14.2: Climate Change and Global Warming

“Climate change” and “global warming” are often used interchangeably but have distinct meanings. Global warming is the long-term heating of Earth’s climate system observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere. Climate change is long-term changes in the average weather patterns that have come to define Earth’s local, regional and global climates.

Global Warming: The Greenhouse Effect

Imagine stepping into a greenhouse. The temperature inside and outside of a greenhouse can be very different. Outside of a greenhouse you can feel the heat from the sun’s rays, but the heat can escape to the atmosphere, however, inside the greenhouse, the sun’s rays are turned to heat and the heat is trapped by the greenhouse. The way a greenhouse traps heat making it warmer inside than outside, is similar to how the greenhouse gases in the Earths atmosphere, such as CO2, trap heat and increase the temperature of the planet.

Earth is surrounded by the earth’s atmosphere, which is an envelope of gases surrounding the earth. The sun provides solar radiation to our planet that passes through the earths atmosphere. When the Sun’s solar rays hit our planet the solar radiation turns to heat. Gases in the atmosphere, such as carbon dioxide, nitrous oxide, methane, and water vapor, block heat from escaping, thus trapping in heat inside our atmosphere.

The trapping of heat is called the Greenhouse effect. Global warming refers to the recent and ongoing global average increase in temperature near the Earth’s surface. Scientists attribute the global warming trend observed since the mid-20^th century to the “greenhouse effect.”

Figure \(\PageIndex{1}\): The Greenhouse effect

Evidence/Indicators of Climate Change

A picture is worth 1,000 words

Nasa has provided a library of “Images of Change” that show show a before and after comparison of land masses, glaciers, oceans, etc to depict how climate change is impacting the world around you. Some of the pictures show more recent changes, for example how the drought has changed Lake Powell, others show changes over time, such as the Muir Glacier Melt in Alaska.

Figure \(\PageIndex{2}\): Muir Glacier Melt, Alaska. 1941 compared to 2004

The Earth’s climate is changing. Observations from around the world show the widespread effects of increasing greenhouse gas concentrations on Earth’s climate. For example:

• High temperature extremes, droughts, and heavy precipitation events are becoming more frequent and intense.
• Glaciers and snow cover are shrinking, and sea ice is retreating.
• Seas are warming, rising, and becoming more acidic, and marine species are moving to new locations in colder waters.
• Flooding is becoming more frequent along the U.S. coastline.
• Growing seasons are lengthening, and areas burned by wildfire are growing.

The Environmental Protection Agency has compiled a key set of indicators^[1] related to the causes and effects of climate change. The climate change indicators provide important information on how and why the climate is changing. These indicators include: greenhouse gases, weather and climate, oceans, snow and ice, health and society, and ecosystem

Greenhouse gases

Ice cores^[2] are like frozen time capsules that allow scientists to reconstruct climate far into the past. Using ice cores, scientists have been able to measure greenhouse gas emissions from over 800,000 years ago. Ice cores provide a direct measurement of greenhouse gases thus are considered a gold standard. As greenhouse gas emissions from human activities increase, they build up in the atmosphere and warm the climate, leading to many other worldwide changes in the atmosphere, on land, and in the oceans. Greenhouse gases from human activities are the most significant driver of observed climate change since the mid-20th century.

Summary of key points:

• In the United States, greenhouse gas emissions caused by human activities increased by 2 percent from 1990 to 2019. Since 2005, however, total U.S. greenhouse gas emissions have decreased by 12 percent. Carbon dioxide accounts for most of the nation’s emissions and most of the increase since 1990. Transportation is the largest source of greenhouse gas emissions in the United States, followed by electricity generation. Emissions per person have decreased slightly in the last few years.
• Worldwide, net emissions of greenhouse gases from human activities increased by 43 percent from 1990 to 2015. Emissions of carbon dioxide, which account for about three-fourths of total emissions, increased by 51 percent over this period. As with the United States, the majority of the world’s emissions result from transportation, electricity generation, and other forms of energy production and use.
• Concentrations of carbon dioxide and other greenhouse gases in the atmosphere have increased since the beginning of the industrial era. Almost all of this increase is attributable to human activities. Historical measurements show that the current global atmospheric concentrations of carbon dioxide are unprecedented compared with the past 800,000 years, even after accounting for natural fluctuations.
• From 1990 to 2019, the total warming effect from greenhouse gases added by humans to the Earth’s atmosphere increased by 45 percent. The warming effect associated with carbon dioxide alone increased by 36 percent.

Figure \(\PageIndex{3}\): Global Atmospheric Concentrations of Greenhouse Gases

Weather and climate

Rising global average temperature is associated with widespread changes in weather patterns. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere “behaves” over relatively long periods of time.

Summary of Key Points

• Average temperatures have risen across the contiguous 48 states since 1901, with an increased rate of warming over the past 30 years.
• Eight of the top 10 warmest years on record have occurred since 1998. All of the top 10 warmest years on record worldwide have occurred since 2005.
• Since 1896, average winter temperatures across the contiguous 48 states have increased by nearly 3°F. Spring temperatures have increased by about 2°F, while summer and fall temperatures have increased by 1.4°F.
• Record-setting daily high temperatures have become more common than record lows. Since the 1970s, unusually hot summer days (highs) and unusually hot summer nights (lows) have become more common over the last few decades. Unusually cold winter temperatures have become less common—particularly very cold nights (lows).
• Heat waves are occurring three times more often than they did in the 1960s—about six per year compared with two per year. The average heat wave season is 47 days longer, and individual heat waves are lasting longer and becoming more intense.
• Since 1901, precipitation has increased at an average rate of 0.1 inches per decade over land areas worldwide. However, shifting weather patterns have caused certain areas, such as the Southwest, to experience less precipitation than usual.
• The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s but has risen substantially since then. Nationwide, nine of the top 10 years for extreme one-day precipitation events have occurred since 1996.
• Floods have generally become larger across parts of the Northeast and Midwest and smaller in the West, southern Appalachia, and northern Michigan. Large floods have become more frequent across the Northeast, Pacific Northwest, and parts of the northern Great Plains, and less frequent in the Southwest and the Rockies.
• Drought trends vary by region, as the West has generally experienced more drought while the Midwest and Northeast have become wetter.
• The southwestern United States is particularly sensitive to changes in temperature and thus vulnerable to drought, as even a small decrease in water availability in this already arid region can stress natural systems and further threaten water supplies. Several measures indicate persistent and more severe drought conditions in recent years.

Oceans

Covering about 70 percent of the Earth’s surface, the world’s oceans have a two-way relationship with weather and climate. The oceans influence the weather on local to global scales, while changes in climate can fundamentally alter many properties of the oceans.

Summary of Key Points

• The amount of heat stored in the ocean has increased substantially since the 1950s. Ocean heat content not only determines sea surface temperature, but also affects sea level and currents.
• Sea surface temperatures have been consistently higher during the past three decades than at any other time since reliable observations began in the late 1800s.
• When averaged over all of the world’s oceans, sea level has risen at a rate of roughly six-tenths of an inch per decade since 1880. Changes in sea level relative to the land vary by region. Along the U.S. coastline, sea level has risen the most along the Mid-Atlantic coast and parts of the Gulf coast, where several stations registered increases of more than 8 inches between 1960 and 2020. Sea level has decreased relative to the land in parts of Alaska and the Pacific Northwest.
• As sea level rises, dry land and wetlands can turn into open water. Along many parts of the Atlantic coast, this problem is made worse by low elevations and land that is already sinking. Between 1996 and 2011, the coastline from Florida to New York lost more land than it gained.
• Flooding is becoming more frequent along the U.S. coastline as sea level rises. Every site measured has experienced an increase in coastal flooding since the 1950s. The rate is accelerating at most locations along the East and Gulf coasts. The East Coast suffers the most frequent coastal flooding and has generally experienced the largest increases in the number of flood days.
• The ocean has become more acidic over the past few decades because of increased levels of atmospheric carbon dioxide, which dissolves in the water. Higher acidity affects the balance of minerals in the water, which can make it more difficult for certain marine animals to build their protective skeletons or shells.

Snow and Ice

The Earth’s surface contains many forms of snow and ice, including sea, lake, and river ice; snow cover; glaciers, ice caps, and ice sheets; and frozen ground. Climate change can dramatically alter the Earth’s snow- and ice-covered areas because snow and ice can easily change between solid and liquid states in response to relatively minor changes in temperature.

Summary of Key Points

• The part of the Arctic Ocean covered by ice is typically smallest in September, after the summer melting season. Arctic sea ice has decreased over time, and in September 2020 it was the second smallest ever recorded. The length of the melt season for Arctic ice has grown, and the ice has also become thinner, which makes it more vulnerable to further melting.
• Antarctic sea ice has increased slightly overall since 1979, though it has decreased in the last few years. Slight increases in Antarctic sea ice are outweighed by the loss of sea ice in the Arctic during the same time period.
• Since 1992, the giant ice sheets that cover Greenland and Antarctica have each lost more than 100 billion metric tons of ice per year on average. The total amount of ice lost from 1992 to 2018 was enough to raise sea level worldwide by an average of roughly seven-tenths of an inch.
• Glaciers in the United States and around the world have generally shrunk since the 1960s, and the rate at which glaciers are melting has accelerated over the last decade.
• Lakes in the northern United States are freezing later and thawing earlier compared with the 1800s and early 1900s.
• Since the early 1970s, all five of the Great Lakes have experienced a long-term decrease in the maximum area that freezes each year, but the decrease is only statistically meaningful in one lake (Superior). The number of frozen days per year has also decreased for all five lakes since the early 1970s.
• Total snowfall has decreased in most parts of the country since widespread records began in 1930. One reason for this decline is that nearly 80 percent of the locations studied have seen more winter precipitation fall in the form of rain instead of snow.
• Between 1972 and 2020, the average portion of North America covered by snow decreased at a rate of about 1,870 square miles per year, based on weekly measurements taken throughout the year. The length of time when snow covers the ground has become shorter by nearly two weeks since 1972, on average.
• The amount of snow on the ground (snowpack) in early spring decreased at 86 percent of measurement sites in the western United States between 1955 and 2020 and snowpack at all sites declined by an average of 19 percent during this time period.
• About 80 percent of Alaska’s land is underlain by permafrost, which refers to rock or soil with ice that stays frozen for two or more years. Between 1978 and 2020, permafrost temperatures increased at 14 out of 15 long-term monitoring sites in Alaska.
• The number of days per year with unfrozen ground has increased at an average rate of about four days per decade in both the contiguous 48 states and Alaska. Unfrozen days have generally increased across North America, with some variability by region.

Health and Society

Changes in the Earth’s climate can affect public health, agriculture, water supplies, energy production and use, land use and development, and recreation.

Summary of Key Points

• Since 1979, more than 11,000 Americans were reported to have died as a direct result of heat-related illnesses such as heat stroke, and even more died where heat was a contributing factor.
• From 2001 to 2010, a total of about 28,000 heat-related hospitalizations were recorded across 20 states.
• Since 1979, more than 19,000 Americans were reported to have died from exposure to cold temperatures, and even more died where exposure to cold was a contributing factor.
• As the U.S. climate has warmed in recent years Americans in the North and West are using less energy for heating and more energy for air conditioning, while much of the Southeast has experienced the opposite results.
• Since 1973, the average amount of electricity used by Americans at home during the summer has nearly doubled, but it appears to have leveled somewhat in recent years. Conversely, the average American’s winter use of natural gas (the most common home heating fuel) has decreased since 1974.
• Nationwide, the rate of reported cases of Lyme disease has nearly doubled since 1991. The number and distribution of reported cases of Lyme disease have increased in the Northeast and upper Midwest over time, where some states now report 50 to 110 more cases of Lyme disease per 100,000 people than they did in 1991.
• The length of the growing season for crops and other plants has increased in almost every state. States in the Southwest (e.g., Arizona and California) have seen the most dramatic increase. In contrast, the growing season has become slightly shorter in two states (Alabama and Georgia). The observed changes reflect earlier spring warming as well as later arrival of fall frosts. The length of the growing season has increased more rapidly in the West than in the East.
• Since the late 1940s, the annual number of growing degree days increased at 75 percent of the locations measured across the contiguous 48 states. The average change was a 9 percent increase. The largest increases occurred in the West and the Northeast.
• The length of ragweed pollen season has increased at 10 out of 11 locations studied in the central United States and Canada since 1995 allowing ragweed plants to produce pollen later into the year, potentially prolonging the allergy season for millions of people.

Ecosystems

Ecosystems provide humans with food, clean water, and a variety of other services that can be affected by climate change.

Summary of Key Points

• Since 1983, the United States has had on average 70,000 recorded wildfires per year. The top 10 years with the largest acreage burned since 1983, have all occurred since 2004 with many of the largest increases occurring in western states.
• Over the past 79 years, minimum, maximum, and average streamflows have changed in many parts of the country—some higher, some lower. Among rivers and streams strongly influenced by snowmelt, peak flow from winter-spring runoff is happening at least five days earlier than it did in the mid-20th century.
• From 1960 through 2014, water temperature increased at 79 percent of the stream sites measured in the Chesapeake Bay region.
• Between 1960 and 2020, water temperatures increased by 1.9°F in the Snake River at a site on tribal lands in eastern Washington. Several species of salmon use the Snake River to migrate and spawn, and these salmon play an important role in the diet, culture, religion, and economy of the region’s Native Americans.
• Since 1985, summer surface water temperatures have increased in 32 of the 34 lakes with long-term records that this indicator tracks across North America.
• Water levels in most of the Great Lakes appear to have declined slightly over the last few decades.
• Long-term studies have found that bird species in North America have shifted their wintering grounds northward by an average of more than 40 miles since 1966, with several species shifting by hundreds of miles. Some birds shift their range or alter their migration habits to adapt to changes in temperature or other environmental conditions.
• Shifts in location have occurred among several economically important fish and shellfish species that have moved about 20 miles, and 21 feet deeper in the water, between 1982 and 2018. For example, American lobster, black sea bass, and red hake in the Northeast have moved northward by an average of 113 miles.
• Leaf growth and flower blooms are examples of natural events whose timing can be influenced by climate change. In general, leaf and bloom events are happening earlier throughout the North and West but later in much of the South. for example, Cherry Blossom Bloom Dates in Washington, D.C. Peak bloom dates of the iconic cherry trees in Washington, D.C., recorded since the 1920s, indicate that cherry trees are blooming slightly earlier than in the past. Bloom dates are key to planning the Cherry Blossom Festival, one of the region’s most popular spring attractions.

Figure \(\PageIndex{3}\): Ten Indicators of a Warming World

Exercises: Data, data, and more data

Are you curious how the U.S. compares to other countries in the various elements of climate change? Do you like looking at data and graphs? If so, check out “Our World in Data” to view extensive data and graphs on climate change for the U.S. and around the world.

Impacts of Climate Change on Human Health

Climate change is a significant threat to the health of the American people and every American is vulnerable to the health impacts associated with climate change. The U.S. Global Change Research Program’s (USGCRP’s) National Climate Assessment (NCA) process has published a scientific assessment on the impact of climate change on human health^[3].

With climate change, the frequency, severity, duration, and location of weather and climate phenomena, like rising temperatures, heavy rains and droughts, and some other kinds of severe weather, are changing. This means that areas already experiencing health-threatening weather and climate phenomena are likely to experience worsening impacts, it also means that some locations will experience new climate-related health threats. Climate changes that impact human health include: wildfires, heat waves, drought, cold waves and winter storms, sea level changes, hurricanes, floods, extreme precipitation, and rising temperatures.

Figure \(\PageIndex{4}\): Major U.S. national and regional climate trends

Changes in the environment may have numerous impacts to existing health conditions and the development of new diseases.

Table \(\PageIndex{1}\): Health Risks Associated with Climate Change
Source: Adapted from Impacts of Global Climate Change on Health: Climate Change and Health

Health Conditions	Possible Influences of Climate Change
ALZHEIMER’S DISEASE	Persons with cognitive impairments are vulnerable to extreme weather events that require evacuation or other emergency responses.
ASTHMA	Asthma is exacerbated by changes in pollen season and allergenicity and in exposures to air pollutants affected by changes in temperature, humidity, and wind.
CHRONIC OBSTRUCTIVE PULMONARY DISEASE ( COPD )	COPD patients are more sensitive than the general population to changes in ambient air quality associated with climate change.
DIABETES	Diabetes increases sensitivity to heat stress; medication and dietary needs may increase vulnerability during and after extreme weather events.
CARDIOVASCULAR DISEASE	Cardiovascular disease increases sensitivity to heat stress.
MENTAL ILLNESS	Mental illness may impair responses to extreme events ; certain medications increase sensitivity to heat stress.
OBESITY	Obesity increases sensitivity to high ambient temperatures.
DISABILITY	Persons with disabilities may find it hard to respond when evacuation is required and when there is no available means of transportation or easy exit from residences.

The following sections provide key findings from the “The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.” In the official report, each key finding is notated with a level of confidence, from medium to high levels of confidence based on scientific analyses.

Temperature-Related Death and Illness

Key findings:

• By the end of the century, an increase of thousands to tens of thousands of premature heat-related deaths in the summer and a decrease of premature cold-related deaths in the winter are projected each year as a result of climate change.
• Days that are hotter than usual in the summer or colder than usual in the winter are both associated with increased illness and death.
• Some populations are more vulnerable to death or illness due to high temperatures such as children, older adults, people working outdoors, the socially isolated and economically disadvantaged, those with chronic illnesses, as well as some communities of color.

Air Quality Impacts

Key Findings

• Changes in the climate affect the air we breathe, both indoors and outdoors.
• Climate-driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms
• Changes in climate, specifically rising temperatures, altered precipitation patterns, and increasing concentrations of atmospheric carbon dioxide, are expected to contribute to increases in the levels of some airborne allergens and associated increases in asthma episodes and other allergic illnesses .
• Climate change is projected to increase the number and severity of naturally occurring wildfires in parts of the United States, increasing emissions of particulate matter and ozone precursors and resulting in additional adverse health outcomes.

Extreme Events

Key Findings

• Extreme weather events can cause death or injury during an event (for example, drowning during floods), however health impacts can also occur before or after an extreme event, as individuals may be involved in activities that put their health at risk, such as disaster preparation and post-event cleanup.
• Climate change will increase exposure risk in some regions of the United States due to projected increases in the frequency and/or intensity of drought, wildfires, and flooding related to extreme precipitation and hurricanes.
• Many types of extreme events related to climate change cause disruption of infrastructure, including power, water, transportation, and communication systems, that are essential to maintaining access to health care and emergency response services and safeguarding human health.
• Climate change will increase exposure risk to coastal flooding due to increases in extreme precipitation and in hurricane intensity and rainfall rates, as well as sea level rise and the resulting increases in storm surge

Vector-Borne Diseases

Key Findings

• Vector-borne diseases are illnesses that are transmitted by vectors, which include mosquitoes, ticks, and fleas.
• Climate change is expected to alter the geographic and seasonal distributions of existing vectors and vector-borne diseases.
• Ticks capable of carrying the bacteria that cause Lyme disease and other pathogens will show earlier seasonal activity and a generally northward expansion in response to increasing temperatures associated with climate change. Longer seasonal activity and expanding geographic range of these ticks will increase the risk of human exposure to ticks.
• Alterations in the distribution, abundance, and infection rate of mosquitoes will influence human exposure to bites from infected mosquitoes, which is expected to alter risk for human disease.
• Vector-borne pathogens are expected to emerge or reemerge due to the interactions of climate factors with many other drivers, such as changing land-use patterns.

Water-Related Illness

Key Findings

• Increases in water temperatures associated with climate change will alter the seasonal windows of growth and the geographic range of suitable habitat for freshwater toxin-producing harmful algae, certain naturally occurring Vibrio bacteria, and marine toxin-producing harmful algae. These changes will increase the risk of exposure to waterborne pathogens and algal toxins that can cause a variety of illnesses.
• Runoff from more frequent and intense extreme precipitation events will increasingly compromise recreational waters, shellfish harvesting waters, and sources of drinking water through increased introduction of pathogens and prevalence of toxic algal blooms. As a result, the risk of human exposure to agents of water-related illness will increase.
• Increases in some extreme weather events and storm surges will increase the risk that infrastructure for drinking water, wastewater, and stormwater, will fail due to either damage or exceedance of system capacity, especially in areas with aging infrastructure. As a result, the risk of exposure to water-related pathogens, chemicals, and algal toxins will increase in recreational and shellfish harvesting waters, and in drinking water where treatment barriers break down.

Food Safety, Nutrition, and Distribution

Key Findings

• Climate change, including rising temperatures and changes in weather extremes, is expected to increase the exposure of food to certain pathogens and toxins. This will increase the risk of negative health impacts
• Elevated sea surface temperatures will lead to greater accumulation of mercury in seafood, while increases in extreme weather events will introduce contaminants into the food chain. Rising carbon dioxide concentrations and climate change will alter incidence and distribution of pests, parasites, and microbes, leading to increases in the use of pesticides and veterinary drugs
• The nutritional value of agriculturally important food crops, such as wheat and rice, will decrease as rising levels of atmospheric carbon dioxide continue to reduce the concentrations of protein and essential minerals in most plant species.
• Increases in the frequency or intensity of some extreme weather events associated with climate change will increase disruptions of food distribution by damaging existing infrastructure or slowing food shipments. These impediments lead to increased risk for food damage, spoilage, or contamination, which will limit availability of and access to safe and nutritious food depending on the extent of disruption and the resilience of food distribution infrastructure.

Mental Health and Well-Being

Key Findings

• Many people exposed to climate-related or weather-related disasters experience stress and serious mental health consequences.
• Specific groups of people are at higher risk for distress and other adverse mental health consequences from exposure to climate-related or weather-related disasters. These groups include children, the elderly, women (especially pregnant and post-partum women), people with preexisting mental illness, the economically disadvantaged, the homeless, and first responders
• Many people will experience adverse mental health outcomes and social impacts from the threat of climate change, the perceived direct experience of climate change, and changes to one’s local environment.
• Increases in extreme heat will increase the risk of disease and death for people with mental illness, including elderly populations and those taking prescription medications that impair the body’s ability to regulate temperature.

Populations of Concern

Key Findings

• Across the United States, people and communities differ in their exposure, their inherent sensitivity, and their adaptive capacity to respond to and cope with climate change related health threats [Very High Confidence]. Vulnerability to climate change varies across time and location, across communities, and among individuals within communities.
• People experience different inherent sensitivities to the impacts of climate change at different ages and life stages. For example, the very young and the very old are particularly sensitive to climate-related health impacts.
• Climate change threatens the health of people and communities by affecting exposure, sensitivity, and adaptive capacity. Social determinants of health, such as those related to socioeconomic factors and health disparities, may amplify, moderate, or otherwise influence climate-related health effects, particularly when these factors occur simultaneously or close in time or space.
• The use of geographic data and tools allows for more sophisticated mapping of risk factors and social vulnerabilities to identify and protect specific locations and groups of people