Global Change: An Overview

Almost 7 billion people now live on Earth. Rapid growth of the human population, especially over the last 300 years, is one of the most remarkable trends in population change ever observed. Demographers project that world population will rise to 9 billion by 2050 and level off somewhere between 9–12 billion people by the end of the century. In many modern societies, more people require more resources, such as crops, seafood, forest products, energy, and minerals and increasingly larger economies to support economic development and rising standards of living. Population growth and the increased demand for natural resources is therefore a major factor driving global environmental change (Figure 1).

The population story is more complex, however, because there is not a simple relationship between the number of people and the amount of resources consumed. Affluence, or the wealth per person, and the social norms of consumption are also important. For example, the populations of China and India are roughly 1.32 and 1.14 billion people, respectively — about four times that of the US. However, the energy consumption per person in the US is six times larger than that of a person in China, and 15 times that of a person in India. Because the demand for resources like energy is often greater in wealthy, developed nations like the US, this means that countries with smaller populations can actually have a greater overall environmental impact. Over much of the past century, the US was the largest greenhouse gas emitter because of high levels of affluence and energy consumption. In 2007, China overtook the US in terms of overall CO₂ emissions as a result of economic development, increasing personal wealth, and the demand for consumer goods, including automobiles.

Worldwide, fossil fuels (oil, coal, and natural gas) dominate our energy consumption, accounting for 85% of all energy used. As mentioned previously, the rapid rise of fossil fuels is a relatively recent phenomenon, developing in the nineteenth century with the discovery of oil and the industrialization of economies, and expanding rapidly in the twentieth century with increased economic development and rising populations and affluence. From 1860–1991, energy use per person rose more than 93 fold compared to a world population increase of four fold, indicating that rising affluence and consumption are driving energy demand (Cohen 1995).

Burning fossil fuels releases about 8.5 billion tons of carbon (as CO₂) into the atmosphere each year, causing its concentration to increase and Earth’s greenhouse warming to strengthen, which leads to rising global air temperatures. Since 1880, average global air temperature has risen approximately 0.9°C. The top five CO₂-emitting countries/regions are China, US, EU, Russia, and India, which together account for two thirds of global emissions.

Global change scientists use climate models to determine how added greenhouse gases affect changes in air temperature and precipitation. If fossil fuel burning continues at current rates, global temperatures may rise by as much as 4°C by the year 2100 (IPCC 2007). Precipitation changes are expected to lead to increased rainfall in mid-to-high latitude regions, but increased droughts are projected for subtropical regions (IPCC 2007).

Landscapes are changing worldwide, as natural land covers like forests, grasslands, and deserts are being converted to human-dominated ecosystems, including cities, agriculture, and forestry (Figure 2). Between 2000–2010, approximately 13 million hectares of land (an area the size of Greece) were converted each year to other land cover types (FAO 2010). Developed regions like the US and Europe experienced significant losses of forest and grassland cover over the past few centuries during phases of economic growth and expansion. More recently, developing nations have experienced similar losses over the past 60 years, with significant forest losses in biologically diverse regions like Southeast Asia, South America, and Western Africa.

Land use changes affect the biosphere in several ways. They often reduce native habitat, making it increasingly difficult for species to survive. Some land use changes, such as deforestation and agriculture, remove native vegetation and diminish carbon uptake by photosynthesis as well as hasten soil decomposition, leading to additional greenhouse gas release. Almost 20% of the global CO₂ released to the atmosphere (1.5–2 billion tons of carbon) is thought to come from deforestation.

One of the byproducts of economic development has been the production of pollution — products and waste materials that are harmful to human and ecological health. The rise of pollution corresponds to the increased use of petroleum in the twentieth century, as new synthetic products such as plastics, pesticides, solvents, and other chemicals, were developed and became central to our lives. Many air pollutants, including nitrogen and sulfur oxides, fine particulates, lead, carbon monoxide, and ground-level ozone come from coal and oil consumption by power plants and automobiles. Heavy metals, such as mercury, lead, cadmium, and arsenic, are produced from mining, the burning of fossil fuels, and the manufacture of certain products like metals, paints, and batteries.

Aquatic ecosystems such as rivers, lakes, and coastal oceans have traditionally been used for pollution disposal from industry and sewage treatment plants, but they have also been subject to unintentional runoff from upland watersheds, such as nitrogen and phosphorus loss from agricultural soils and home septic systems as well as plastics washed into rivers and oceans from storm sewer systems. We often don’t think of nutrients like nitrogen and phosphorus as pollutants. However, humans now add more nitrogen to the biosphere through fertilizers than is added naturally each year by all of the nitrogen-fixing bacteria on the planet (Vitousek 1994). The Pacific and Atlantic oceans now have garbage patches full of plastic that are possibly as large as the continental US. These are strong indicators of global change — humanity now dominates the global movement of nitrogen and other materials on Earth.

Human consumption of animals is impacting species worldwide. Over the past 2000 years, the spread of human societies throughout islands in the Pacific Ocean led to the overhunting of many bird species. As many as 2000 species may have gone extinct, representing 20% of all known bird species, and an extinction rate 100–1000 times greater than natural rate of species loss over geological history (Pimm et al. 1995, Steadman 1995). As a result of industrialized fishing, the populations of many seafood species, including marlins, tunas, swordfish, codfish, sailfish, and sharks have declined 80-90%, pushed to the brink of extinction over the past half century (Baum et al. 2004, Myers & Worm 2003). The rise of the “bushmeat trade” in tropical forests is leading to a decline in threatened and endangered primate species where hunters gain easy access to forests by logging roads.

Climate change is another threat affecting populations. Corals are tropical marine animals related to jellyfish that harbor unicellular photosynthetic algal symbionts, which help the corals acquire energy. The combination of high solar radiation and rising temperatures is causing corals to “bleach,” whereby they expel their algae. Some corals never recover from this kind of disturbance. Bleaching events are increasing in frequency in many important coral systems in the world (Hoegh-Guldberg 1999). Corals and many other marine organisms build calcium carbonate (CaCO₃) shells or external skeletons from seawater. Increased CO₂ in the atmosphere is dissolving into oceans, making seawater more acidic, and causing the CaCO₃ to dissolve. The combined effects of bleaching and ocean acidification have led many marine scientists to predict the significant decline of coral reefs worldwide (Hoegh-Guldberg et al. 2007). The Great Barrier Reef in Australia — the largest biological structure on earth — may be largely dead by 2050 (Figure 3).

Land use changes are diminishing the habitat of species worldwide. In the United States, urbanization, agriculture, tourism, ranching, and roads are leading causes of species endangerment (Czech et al. 2000). The greater prairie chicken in the American Midwest is a classic example. This species lives in native tallgrass prairie ecosystems, which were eliminated as settlers converted them to agricultural fields and cities from 1850–1870. In Illinois, for example, tallgrass prairies declined in abundance from 8.5 million hectares to 985 hectares, supporting only two populations of 46 birds by the 1990s (Westemeier et al. 1998). In heavily deforested tropical regions like Singapore in Southeast Asia, around 40% of native butterfly, bird, fish, and mammal species have gone extinct as a result of deforestation (Brook et al. 2003).

Pollution has caused diverse impacts on species populations. Some chemical pollutants, such as the pesticide DDT, have been discovered to disrupt animal hormone systems, causing reproductive failure in many bird, fish, and reptile species. Nitrogen and phosphorus runoff from agricultural fields travels to lakes and down rivers to coastal oceans, where nutrient-poor waters become fertilized with excess nutrients. This results in a population explosion of phytoplankton, which, when they die, creates an abundant food source for bacteria. As bacteria consume the decaying algae, they also use up the oxygen supply in the water, creating anoxic (no oxygen) or hypoxic (low oxygen) conditions. We call this ecosystem response to nutrient pollution “eutrophication.” Animal species such as fish, crabs, and oysters, which require abundant oxygen to survive, die off under these conditions. Indeed, the mouths of most of the world’s major river systems are now classified as “dead zones” (Diaz and Rosenberg 2008).

Organisms can respond to environmental change by moving to new locations that are physiological suitable. This is a concern with respect to climate warming, as suitable ranges for species are now moving northwards. There is evidence that tree species, such as spruce, have been shifting northwards in latitude and upwards in altitude in response to global warming. Warming is also changing the seasons, with the earlier arrival of spring causing some bird and butterfly species to migrate northwards earlier in the year, as well as some plant species to bloom earlier (Walther et al. 2002)

Human alterations to the environment are causing some species to become evolutionarily adapted to new conditions (Visser 2008). Because people generally harvest large plants and animals that often produce the most offspring, selection favors early reproductive maturity and organisms with smaller body sizes (Darimont et al. 2009). This could lead to lower reproduction, species fitness, and cause populations to decline faster than expected. Other long-term studies of bird populations offer insights into how climate change affects fitness. As temperature warms and spring comes earlier in the year, insect populations (a food source for birds) peak sooner, creating a selective force on bird populations that favors individuals who lay their eggs earlier in the year. Studies testing this hypothesis have found that some bird populations do not adapt egg laying to warming, leading to lower fitness (Visser et al. 1998), whereas other studies have found rapid changes in egg laying that keep up with shifting insect populations (Charmantier et al. 2008). Some scientists argue that many species will not be able to adapt fast enough to new environmental conditions, possibly leading to 15–37% of species going extinct (Thomas et al. 2004).

Climate change and pollution pose a number of potential risks. One of the most commonly cited examples is the potential spread and rise of infectious diseases, such as the mosquito-borne tropical diseases, malaria and Dengue fever. Warming temperatures are expected to increase the geographic range suitable for the mosquito species that transmit these diseases, and some scientists have warned that this could mean a spread of tropical diseases into areas that traditionally do not experience them, such as North America (Kuhn et al. 2005). However, other scientists have argued that the range of malaria actually contracted during twentieth-century warming (Gething et al. 2010). The reason, they argue, is that control measures, such as insecticides, bed nets, and medical treatments, are being deployed effectively to combat the diseases. Exposure to extreme heat is another risk to societies. The heat waves of 1995 in Chicago and 2003 in France caused 700 and 14,800 deaths, respectively. Poor sanitation increases the risk of cholera and other diarrheal diseases where floods contaminate water supplies with untreated sewage. The risk of these diseases rises with the increased occurrence of extreme precipitation events. Other causes of concern include increased respiratory illnesses resulting from ground-level ozone (smog) and fine particulates from vehicle exhaust and indoor cooking fires in urban and suburban areas.

Changing climate has the potential to increase risks from sea level rise, extreme storm events, and drought. About 25% of the world’s population lives with 100 km of the coast. In 2007, the IPCC projected an 18–59 cm sea level rise by 2100, but many scientists argue that this range is too low, and that sea level rise could be as great as 1–6 m (Kopp et al 2009, Jevrejeva et al. 2010). People living in low lying regions, such as Bangladesh and Pacific island nations (e.g., Tuvalu and the Maldives) are already experiencing the effects of salt water incursion in their agricultural fields and fresh water supplies. Arctic Inuit communities are battling the loss of coastal villages as a result of increased storm surges from sea level rise.

Extreme precipitation events may become more common in mid-to-high latitude regions, consistent with the prediction that warmer air temperatures, as a result of climate warming, will increase the moisture content of the atmosphere (IPCC 2007). Besides catastrophic flooding and associated property damage (Figure 4), extreme storms are a concern for infrastructure, with cities now faced with the prospect of significant costs associated with roads, dams, and levees being washed out by floods.

Meanwhile, drought is gripping subtropical regions worldwide, and precipitation is expected to decline in these regions over the twenty-first century (IPCC 2007). The American Southwest has been experiencing drought conditions since 1999, a duration comparable to other severe historical droughts such as the Dust Bowl of the 1930s in the American Great Plains. Warming is also diminishing the snow packs of the Rocky and Sierra Mountains, further decreasing water supplies to major metropolitan areas. Australia has also experienced severe dryness over the past decade. Droughts in climatically sensitive subtropical regions, such as sub Saharan Africa pose risks for food security for a number of African nations.

Global change scientists have also investigated whether tropical cyclones (hurricanes) are becoming more frequent or severe with climate change. Although observational and modeling studies suggest that hurricanes may become stronger in the future as a result of warmer sea surface temperatures, there is little evidence to suggest that they are becoming more common (Knutson et al. 2010).

Food security is the ability of people to have access to sufficient, nutritious food. Although we currently grow enough food to feed the global human population, a population rising to 9 billion by 2050, combined with climate changes, will strain the capacity of some regions to feed people, thereby raising the risks of food insecurity (Godfray et al. 2010).

Water is very important to food security because it is one of the key determinants of crop yield, and changing climate has the potential to alter precipitation around the world. It takes approximately 1000 tons of water to produce a kilogram of grain, and 2–7 kilograms of grain are required per kilogram of livestock weight gain (Brown 2008), suggesting that food production, and meat production in particular, requires substantial inputs of water from precipitation. Where precipitation is insufficient, irrigation, and the diversion of rivers, lakes, or groundwater is required to supplement rainfall.

The world faces several challenges with respect to water and food security. Warming temperatures and decreased precipitation may cause crop yields to decline, particularly in southern Asia and Africa, leading to rising food prices and increased difficulty in gaining access to food (Parry et al 2005, Lobell et al. 2008). In some of the world’s largest grain producing regions, such as the American Southern Great Plains and the North China Plain, groundwater aquifers are being depleted by as much as 10 feet (3 m) per year as irrigation is outpacing recharge by precipitation (Brown 2008). Collapse of these aquifers would impact globally significant grain supplies. Poorly designed irrigation systems can also destroy important aquatic ecosystems, such as the Aral Sea in Asia, which was once one of the largest lakes in the world but has shrunk to 10% of its original size due to water diversion for agricultural irrigation (Figure 5). Population growth and rising affluence in developing nations and the growing demand for meat further exacerbates water demands.