biomes—from the Arctic conditions in the far north to the tropical humid forests found in southernmost Florida (Figure 10.1.1). Extending across the far north of the continent, the tundra biome experiences annual temperature ranges from -40°C (-40 °F) and 18°C (64°F) and 150 to 250 millimeters (6 to 10 inches) of rain per year. There are few trees due to the short growing season and permafrost, which is ground that has stayed frozen for at least two continuous years. In the tundra summers, the top layer of soil in permafrost thaws only a few inches down, providing a growing surface for the roots of grass and shrub vegetation. Even though there is little precipitation in the tundra biome, it is usually a wet place because the low temperatures cause evaporation of water to be slow. Much of the arctic has rain and fog in the summers, and water gathers in bogs and ponds. Lichens, mosses, grasses, sedges, and shrubs are commonly found in the tundra (Figure 10.1.2).

 

Figure \(\PageIndex{1}\): This physical map shows the geographic distribution of biomes in North America and prominent physical features, like peaks, rivers, lakes, oceans, plateaus, mountains, and tectonic boundaries (CC BY-NC-SA; Wallace via Flickr).

 

Figure \(\PageIndex{2}\): Photograph of the tundra biome in Nunavut, Canada (CC BY 2.0; ADialla via Wikimedia Commons).

 

Two forest biomes are found south of the tundra, with the grassland biome in the central part of the continent. Coniferous forests—so-called because of the presence of evergreen trees that produce cones and needles—are mostly found between the tundra biome to the north and the deciduous forest to the south. Conifers, such as spruces, pines, and firs, are adapted to survive in areas that are very cold or dry (Figure 10.1.3, left). The temperate deciduous forests have broadleaf trees such as oaks, maples, and beeches. The deciduous forest regions are exposed to warm and cold air masses, which cause this area to have four seasons. The temperature varies widely from season to season with cold winters and hot, wet summers. During the fall, trees change color and then lose their leaves. This is in preparation for the winter season. Because it gets so cold, the trees have adapted to the winter by going into a period of dormancy (Figure 10.1.3, middle). Grasslands are generally open and continuous, fairly flat areas of grass (Figure 10.1.3, right). The height of grass correlates with the amount of rainfall it receives. Grasses vary in size from 2.1 meters (7 feet) tall with roots extending down into the soil 1.8 meters (6 feet), to the short grasses growing to a height of only 20 to 25 centimeters (8 to 10 inches) tall. These short grasses can have roots that extend 1 meter (about 3 feet) deep.

 

Figure \(\PageIndex{3}\): Left: Photograph of coniferous forest in Rocky Mountain National Park, Colorado (CC BY-SA 4.0; Sarbjit Bahga via Wikimedia Commons); Middle: Fall foliage in Maine as trees move into winter dormancy (CC BY 2.0; Kimberly Varderman via Flickr); Right: Grassland in Badlands National Park, South Dakota (CC BY-SA 2.5; Wing-Chi Poon via Wikimedia Commons).

 

The parts of the Mojave, Sonoran, Chihuahua, and Great Basin deserts comprise the desert biome in North America. Deserts get about 250 millimeters (10 inches) of rain per year—the least amount of rain of all of the biomes. The temperature in the desert can change drastically from day to night because the air is so dry that heat escapes rapidly at night. Since desert conditions are so severe, the plants that live there need to have adaptations to compensate for the lack of water. Some plants, such as cacti, store water in their stems and use it very slowly, while others like bushes conserve water by growing few leaves or by having large root systems to gather water. Some desert plant species have a short life cycle of a few weeks that lasts only during periods of rain. The cholla plant uses CAM photosynthesis, an alternative pathway to convert energy from the sun into food. Mesophyll cells in the leaves convert carbon dioxide into organic acids. This allows the cholla to conserve water by keeping the stomata (the traditional pathway for photosynthesis) closed during the day (Figure 10.1.4).

 

Figure \(\PageIndex{4}\): Teddybear or “jumping cholla” (Cylindropuntia bigelovii) grow in the transition zone between the lower, warmer Colorado Desert and the higher, cooler Mojave Desert ecosystems (public domain; USGS).

 

The Mediterranean scrubland extends along the California coast from northern Baja through southern Oregon. It is characterized by a climate of dry, hot summers and wet, cool winters. This region is one of the world’s most biodiverse areas. Thirty percent of its vascular plants (plants that have conducting tissue like ferns, flowering plants, and grasses) are endemic—meaning that they are only found naturally in this region. There were more than 5,000 vascular plants in the Mediterranean shrubland compared to about 20,000 in the entire continental United States; 25 percent of the biodiversity of vascular plants are found in the Mediterranean scrubland, which represents only 5 percent of the area of the continental United States.^[1] Due to habitat loss and other threats, Conservation International identified this region as a global biodiversity hotspot. Named the California Floristic Province, only about one-quarter of its habitats remain intact with less than 90 percent of its wetlands destroyed by development and water diversion for agricultural and urban uses.^[2]

Extending from northeast Mexico through the Gulf and Atlantic coasts to southeastern Massachusetts, the North American Coastal Plain is another global biodiversity hotspot found in North America. Wetlands, deciduous forests, and coastal plains are commonly found in this hotspot. When European settlers colonized the Northern American Coastal Plains, they introduced non-native grasses, trees, and grazing animals, which left less than 15 percent of the native savanna and woodland habitats intact. This gave the appearance that the number of endemic species and that the overall biodiversity was lower than it really is. For example, river habitats in this hotspot have particularly high levels of endemic fish species.^[3]

 

Physical Features

North America is divided into a number of physical regions with distinct landforms (Figure 10.1.5). The western part of the continent is marked by the Rocky Mountains. The eastern portion of North America is defined by the ancient Appalachian Mountains, a mountain range that is much less rugged than the Rockies but with no less influence on the history and development of the United States. The interior of the continent is characterized by the Great Plains. To the north is the Canadian Shield, geologically the oldest part of North America, and a sparsely populated area with poor soils. At the southern and eastern edge of the continent is the Gulf-Atlantic Coastal Plain, a relatively flat zone that extends from New York to Texas.

 

Figure \(\PageIndex{5}\): Physical features of North America with low-elevation areas shaded in green and high-elevation areas shaded in dark grey (adapted from public domain; NASA/JPL).

 

Most of North America, to include Mexico, Greenland, and some of the Caribbean, is situated on the North American plate and is thus relatively geologically stable (Figure 10.1.6). One notable exception, however, is the Juan de Fuca Plate, which is subducting under the North American plate near California and Vancouver Island, an area known as the Cascadia subduction zone. Severe earthquakes, generating tsunamis, have occurred here roughly every 500 years; the last major earthquake in the area was in 1700 CE. Just to the south, the San Andreas Fault running along the edge of California forms the boundary between the Pacific Plate and the North American Plate. This is a transform plate boundary, with the two plates sliding past each other horizontally. San Francisco is located on this fault line and the area has experienced numerous earthquakes.

 

Figure \(\PageIndex{6}\): Plate tectonic map of North America labeled with numbers representing the average annual plate movement in millimeters (CC BY-SA 4.0; Alataristarion via Wikimedia Commons).

 

Climate Change

Earth's climate is changing. Multiple lines of evidence show changes in our weather, oceans, ecosystems, and more. Scientists have long established that natural causes alone cannot explain all of these changes. Human activities contribute to climate change, primarily by releasing billions of tons of carbon dioxide (CO₂) and other heat-trapping gases, known as greenhouse gases, into the atmosphere every year. The more greenhouse gases emitted, the more intense future climate changes will be. In 2020, North America accounted for approximately 21 percent of global carbon dioxide emissions (Figure 10.1.7) while representing less than 5% of the global population. This means that on a per person basis, Americans are among the top emitters of greenhouse gases in the world.

 

Figure \(\PageIndex{7}\): Annual carbon dioxide emissions by world region (CC BY 4.0; Our World in Data).

 

Yet, the impacts of greenhouse gas emissions and global climate change are uneven and vary by region. Climate change is already attributable to changes in precipitation patterns, ice loss, sea-level rise, ocean acidification, extreme weather, and more. In North America, these climate impacts vary geographically. Northern areas are projected to become wetter, especially in the winter and spring. Southern areas, especially the Southwest, are projected to become drier. Heavy precipitation events will likely be more frequent, even in areas where total precipitation is projected to decrease. Heavy downpours that currently occur about once every 20 years are projected to occur between twice and five times as frequently by 2100, depending on location. The proportion of precipitation falling as rain rather than snow is expected to increase, except in far northern areas. Thus, generally speaking, there are two Americas of the climate crisis: a parched west and a soaked east.^[4] Figure 10.1.8 shows how annual temperature and precipitation patterns have changed from 1901 to 2020.

 

Figure \(\PageIndex{8}\): Top maps: Annual U.S. temperature compared to the 20th-century average. Bottom maps: Normal annual U.S. precipitation as a percent of the 20th-century average. (public domain; NOAA).

 

Hurricanes in the Gulf of Mexico and along the east coast are likely to increase in intensity and frequency. Cold-season storm tracks are expected to continue to shift northward with the strongest cold-season storms are projected to become stronger and more frequent. There will be changes to precipitation regimes, as well. Figure 10.1.9 shows the predicted precipitation pattern changes in North America modeled with the “higher emissions” future scenario.

 

Figure \(\PageIndex{9}\): Projected future changes in precipitation for the end of this century, compared with 1970-1999, under a higher emissions scenario. For example, in winter and spring, climate models agree that northern areas in the United States are likely to get wetter and southern areas drier. There is less confidence in exactly where the transition between wetter and drier areas will occur. Confidence in the projected changes is highest in the areas marked with diagonal lines. The changes in white areas are not projected to be larger than what would be expected from natural variability. (public domain; National Climate Assessment).

 

In the Canadian Mackenzie Valley, permafrost temperature has increased over the last four decades by at least 2°C. The Arctic is experiencing steep temperature increases, and warming temperatures over lands with permafrost are projected to continue in all emissions scenarios. This is resulting in a sustained thawing of large areas of permafrost. Canada underlain by deep permafrost is projected to decline by 16 percent to 20 percent by 2090, relative to 1990. This thawing of permafrost will affect the hydrology of the tundra region and release methane into the atmosphere, which would contribute to more warming.

 

Climate Justice

North America, and the United States in particular, has benefited more than others from the industries and technologies that are causing climate change. And at the same time, the countries that have benefited the least are more likely to be suffering first and worst because of climate change (See Africa South of the Sahara, Section 3.5). Wealthy, industrialized nations have released most of the greenhouse gas pollution to date — meaning they’ve played an outsized role in causing climate change. Climate justice calls for these countries, along with multinational corporations that have become wealthy through polluting industries, to pay their “climate debt” to the rest of the world. In this view, stopping their greenhouse gas emissions, while hugely important, is not enough to fully pay the debt from over a century of pollution; these actors also have a responsibility to share wealth, technology, and other benefits of industrialization with the countries least responsible for the climate crisis, to help them cope with the effects of climate change and build clean energy systems and industries.

Climate justice is the principle that the benefits reaped from activities that cause climate change and the burdens of climate change impacts should be distributed fairly. Climate justice means that countries that became wealthy through unrestricted carbon emissions have the greatest responsibility to not only stop warming the planet, but also to help other countries adapt to the adverse effects of climate change and develop economically with nonpolluting technologies.

Climate justice also calls for fairness in environmental decision-making. The principle supports centering populations that are least responsible for and most vulnerable to the climate crisis . It also means acknowledging that climate change threatens basic human rights principles, which hold that all people are born with equal dignity and rights, including to food, water, and other resources needed to support human health. Calling for climate justice, rather than climate action, has implications for policymaking, diplomacy, academic study and activism by bringing attention to how different responses to climate change distribute harms and benefits and who gets a role in forming those responses. It calls for a reflection on universal ethics and differential power.

A climate justice perspective also brings attention to inequalities within countries. Within high and low income countries, wealthier people are more likely to enjoy energy-intensive homes, private cars, leisure travel, and other comforts that both exacerbate climate change and buffer them from impacts like extreme heat. Climate change also worsens pre-existing social inequalities stemming from structural racism, socioeconomic marginalization, and other forms of social exclusion. In the U.S., for example, communities of color and immigrant communities are more likely to be located in places where climate risks are more severe, such as in flood zones or urban heat islands.

Watch the video below to learn about the connection of climate justice to Indigenous perspectives.

 

Water Resources

North America has a number of significant rivers, some of which are used for shipping and hydroelectric power. The longest North American river is the Missouri, which forms in Montana and flows into the Mississippi River. The Mississippi River is largely considered to be the most important waterway in terms of commercial transportation. The Port of South Louisiana, located along the Mississippi, is a commercial hub with the largest tonnage of trade goods flowing in and out of the United States. Canada’s surface water resources are considerable, an estimated 7 percent of the world’s renewable water supply. Approximately 60 percent of the country’s freshwater drains to the north, away from the 85 percent of the population living within 300 kilometers of our southern border.

A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean. Figure 10.1.10 shows one set of watershed boundaries in North America.

 

Figure \(\PageIndex{10}\): North America watershed map (public domain; USGS).

 

Dams

A dam is a constructed barrier along a river designed to hold back the flow of surface water, which creates a reservoir (Figure 10.1.11). There are more than 105,000 dams in North America but only a few percent produce hydroelectricity—most were built for flood control, the creation of reservoirs to hold water supplies, or for irrigation. Decades of human alteration through dams and other infrastructure have profoundly affected a host of hydrological and ecological processes. Because dams block the transport of sediment and nutrients downstream, there are many effects on downstream habitats.

 

Figure \(\PageIndex{11}\): Simple diagram of a dam (adapted from public domain; USGS).

 

Over the past several decades, more than 1,100 dams have been removed nationally due to increasing public concern over their safety, an unwillingness to invest scarce resources in infrastructure repair, and a growing interest in restoring degraded ecosystems.^[5][6] Reasons for dam removal include improving water quality, improving native plant communities, and restoring wetlands.

The Elwha River in Washington state provides a case study of the rationale and benefits of dam removal. For millennia, the Elwha River ran wild, connecting mountains and seas in a thriving ecosystem. The river proved to be an ideal habitat for anadromous (sea-run) fish, with eleven varieties of salmon and trout spawning in its waters. These fish thrived in the cold, clear waters of the Elwha River and historically served as an important food source for the Lower Elwha Klallam Tribe living along its banks. American expansion spurred a continual demand for lumber. The growth of the logging industry in the region brought rapid change to the Olympic Peninsula and specifically to the Elwha River with the construction of two dams. However, construction of the dams blocked the migration of salmon upstream, disrupted the flow of sediment downstream, and flooded the historic homelands and cultural sites of the Lower Elwha Klallam Tribe.

For over a century, the web of ecological and cultural connections in the Elwha Valley were broken - then the river's story changed course. In 1992, Congress passed the Elwha River Ecosystem and Fisheries Restoration Act, authorizing dam removal to restore the altered ecosystem and the native anadromous fisheries therein. After two decades of planning, one of the largest dam removals in U.S. history began on September 17, 2011. Six months later the Elwha Dam was gone, followed by the Glines Canyon Dam in 2014. Today, the Elwha River once again flows freely from its headwaters in the Olympic Mountains to the Strait of Juan de Fuca (Figure 10.1.12).

 

Figure \(\PageIndex{12}\): Aerial photograph of the former Lake Aldwell reservoir and the Elwha River 16 months following the removal of the Elwha Dam, Washington State, USA (public domain; USGS).

 

Megadams and Energy Exports from Canada to the United States

All over Europe and North America big damns are no longer being built given grave environmental and social concerns associated with mega dams. Canada appears to be the exception to that.Large dams are still being built across Canada, from Muskrat Falls in Labrador to the generically titled “Site C” in British Columbia, despite cost overruns, outcry from some First Nations and even environmental concerns from the United Nations (Figure 10.1.13).

Figure \(\PageIndex{13}\): Protest sign that says “Site ‘C’ Sucks” (CC BY 2.0; by The Narwhal Canada via Flickr).

Hydroelectric power already supplies 60 percent of the country’s energy. But the dam building isn’t just to feed Canada’s power needs. It’s also become a hot export commodity. As U.S. states look to meet new clean energy targets, imported low-carbon hydropower from across the northern border has become a larger part of the conversation — and the grid. New England already gets 17 percent of its energy from Canadian hydropower, Midwest states around 12 percent and New York 5 percent. That number is likely to increase. A new transmission project to bring 250 megawatts of Canadian hydropower to the United States just came online in Minnesota. Two more are in the works for Massachusetts and New York.

Proponents say large-scale hydropower can help speed up the transition to clean energy, energy sources that have low or zero carbon emissions. Others caution that it comes with a larger environmental cost compared to wind and solar and could open the floodgates for more dam building. All told, over the life cycle of a project, most hydropower is cleaner than fossil fuels, but not as clean as wind and solar. A study in Nature Energy on the projected life-cycle emissions of energy sources put solar at 6 grams of CO2 equivalent per kilowatt hour and wind at 4. The researchers estimated typical hydro at 97, but there’s great variation between sites.^[7]

There are other environmental considerations beyond greenhouse gas emissions, including habitat fragmentation, construction on contaminated ecosystems, and threats to endangered species. Hydropower may be renewable, but it damages ecosystems and threatens biodiversity. With the aim to maintain the ecosystem integrity and the cultural heritage of the region, a group of Cree and Inuit protestors paddled the Hudson River to Manhattan to ask New Yorkers to oppose a power purchase agreement between the state and Quebec and the construction of a second dam in the James Bay hydroelectric project in northern Quebec in 1990. They were successful. Now, 30 years later, a different group of First Nations is making a similar plea.

On October 7, 2020, the First Nations of Pessamit, Wemotaci, Pikogan, Lac Simon and Kitcisakik sent a letter to the U.S. Department of Energy stating their opposition to the Massachusetts transmission line. The groups wrote that one-third of Hydro-Québec’s installed power is “produced in our respective ancestral territories from reservoirs, dams, power plants and various other installations, without prior consultation, without our consent and without compensation.”^[8] Major hydroelectric projects have altered the flow of rivers and in some cases and the food and cultural resources used by Indigenous communities. Thus, they raise questions about who benefits from such megaprojects, at what cost.

 

Aquifers

Below the surface of North America lies a number of aquifers, or underground layers of permeable rock that hold groundwater in natural reserves. The largest of these aquifers is the Ogallala Aquifer located in the central United States stretching from South Dakota down to Texas. This aquifer supplies water to much of the Great Plains, about one-third of all groundwater used for irrigation in the United States. While aquifers are beneficial for irrigation, they replenish their water supplies relatively slowly through rainfall. The high demands for irrigation waters in the Great Plains means has accelerated the mining of groundwater from the Ogallala Aquifer at rates beyond natural replenishment, resulting in aquifer depletion (Figure 10.1.14). Once all of the water is depleted, it will take around 6,000 years to naturally replenish. This is a grave environmental problem, given the importance of groundwater this vital agricultural region of the United States.

 

Figure \(\PageIndex{14}\): Map of the High Plains aquifer water-level changes, predevelopment (about 1950) to 2015; red areas have water-level declines of more than 150 feet and pink areas have water-level declines of 100 to 150 feet (public domain; USGS).

 

Water is already scarce in the American Southwest, so every drop is a precious resource. People in the Southwest are particularly dependent on surface water supplies like Lake Mead, which are vulnerable to evaporation. Thus, even a small increase in temperature (which drives evaporation) or a decrease in precipitation in this already arid region can seriously threaten natural systems and society. Droughts also contribute to increased pest outbreaks and wildfires, both of which damage local economies.Droughts also reduce the amount of water available for generating electricity—for example, at the Hoover Dam, which supplies roughly 8 million people in Arizona, southern California, and southern Nevada with power. Every part of the Southwest experienced higher average temperatures between 2000 and 2020 than the long-term average (1895–2020). Some areas were more than 2°F warmer than average (Figure 10.1.15).

 

Figure \(\PageIndex{15}\): Map of average air temperature from 2000 to 2020 has differed from the long-term average (1895–2020) (public domain; EPA).

 

Large portions of the Southwest have experienced drought conditions since weekly Drought Monitor records began in 2000. For extended periods from 2002 to 2005 and from 2012 to 2020, nearly the entire region was abnormally dry or even drier (Figure 10.1.16). Extended droughts have severe impacts on water supplies of the American southwest and beyond. In California, a state with high value agricultural crops, farmers are turning to pumping more water from its underground water reserves at rates that can result in aquifer depletion. California also diverts water from the Colorado River, a lifeline of the American southwest for more than 30 million people. Increased diversion from the Colorado by American municipalities means that Mexico, a country facing severe water stress, can no longer depend on the flow on a river. Since 2008, the Colorado River Delta has dried and its waters no longer reach the Gulf of California, like it had for millions of years.

 

Figure \(\PageIndex{16}\): Percentage of land area in six southwestern states (Arizona, California, Colorado, Nevada, New Mexico, and Utah) classified under drought conditions from 2000 through 2020 (public domain; EPA).

 

 

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References:

[1] Mooney, Harold A. 1988. Lessons from Mediterranean-Climate regions. In Biodiversity, E.O. Wilson, F.M. Peter (Eds.). National Academies Press.

[2] Conservation International. 2022. California Floristic Province.

[3] Conservation International. 2022. North American Coastal Plain.

[4] Popovich, N. and Bhatia, A. (2021). These maps tell the story of two Americas: one parched, one soaked. The New York Times.

[5] O’Connor J.E., Duda J.J., Grant G.E. 2015. 1000 dams down and counting. Science, 348: 496–497.

[6] Service R.F. 2011. Will busting dams boost salmon? Science, 334: 888–892.

[7] Pehl, M., A. Arvesen, F. Humpenöder, A. Popp, E.G. Hertwich, & G. Luderer. 2017. Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling. Nature Energy, 2: 939-945.

[8] ICT. 2020, October 8. Opposition to the New England clean energy connect electricity transmission line project to Massachusetts.

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Attributions:

“Biomes and Physical Features” is adapted from World Regional Geography by Royal Berglee (CC BY-NC-SA 3.0); World Regional Geography by Caitlin Finlayson (CC BY-NC-SA 3.0); Introduction to World Regional Geography by R. Adam Dastrup (CC BY-NC-SA 3.0); and Mission: Biomes by the NASA Earth Observatory (public domain).

“Climate change” is adapted from Climate Change Science and Climate Change Indicators by the U.S. Environmental Protection Agency (public domain); Canada’s Changing Climate Report by E. Bush and D.S. Lemmen (editors) for the Government of Canada (public domain); and Climate Justice by Mariana Arcaya and Elizabeth Gribkoff (editors) for the Climate Portal (CC BY-NA-SA 4.0).

“Water Resources” is adapted from World Regional Geography by Caitlin Finlayson (CC BY-NC-SA 3.0); Hydrology of Canada by the government of Canada (public domain); River restoration by dam removal: Enhancing connectivity at watershed scales by F.J. Magilligan et al. (CC BY 4.0); Elwha River Restoration by the National Park Service (public domain); and Promise or Peril? Importing Hydropower to Fuel the Clean Energy Transition by Tara Lohan (CC BY-SA 2.0).