Global Warming

Global warming is the term applied specifically to indicate rising average global air temperatures. This rise in temperature has the potential to cause drastic changes in climate and weather patterns worldwide by disrupting the equilibrium between incoming solar energy and the thermal energy that is reradiated away from the surface. As part of a worldwide trend of human-driven climate change, global warming has been widely identified as a serious threat to human communities and the entire natural environment.

Overview

"Global warming" is the term for the rise in Earth's average atmospheric temperature. It is known that Earth's average global temperature has been rising slowly over time, a trend that has been particularly noted since the beginning of the Industrial Revolution in the second half of the eighteenth century. It is generally conceded that increases in air temperature could alter precipitation patterns, change growing seasons, result in coastal flooding, and turn some currently fertile areas into deserts. Although the global climate is an exceedingly complex system reliant on many factors, including long-term cycles and natural events, there is a strong scientific consensus that human activity is responsible at least in part for global warming and related climate change.

When the ground surface is heated by sunlight, it emits much of the solar energy it has absorbed as infrared radiation. Gases in the atmosphere absorb the reradiated infrared radiation and prevent it from escaping into space. This is called the "greenhouse effect" because it was once believed that the glass panes of greenhouses captured the infrared radiation given off by the soil inside the greenhouse. Although it has since been shown that greenhouses work by trapping heated air and not allowing it to blow away, the name has stuck.

The atmosphere eventually releases its contained heat into space and the amount of heat stored in the atmosphere remains fairly constant as long as the composition of gases in the atmosphere does not change. Some gases, including carbon dioxide, water vapor, and methane, capture and store infrared radiation, or heat, more efficiently than others. These are called "greenhouse gases." If the composition of the atmosphere changes to include more of these greenhouse gases, the atmosphere retains more heat and becomes warmer.

Global levels of greenhouse gases have been steadily increasing. In 1990 they were more than 14 percent higher than in 1960. According to the National Oceanic and Atmospheric Administration (NOAA), which measures long-lived greenhouse gases and publishes an Annual Greenhouse Gas Index (AGGI), greenhouse gases increased 45 percent from 1990 to 2019, and of that, carbon dioxide accounted for the largest percentage of the increase. These numbers, according to the NOAA, were directly proportional to a similar rise in average global temperature. Meteorological records show that from 1975 to 2017, the average global temperature rose at a rate of 1.5–1.8 degrees Celsius per century. In comparison, the difference between the average global temperature in the 1990s and in the last ice age was approximately 10 degrees Celsius, and it is estimated that a drop of as little as 4 to 5 degrees Celsius could trigger the formation of continental glaciers. Therefore, the rise in average temperatures is significant and already appears to be causing changes in the global climate. Documented changes include the net loss of ice due to melting glaciers and slowly rising sea levels as the ocean gets warmer and its waters expand. Also, measurements of plant activity indicate that the annual growing season has become approximately two weeks longer in the middle latitude regions.

Such observations led to increasing scientific attention to global warming and its potential disruptive effects. In the 2010s researchers warned that human-induced climate change was accelerating global warming to unprecedented levels. The year 2014 broke heat records but was quickly surpassed by 2015, which showed an exceptional average temperature increase over the previous year. In early 2016, climate scientists from organizations throughout the world (including NASA and NOAA) confirmed that 2015 had been the hottest year on historical record to that point, and that almost all of the ten warmest years on record had occurred since 2000. Along with direct human-induced causes, scientists also attributed this extreme rise in temperature in part to the El Niño weather pattern, which was one of the largest seen in one hundred years. El Niño's contribution to global warming continued into 2016, which in turn overtook 2015 as the hottest year on record.

According to NOAA and other observers, the warming trend continued over the next several years. Although 2016 for a time remained the hottest year on record, each of the next six years was still ranked in the top ten. Some scientists did consider 2020 to match the 2016 record, despite a significant but short-lived decrease in greenhouse emissions due to the outbreak of the COVID-19 pandemic. At that point, NOAA also stated that for forty-four years in a row, the land and ocean temperatures worldwide had exceeded the average for the twentieth century. The year 2023 then broke the record for hottest ever, with NOAA, the World Meteorological Organization, and other experts noting that it did so by an especially large margin.

Effects of Global Warming

A common misunderstanding is that global warming simply means that winters will be less cold and summers will be hotter, while everything else will be basically the same. However, Earth's global weather system is very complex, and scientists stress that higher global temperatures will result in many significant changes in climate and in weather patterns, including some that might seem counterintuitive. For example, global warming could result in shorter but colder winters in some places.

This complexity makes it difficult to predict precisely what effects will occur, but observation of the changing climate and scientific studies allow researchers to make some estimates and generalizations. Summers will be hotter, with more severe heat waves. Because hot air holds more moisture than cool air, rain will fall less frequently in the summer. Droughts can be expected to be more common and more severe. This is a particularly troubling problem in areas where homes are typically built without air conditioning. Heat waves are particularly dangerous for people who are very young, older people, and people who are sick. More frequent heat waves will cause increases in the use of air conditioning, which requires more energy consumption that will in all likelihood lead to the release of additional greenhouse gases unless renewable energy sources are more widely adopted.

Global warming also likely helps produce more severe weather patterns and extreme weather events in general. A 2023 report from the American Meteorological Society (AMS) described how climate change and rising temperatures helped fuel increasingly frequent instances extreme flooding, drought, and heat waves in previous decades. The AMS researchers recommended further reductions in greenhouse gas emissions as a way to slow or reverse this trend.

Another major impact of global warming is ocean level rise, both because water expands as it becomes warmer and because of the additional water that enters the oceans due to melting glaciers on Greenland and Antarctica. These terrestrial icecaps are as much as 4 kilometers thick and lie atop land rather than ocean water. In 2020 NASA scientists reported that data acquired from a satellite using advanced laser technology indicated that ice loss from both Antarctica and Greenland had contributed 14 millimeters of sea level rise over sixteen years beginning in 2003. (The melting of the ice in the northern polar areas will have a limited impact on rising ocean levels because that ice, unlike the ice on Greenland and Antarctica, is already in the ocean.)

By 2023, according to NOAA, global mean sea level had reached 101.2 millimeters (4 inches) above 1993 levels, the highest annual average since 1993, when modern satellite record-keeping began. Alongside rising sea levels, researchers also noted increased ocean temperatures by that point. Ocean temperatures rose an average of .13 degrees Fahrenheit per decade, leading to a total increase of 1.5 degrees Fahrenheit by the early 2020s. Warming ocean temperatures can themselves further affect weather patterns. The heating also has a detrimental impact on many marine species.

The negative consequences of global warming on biodiversity is another major area of concern. In 2019, for example, the United Nations reported that climate change was one of the causes of an unprecedented decline of species population and an increasing rate of species extinction in the late 2010s. In addition to directly affecting animals and plants through temperature changes, global warming impacts species by changing the way humans interact with the natural environment. For example, increasing drought may cause humans to tap into new water sources, disrupting the species that already relied on that water. Similarly, the impact of invasive species is potentially heightened as global warming increases the range of certain animals and plants and shrinks that of others.

Causes of Global Warming

The vast majority of scientists are convinced, based on the abundant evidence, that global warming is occurring. While climate change skepticism is often a high-profile subject, serious researchers agree that the warming trend is real. However, there is still some dispute over which factors most drive the complex phenomenon. Some scientists point to gradual climatic cycles over extremely long periods of time, or other little-understood natural processes. Yet most experts believe anthropocentric climate change is a crucial component, especially in the unprecedented increase in temperatures observed since the beginning of the Industrial Revolution.

One significant factor in global warming, the rise in greenhouse gases, is strongly linked to the activities of humankind. Burning forests to clear land and operating factories and automobiles produce carbon dioxide and water vapor. Livestock herds and rotting vegetation release methane, and fertilizers used on farms also release greenhouse gases. Power plants that consume fossil fuels such as coal, oil, or natural gas release massive amounts of carbon dioxide into the air. The United Nations' Intergovernmental Panel on Climate Change (IPCC), made up of experts from around the world, determined that the probability of such human activity playing a role in global warming was over 95 percent.

While human activity accounts for the rapid speed at which climate change is occurring, to a lesser degree the warming trend can be linked to natural, cyclical changes in climate. The last ice age ended recently in geologic terms, and a number of changes are still taking place as the globe recovers from the presence of huge ice sheets on its surface. It is possible that the world's climate is still warming up from the last ice age. History provides examples of dramatic short-term changes in climate, including one period from 1617 to 1650 that was so unusually cold that it is called the "Little Ice Age." While a few researchers have suggested that solar energy emissions may be responsible for global warming, the scientific consensus holds that this theory does not fit with modern observed evidence, as solar energy has remained relatively constant since 1750 and all layers of the atmosphere have not warmed as would be expected if the sun were responsible.

Study of Global Warming

The greenhouse gas theory first emerged in the late nineteenth century, and by the mid-twentieth century increasing scientific attention was paid to the global climate. By the 1990s the scientific community widely recognized the occurrence and potential impact of global warming, and the study of the phenomenon grew steadily. However, skepticism prevailed in part due to the innate challenge in studying something as complex as the global climate system, which operates on many different scales of time. It may seem straightforward to record temperatures for a number of years and then compare them. However, detailed records on the weather have not been kept for more than a few decades in many areas, and even the oldest, most reliable written accounts provide only a glimpse at larger-scale cycles and trends. Scientists are therefore forced to rely on interpretations of historical accounts and the clues left in fossil records. Tree rings, old ice from deep within glaciers, and sedimentary deposits are examined to provide data about the global climate in the past.

Another complication is that existing records must be reviewed carefully to identify local changes that may not reflect global ones. For example, as towns grow into cities, the temperature climbs simply because larger cities are warmer than smaller ones, a phenomenon known as the urban heat island effect. Measurements taken years ago in a more rural environment should be lower than those taken after the population around the measuring station increased. This problem can be overcome with balloons. By sending instruments high in the atmosphere on weather balloons, air temperatures can be measured without being affected by urbanization. Data recorded this way show a consistent rise in global temperature.

Measurements of the level of greenhouse gases in the atmosphere are also affected by local factors such as urbanization. As a small town becomes a city, levels can be expected to rise. However, recording stations located in regions far removed from cities and factories also show an increase in the level of greenhouse gases. One station in Hawaii showed a steady rise in the amount of carbon dioxide present in the air since early 1958, with similar reports coming from stations in Point Barrow, Alaska, and Antarctica.

Another important variable in looking at global warming is sea surface temperature. Measurements can be skewed by local effects that have no impact on the global climate. One method used to make detailed measurements of seawater temperature is to broadcast a particular frequency of sound through the water and measure it at distant locations. The speed and frequency of the sound are affected by the temperature of the water.

However, more accurate and more global data became available through NASA's Earth Science program (formerly Mission to Planet Earth). The program placed satellites in orbit to study Earth and make a variety of detailed measurements, many of which are attributed to factors that contribute to global warming. These data have helped immensely in enabling scientists to understand the global climate and the changes it is undergoing.

Similarly, international efforts such as the IPCC (founded in 1988) have helped both generate data on global warming and study its potential effects. That organization's outlook grew more dire as research on the subject increased greatly in the twenty-first century. In 2018, the IPCC published a report stating that in order to avoid disastrous consequences including increased fires, floods, extreme heat, and drought, the increase in the global temperature had to be kept below 1.5 degrees Celsius relative to preindustrial levels. An increase of just half a degree more than that, the report said, would lead to 50 percent more people being exposed to water stress and 10 million more people being affected by sea level rise by 2100. At the time of the report's publication, the global average temperature was already 1 degree Celsius above preindustrial levels.

As temperatures continued to rise into the 2020s, researchers around the world also continued to study the processes, causes, and effects of global warming. Some of these studies sought to project likely scenarios for the extent of global warming based on earlier trends and current carbon dioxide emissions levels. In March 2023 the IPCC released a new report which estimated that average temperatures around the world would rise 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels during the first half of the 2030s due to ongoing consumption of fossil fuels such as natural gas and coal.

Mitigation Efforts

Environmental and climate advocacy groups such as the IPCC argue that people must take action to preserve the environment, implementing large-scale economic reforms, including investments in renewable energy, as well as behavioral changes to slow global warming and avoid catastrophic consequences. However, many politicians, businesses, and other groups oppose such changes due to their potential short-term impact on the economy and society. For this reason, global warming (and climate change in general) emerged as a point of debate around the world in the late twentieth century, and became a major political issue in the early twenty-first century. Along these lines, during this time some news outlets changed the terminology used to describe global warming, including phrases such as "climate crisis" and "climate emergency" to highlight the growing urgency of the situation.

A number of international agreements overseen by the United Nations Framework Convention on Climate Change (UNFCCC), including the 1992 Kyoto Protocol and 2015 Paris Climate Agreement, sought to build global consensus on fighting climate change and set clear goals for emissions reductions. Some nations also enacted their own guidelines aimed at mitigating global warming. However, many countries struggled to stay on track in in efforts to meet climate goals such as net-zero emissions. Debate also emerged around environmental justice in relation to global warming and other climate change impacts, with some activists suggesting that developed countries who were historically responsible for the vast majority of carbon dioxide emissions should bear more of the financial burden of addressing the problem.

Principal Terms

climate: the long-term weather patterns for a region, distinct from the day-to-day weather patterns

greenhouse effect: the net accumulation of heat held in the atmosphere as thermal energy by certain gases

greenhouse gas: any atmospheric gas that absorbs infrared radiation, preventing it from being radiated back into space; examples include carbon dioxide, methane, and water vapor

infrared radiation: energy emitted as electromagnetic radiation having wavelengths between 0.75 micron and 1 millimeter, frequently confused with heat but actually a different phenomenon

Bibliography

Black, Brian, and Gary J. Weisel. Global Warming. Greenwood, 2011.

Brownstein, Ronald. "The Unbearable Summer." The Atlantic, 8 Sept. 2021, www.theatlantic.com/politics/archive/2021/08/summer-2021-climate-change-records/619887. Accessed 15 Apr. 2023.

"The Causes of Climate Change." NASA, science.nasa.gov/climate-change/causes/. Accessed 6 Sept. 2024.

Fourth National Climate Assessment. US Global Change Research Program, 2018. www.globalchange.gov/nca4. Accessed 15 Apr. 2023.

Gillis, Justin. "2015 Was Hottest Year in Historical Record, Scientists Say." The New York Times, 20 Jan. 2016, www.nytimes.com/2016/01/21/science/earth/2015-hottest-year-global-warming.html. Accessed 3 Feb. 2016.

Global Warming of 1.5 ºC. UN Intergovernmental Panel on Climate Change, 2018. www.ipcc.ch/site/assets/uploads/sites/2/2019/06/SR15‗Full‗Report‗High‗Res.pdf. Accessed 15 Apr. 2023.

"Global Temperature." NASA, climate.nasa.gov/vital-signs/global-temperature/?intent=121. Accessed 6 Sept. 2024.

Lindsey, Rebecca. "Climate Change: Annual Greenhouse Gas Index." Climate.gov, NOAA, 17 June 2022, www.climate.gov/news-features/understanding-climate/climate-change-annual-greenhouse-gas-index. Accessed 6 Sept. 2024.

Lindsey, Rebecca. "Climate Change: Global Sea Level." Climate.gov, 19 Apr. 2022, www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level. Accessed 29 Sept. 2023.

Lucero, Olivia C., and Carter B. Keyes. New Developments in Global Warming Research. Nova Science, 2013.

Plumer, Brad. "Climate Change Is Speeding Toward Catastrophe. The Next Decade Is Crucial, UN Panel Says." The New York Times, 20 Mar. 2023, www.nytimes.com/2023/03/20/climate/global-warming-ipcc-earth.html. Accessed 29 Sept. 2023.

Ramsayer, Kate. "NASA Space Laser Missions Map 16 Years of Ice Sheet Loss." NASA, 30 Apr. 2020, climate.nasa.gov/news/2981/nasa-space-laser-missions-map-16-years-of-ice-sheet-loss. Accessed 15 Apr. 2023.

Schneider, Stephen H., and Terry L. Root. Wildlife Responses to Climate Change: North American Case Studies. Island, 2013.

"UN Report: Nature's Dangerous Decline 'Unprecedented'; Species Extinction Rates 'Accelerating.'" Sustainable Development, United Nations, 6 May 2019, www.un.org/sustainabledevelopment/blog/2019/05/nature-decline-unprecedented-report. Accessed 15 Apr. 2023.

"What Is Climate Change?" United Nations, www.un.org/en/climatechange/what-is-climate-change. Accessed 19 Aug. 2024.

"What Is the Difference Between Global Warming and Climate Change?" USGS, www.usgs.gov/faqs/what-difference-between-global-warming-and-climate-change. Accessed 6 Sep. 2024.