Although the terms global warming and climate change are often used interchangeably, they apply to different phenomena. Climate change refers to changes in Earth's temperature, humidity, air pressure, wind, clouds, and precipitation patterns over time. Global warming is a contributing factor to climate change and refers specifically to the effect of greenhouse gases on Earth's average surface temperature. When discussing rising temperatures caused by greenhouse gases, global warming is an acceptable term. However, climate change is the more accurate term when describing other long-term changes to the planet's weather patterns.

The term global warming first appeared in geochemist Wallace Broecker's 1975 Science magazine article, "Climatic Change: Are We on the Brink of a Pronounced Global Warming?" Scientists began studying the effect of greenhouse gases on Earth's climate more than a century prior, however, as early as 1820. It was during this time that French scientist Joseph Fourier first discovered that Earth's atmosphere functions to retain the sun's heat. In the early 1900s, Serbian astrophysicist Milutin Milankovitch also identified the long-term climate effects of natural fluctuations in Earth's orbit as well as the tilt and precession of its axis. Since then, scientists and policymakers have worked to better understand the workings of the atmosphere, as well as how to respond to challenges created by climate change.

Causes of Climate Change

Earth's atmosphere contains various gases that act as a blanket to trap heat from the sun and prevent it from escaping back into space. This process is known as the greenhouse effect, and the gases are referred to as greenhouse gases. The main greenhouse gases that occur in nature are carbon dioxide, methane, and nitrous oxide. Without the greenhouse effect, the planet would be too cold to support life. Over time, the amount of greenhouse gases trapped in Earth's atmosphere has increased significantly, causing worldwide temperatures to rise.

Natural processes on Earth constantly create and destroy greenhouse gases. The decay of plant and animal matter, for example, produces carbon dioxide, which plants then absorb during photosynthesis. This natural cycle keeps the level of carbon dioxide in the atmosphere fairly stable. Shifts in the planet's crust and changes in ocean patterns impact weather, as do fluctuations in the sun's output of radiation. Volcanic activity also affects the climate because eruptions discharge greenhouse gases and other pollutants into the atmosphere. Climate change scientists at the National Aeronautics and Space Administration (NASA) and other federal and international agencies recognize that these natural factors continue to play a role in climate change but contend that the impact of these factors alone does not explain the substantial rise in Earth's temperature. Natural causes of climate change are referred to as naturogenic, while human-made causes of climate change are referred to as anthropogenic.

Earth's vegetation releases and absorbs more than two hundred billion metric tons of carbon dioxide annually. Human activities, such as the burning of fossil fuels, add an extra seven billion metric tons per year. Over time, these additions have had a dramatic effect on the atmosphere. In the past 150 years, the concentration of carbon dioxide in the atmosphere has risen by more than 30 percent. Deforestation has also played a role in this increase by eliminating forests that would otherwise absorb tons of carbon dioxide.

Increased levels of other greenhouse gases such as nitrous oxide and methane have also resulted from human activities. Several agricultural and industrial activities, such as the use of certain fertilizers in agriculture, produce nitrous oxide. Methane emissions come from the production of fossil fuels, from landfills, and from livestock. These gases may cause even more harm than carbon dioxide, even though less of them exist, because they have a much greater effect per pound on Earth's temperature. Methane, for example, is a greenhouse gas that is twenty-one times as potent as carbon dioxide. Beginning in October 2015, a methane gas leak from a California storage facility vented about five billion cubic feet of gas into the atmosphere. The leak took more than three months to seal and was finally capped on February 18, 2016. The incident constituted the largest accidental discharge of greenhouse gases in US history, releasing the equivalent of the yearly exhaust emissions from 572,000 automobiles.

Humans have created and released greenhouse gases that do not occur in nature. These include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). These gases, released during such industrial processes as aluminum production and electrical transmission, have thousands of times greater effect on the planet's temperature than carbon dioxide.

Climate Change Predictions

Climate science measures changes that occur to a large geographical area over a long period of time, making it difficult to provide definitive answers to climate change questions. However, multiple studies have been conducted since the 1990s to determine how the scientific community collectively views anthropogenic climate change. These studies included surveys as well as analyses of peer-reviewed articles and have concluded that at least 97 percent of actively publishing climate scientists around the world agree that human activities have contributed to rising global temperatures.

Though surveys indicate that many Americans remain skeptical toward arguments and evidence put forth by climate scientists, reports suggest that belief in global warming has grown steadily in the twenty-first century. According to annual polls conducted by Gallup, the public's belief that global warming is caused by human activity, that climate change has begun to take effect, and that global warming will soon pose a serious threat has increased since 2001. Researchers have observed a strong correlation between people's political affiliations and their levels of concern regarding global warming and acceptance of climate science. A total of 64 percent of all Americans polled reported a belief that human activity contributes to global warming in 2018, which included 89 percent of Democrats, 62 percent of independents, and 35 percent of Republicans. Similarly, 91 percent of Democrats responded that they worried significantly about global warming, compared to 62 percent of independents and 33 percent of Republicans. Democrats also reported much higher confidence (86 percent) that scientists have reached a consensus that global warming is real than Republicans (42 percent). Gallup's polling further indicates that people age 55 and older are more likely than younger respondents to believe that the media exaggerates the threat of climate change.

Skeptics of global warming and climate change have noted that Earth has experienced cyclical changes to its climate patterns for millennia and that recent climatic shifts are not as severe as indicated or necessarily a consequence of human activity alone. Climate scientists contend that such skepticism may stem from an unwillingness to face the scope of the threat posed to the planet by human activity. Additionally, conservative donors, including several foundations established by wealthy families, have contributed large amounts of money to organizations that promote climate change denial. Foundations linked to the prominent Koch family, for example, donated over $100 million to such organizations between 1997 and 2015, as reported by the environmental organization Greenpeace.

Even if all members of society agreed that global temperatures are rising and humans are the cause, the challenges created by climate change lack an obvious solution. No climate model formulated by scientists to chart climate patterns has had 100 percent accuracy in predicting changes. However, scientists continue to refine their methods to produce more reliable data. Most climate models failed to predict a slowdown in rising temperatures at the beginning of the twenty-first century. Some predictions have also underestimated threats. In its initial assessment of rising sea levels in 1990, the Intergovernmental Panel on Climate Change (IPCC) originally anticipated a sea level rise of 1.9 millimeters per year from that year onward. However, studies by NASA have revealed that sea levels have in fact risen at a rate of 3.2 millimeters per year. The science of climate change is so complex that some actions that seem helpful may cause damage in the long term. Some of the most potent greenhouse gases, HFCs and PFCs, are commonly used as replacements for other chemicals called chlorofluorocarbons (CFCs), which were phased out between 1989 and 1996 because they damaged the ozone layer. Consequently, the same process that solved one environmental problem—ozone damage—contributed to another.

Effects of Global Warming

The consequences of global warming remain an issue of great debate and uncertainty, and some researchers predict dramatic and serious problems for future generations. Warmer oceans could result in stronger and more frequent hurricanes. As temperatures climb, some regions could experience frequent heat waves and devastating droughts and wildfires. During the 1990s and first decade of the twenty-first century, many areas in the United States endured record-breaking heat and drought. In 2012, severe drought plagued the Wheat Belt of the United States, located in the North American Great Plains. In the beginning of 2013, Australia experienced a heat wave that caused hundreds of wildfires throughout the country. Climate change has also been linked to the severe drought that occurred in California between 2011 and early 2017. In 2018 California has further endured massive wildfires that have led to the displacement of thousands of residents, widespread destruction of property, and the deaths of at least eight people. Scientists have attributed the fires, which included the largest wildfire in California history to date, to the presence of extremely dry vegetation, brought on by rising temperatures, that created conditions that allowed the fires to spread rapidly and burn intensely. California governor Jerry Brown lamented that such fires have become increasingly common in the southwestern United States and warned that fires would likely become more intense as climate change continues.

Many coastal areas around the world could also face severe flooding due to rising sea levels. Low-lying islands in the Pacific Ocean would eventually become uninhabitable. Within the past century, sea level has risen by four to eight inches worldwide. Some of these effects were felt in 2012 when a super storm known as Superstorm Sandy hit the Eastern coast of the United States, and a typhoon in the Philippines claimed the lives of more than one thousand people. The hurricane season of 2017 proved to be the costliest hurricane season since 1900, with severe weather and rising sea levels leading to tragic loss of life and over $215 billion of property damage in Florida, Texas, and Puerto Rico as well as other states in the Southeast and several countries located in Central America and the Caribbean.

Global warming could also have a major impact on habitats. Some areas well suited to farming might become too dry or too wet to support agriculture. Long periods of drought could turn fertile lands into deserts with little vegetation. Plants and animals might not be able to survive the rapid changes caused by global warming and could become extinct. Over the long term, such changes would result in a loss of biodiversity on the planet. Some ecosystems, such as coral reefs and coastal mangrove swamps, are likely to disappear completely.

Furthermore, people would also face serious problems. Loss of farmland, for example, would cause disruptions in the food supply, bringing about famine in many areas. Scientists have noted that various species of disease-carrying mosquitoes have expanded their habitats to areas where they could not have lived before the rise in atmospheric temperature. More frequent and intense heat waves could result in more heat-related deaths, and changes in air quality could also affect human health. A 2018 study in the environmental science journal Nature Climate Change determined that rising temperatures are contributing to increased rates of suicide, anticipating that the trend will continue without a widespread response to climate change that addresses the impact of humans on the environment.

International Response

Economic and political issues inform how governments choose to respond to anthropogenic climate change. To reduce global warming in years to come, nations may need to implement policies with the potential to inhibit their economies. Efforts to further tighten restrictions on greenhouse emissions may include a market-based system such as a cap-and-trade program that limits a firm's total greenhouse gas output but allows a firm to purchase additional emissions credits. Regulations that place higher industry standards on performance and technology provide another way to reduce emissions. Both approaches, however, have the potential to reduce current production capacity, foreign investment, and household purchasing power, as well as lead to higher consumer prices. For these reasons, governments have encountered great difficulty in agreeing on a global plan to deal with Earth's changing climate.

Wealthier countries produce significantly larger amounts of greenhouse gases than poorer countries, thus contributing more to the process of global warming. At the same time, the negative effects of climate change impact developing countries to a greater degree than developed countries. Therefore, many people believe that industrialized nations should take responsibility for reducing emissions of these gases. The leaders of these nations, however, have been resistant to this idea.

Since the 1995, the United Nations has hosted annual conferences to discuss climate change as part of the United Nations Framework Convention on Climate Change (UNFCCC), which was drafted in 1992 and ratified in 1994. In 1997, delegates gathered in Kyoto, Japan, to negotiate an international treaty known as the Kyoto Protocol. This treaty required industrialized countries to reduce their emissions of greenhouse gases by a certain percentage over a five-year period. The treaty was strongly supported by the European Union and other developed countries. The United States opposed the agreement, however, claiming that it could harm the US economy and was unfair because huge emitters such as China and India were not required to cut emissions because they were considered developing countries. As of August 2018, 192 parties, composed of 191 countries and the European Union as a political-economic union, had ratified the Kyoto Protocol. Notable exceptions from the list of signatories include the United States, which has never ratified the treaty, and Canada, which announced its withdrawal from the agreement in 2011. Amendments proposed in 2012, which would expand the protocol and extend it until 2020, have not been put in force because they have not been ratified by a minimum of 144 signatories.

In 2015, world leaders crafted new climate goals at the UNFCCC conference in Paris, France. This new agreement aimed to limit the rise in global temperatures to less than 2 degrees Celsius (approximately 3.6 degrees Fahrenheit) above preindustrial levels and provide countries with the tools needed to best combat climate change. President Barack Obama played a central role in brokering the Paris Agreement and pushed for greater environmental restrictions during his presidency. On November 4, 2016, the Paris Agreement went into effect with the commitment of the United States and seventy-three other parties.

Changing Political Landscape and Withdrawal from the Paris Agreement

In his 2015 State of the Union address, Obama stated that "no challenge poses a greater threat to future generations than climate change." President Obama's successor, Donald Trump, however, holds an opposing view of climate change. Prior to his election, Trump made several public statements casting doubt on the validity of climate science, including a social media post in which he referred to global warming as a hoax perpetrated by the Chinese government to make US manufacturing less competitive. Upon first arriving in office in January 2017, President Trump ordered a media blackout on federal agencies discussing climate change. In June 2017 Trump withdrew the United States from the Paris Agreement, drawing international criticism as well as objections from domestic sources. Shortly after the president's announcement, a bipartisan coalition of governors, tribal leaders, mayors, and business leaders pledged their commitment to work toward the goals of the Paris Agreement by signing on to the We Are Still In declaration. As of August 2018, the coalition included nearly 2,700 signatories, including ten state governors. On the one-year anniversary of Trump's announcement, Bloomberg Philanthropies, led by former New York City mayor Michael Bloomberg, announced that it was directing $70 million toward the American Cities Climate Challenge, in which twenty "leadership cities" would work at an accelerated pace to meet their carbon reduction goals.

During his presidency, Trump has appointed several individuals to cabinet positions and leadership roles in federal agencies known to resist evidence of climate change, including climate skeptics Ryan Zinke as secretary of the interior, Rex Tillerson as secretary of state, and Mike Pompeo as Tillerson's replacement following the former secretary's dismissal. Trump's appointment of Scott Pruitt, who repeatedly filed lawsuits against the Environmental Protection Agency (EPA) while serving as Oklahoma's attorney general, as administrator of the EPA drew sharp criticism from environmentalists. In accordance with Trump's political directives, Pruitt rescinded several policies implemented by the previous administration meant to address climate change, including the Clean Power Plan, which aimed to reduce emissions at coal-burning power plants and promote investment in alternative energy sources. In Pruitt's first year with the agency, over 700 employees resigned from their positions, including 200 scientists, many of whom were replaced by associates of Pruitt with ties to industries that would benefit from weakened environmental protections. Despite public disapproval of Pruitt, Trump welcomed Pruitt's efforts to roll back environmental regulations and praised his job performance often in public. Although Pruitt resigned in July 2018 in the wake of multiple allegations related to abuse of power and waste of taxpayer money, critics warned that the controversial policies implemented under Pruitt would likely continue under his successor Andrew Wheeler, a former lobbyist for the coal and chemical industries.