Global warming is expected to have far-reaching, long-lasting and, in many cases, devastating consequences for planet Earth.
Global warming, the gradual heating of Earth's surface, oceans and atmosphere, is caused by human activity, primarily the burning of fossil fuels that pump carbon dioxide (CO2), methane and other greenhouse gases into the atmosphere.
Despite political controversy about climate change, a major report released Sept. 27, 2013, by the Intergovernmental Panel on Climate Change (IPCC) stated that scientists are more certain than ever of the link between human activities and global warming. More than 197 international scientific organizations agree that global warming is real and has been caused by human action.
Already, global warming is having a measurable effect on the planet.
"We can observe this happening in real time in many places. Ice is melting in both polar ice caps and mountain glaciers. Lakes around the world, including Lake Superior, are warming rapidly — in some cases faster than the surrounding environment. Animals are changing migration patterns and plants are changing the dates of activity," such as trees budding their leaves earlier in the spring and dropping them later in the fall, Josef Werne, a professor of geology and environmental science at the University of Pittsburgh, told Live Science.
Here is an in-depth look at the changes wrought by global warming.
Increase in average temperatures and temperature extremes
One of the most immediate and obvious effects of global warming is the increase in temperatures around the world. The average global temperature has increased by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past 100 years, according to the National Oceanic and Atmospheric Administration (NOAA).
Since record keeping began in 1895, the hottest year on record worldwide was 2016, according to NOAA and NASA data. That year Earth's surface temperature was 1.78 degrees F (0.99 degrees C) warmer than the average across the entire 20th century. Before 2016, 2015 was the warmest year on record, globally. And before 2015? Yep, 2014. In fact, 16 of the 17 warmest years on record have happened since 2001, according to NASA.
For the contiguous United States and Alaska, 2016 was the second-warmest year on record and the 20th consecutive year that the annual average surface temperature exceeded the 122-year average since record keeping began, according to NOAA.
Extreme weather events
Extreme weather is another effect of global warming. While experiencing some of the hottest summers on record, much of the United States has also been experiencing colder-than-normal winters.
Changes in climate can cause the polar jet stream — the boundary between the cold North Pole air and the warm equatorial air — to migrate south, bringing with it cold, Arctic air. This is why some states can have a sudden cold snap or colder-than-normal winter, even during the long-term trend of global warming, Werne explained.
"Climate is, by definition, the long-term average of weather, over many years. One cold (or warm) year or season has little to do with overall climate. It is when those cold (or warm) years become more and more regular that we start to recognize it as a change in climate rather than simply an anomalous year of weather," he said.
Global warming may also lead to extreme weather other than cold or heat extremes. For example, hurricane formations will change. Though this is still a subject of active scientific research, current computer models of the atmosphere indicate that hurricanes are more likely to become less frequent on a global basis, though the hurricanes that do form may be more intense.
"And even if they become less frequent globally, hurricanes could still become more frequent in some particular areas," said atmospheric scientist Adam Sobel, author of "Storm Surge: Hurricane Sandy, Our Changing Climate, and Extreme Weather of the Past and Future" (HarperWave, 2014). "Additionally, scientists are confident that hurricanes will become more intense due to climate change." This is because hurricanes get their energy from the temperature difference between the warm tropical ocean and the cold upper atmosphere. Global warming increases that temperature difference.
"Since the most damage by far comes from the most intense hurricanes — such as typhoon Haiyan in the Philippines in 2013 — this means that hurricanes could become overall more destructive," said Sobel, a Columbia University professor in the departments of Earth and Environmental Sciences, and Applied Physics and Applied Mathematics. (Hurricanes are called typhoons in the western North Pacific, and they're called cyclones in the South Pacific and Indian oceans.)
Lightening is another weather feature that is being affected by global warming. According to a 2014 study, a 50 percent increase in the number of lightning strikes within the United States is expected by 2100 if global temperatures continue to rise. The researchers of the study found a 12 percent increase in lightning activity for every 1.8 degree F (1 degree C) of warming in the atmosphere.
NOAA established the U.S. Climate Extremes Index (CEI) in 1996 to track extreme weather events. The number of extreme weather events that are among the most unusual in the historical record, according to the CEI, has been rising over the last four decades.
Scientists project that extreme weather events, such as heat waves, droughts, blizzards and rainstorms will continue to occur more often and with greater intensity due to global warming, according to Climate Central. Climate models forecast that global warming will cause climate patterns worldwide to experience significant changes. These changes will likely include major shifts in wind patterns, annual precipitation and seasonal temperatures variations.
In addition, because high levels of greenhouse gases are likely to remain in the atmosphere for many years, these changes are expected to last for several decades or longer, according to the U.S. Environmental Protection Agency (EPA). In the northeastern United States, for example, climate change is likely to bring increased annual rainfall, while in the Pacific Northwest, summer rainfall is expected to decrease, the EPA said.
One of the primary manifestations of climate change so far is melt. North America, Europe and Asia have all seen a trend toward less snow cover between 1960 and 2015, according to 2016 research published in the journal Current Climate Change Reports. According to the National Snow and Ice Data Center, there is now 10 percent less permafrost, or permanently frozen ground, in the Northern Hemisphere than there was in the early 1900s. The thawing of permafrost can cause landslides and other sudden land collapses. It can also release long-buried microbes, as in a 2016 case when a cache of buried reindeer carcasses thawed and caused an outbreak of anthrax.
One of the most dramatic effects of global warming is the reduction in Arctic sea ice. Sea ice hit record-low extents in both the fall and winter of 2015 and 2016, meaning that at the time when the ice is supposed to be at its peak, it was lagging. The melt means there is less thick sea ice that persists for multiple years. That means less heat is reflected back into the atmosphere by the shiny surface of the ice and more is absorbed by the comparatively darker ocean, creating a feedback loop that causes even more melt, according to NASA's Operation IceBridge.
Glacial retreat, too, is an obvious effect of global warming. Only 25 glaciers bigger than 25 acres are now found in Montana's Glacier National Park, where about 150 glaciers were once found, according to the U.S. Geological Survey. A similar trend is seen in glacial areas worldwide. According to a 2016 study in the journal Nature Geoscience, there is a 99 percent likelihood that this rapid retreat is due to human-caused climate change. Some glaciers retreated up to 15 times as much as they would have without global warming, those researchers found.
Sea levels and ocean acidification
In general, as ice melts, sea levels rise. In 2014, the World Meteorological Organization reported that sea-level rise accelerated 0.12 inches (3 millimeters) per year on average worldwide. This is around double the average annual rise of 0.07 in. (1.6 mm) in the 20th century.
Melting polar ice in the Arctic and Antarctic regions, coupled with melting ice sheets and glaciers across Greenland, North America, South America, Europe and Asia, are expected to raise sea levels significantly. And humans are mostly to blame: In the IPCC report released on Sept. 27, 2013, climate scientists said they are at least 95 percent certain that humans are to blame for warming oceans, rapidly melting ice and rising sea levels, changes that have been observed since the 1950s.
Global sea levels have risen about 8 inches since 1870, according to the EPA, and the rate of increase is expected to accelerate in the coming years. If current trends continue, many coastal areas, where roughly half of the Earth's human population lives, will be inundated.
Researchers project that by 2100, average sea levels will be 2.3 feet (.7 meters) higher in New York City, 2.9 feet (0.88 m) higher at Hampton Roads, Virginia, and 3.5 feet (1.06 m) higher at Galveston, Texas, the EPA reports. According to an IPCC report, if greenhouse gas emissions remain unchecked, global sea levels could rise by as much as 3 feet (0.9 meters) by 2100. That estimate is an increase from the estimated 0.9 to 2.7 feet (0.3 to 0.8 meters) that was predicted in the 2007 IPCC report for future sea-level rise.
Sea level isn't the only thing changing for the oceans due to global warming. As levels of CO2 increase, the oceans absorb some of that gas, which increases the acidity of seawater. Werne explains it this way: "When you dissolved CO2 in water, you get carbonic acid. This is the same exact thing that happens in cans of soda. When you pop the top on a can of Dr Pepper, the pH is 2 — quite acidic."
Since the Industrial Revolution began in the early 1700s, the acidity of the oceans has increased about 25 percent, according to the EPA. "This is a problem in the oceans, in large part, because many marine organisms make shells out of calcium carbonate (think corals, oysters), and their shells dissolve in acid solution," said Werne. "So as we add more and more CO2 to the ocean, it gets more and more acidic, dissolving more and more shells of sea creatures. It goes without saying that this is not good for their health."
If current ocean acidification trends continue, coral reefs are expected to become increasingly rare in areas where they are now common, including most U.S. waters, the EPA reports. In 2016 and 2017, portions of the Great Barrier Reef in Australia were hit with bleaching, a phenomenon in which coral eject their symbiotic algae. Bleaching is a sign of stress from too-warm waters, unbalanced pH or pollution; coral can recover from bleaching, but back-to-back episodes make recovery less likely.
Plants and animals
The effects of global warming on the Earth's ecosystems are expected to be profound and widespread. Many species of plants and animals are already moving their range northward or to higher altitudes as a result of warming temperatures, according to a report from the National Academy of Sciences.
"They are not just moving north, they are moving from the equator toward the poles. They are quite simply following the range of comfortable temperatures, which is migrating to the poles as the global average temperature warms," Werne said. Ultimately, he said, this becomes a problem when the rate of climate change velocity (how fast a region changes put into a spatial term) is faster than the rate that many organisms can migrate. Because of this, many animals may not be able to compete in the new climate regime and may go extinct.
Additionally, migratory birds and insects are now arriving in their summer feeding and nesting grounds several days or weeks earlier than they did in the 20th century, according to the EPA.
Warmer temperatures will also expand the range of many disease-causing pathogens that were once confined to tropical and subtropical areas, killing off plant and animal species that formerly were protected from disease.
These and other effects of global warming, if left unchecked, will likely contribute to the disappearance of up to one-half of Earth's plants and one-third of animals from their current range by 2080, according to a 2013 report in the journal Nature Climate Change.
As dramatic as the effects of climate change are expected to be on the natural world, the projected changes to human society may be even more devastating.
Agricultural systems will likely be dealt a crippling blow. Though growing seasons in some areas will expand, the combined impacts of drought, severe weather, lack of accumulated snowmelt, greater number and diversity of pests, lower groundwater tables and a loss of arable land could cause severe crop failures and livestock shortages worldwide.
North Carolina State University also notes that carbon dioxide is affecting plant growth. Though CO2 can increase the growth of plants, the plants may become less nutritious.
This loss of food security may, in turn, create havoc in international food markets and could spark famines, food riots, political instability and civil unrest worldwide, according to a number of analyses from sources as diverse as the U.S Department of Defense, the Center for American Progress and the Woodrow Wilson International Center for Scholars.
In addition to less nutritious food, the effect of global warming on human health is also expected to be serious. The American Medical Association has reported an increase in mosquito-borne diseases like malaria and dengue fever, as well as a rise in cases of chronic conditions like asthma, most likely as a direct result of global warming. The 2016 outbreak of Zika virus, a mosquito-borne illness, highlighted the dangers of climate change. The disease causes devastating birth defects in fetuses when pregnant women are infected, and climate change could make higher-latitude areas habitable for the mosquitos that spread the disease, experts said. Longer, hotter summers could also lead to the spread of tick-borne illnesses.
Climate Change and Health Effects
In the United States disease occurrence is not so strongly tied to the local climate or environmental factors. True, more Americans get the flu or colds during the winter months, but in other parts of the world, climate plays a much more significant role.
Some regions already have thousands of malnourished people. Droughts and floods in these areas cause even further hardship and suffering. Without the appropriate infrastructure any type of environmental change makes adaptive measures more difficult. Severe weather events such as hurricanes can destroy entire cities in some parts of the world, and the people find it difficult, if not impossible, to recover from such events.
Human health suffers under these environmental conditions. Water supplies are contaminated, sewage in city streets carry disease, and a lack of medical care can result in thousands of deaths.
Read below about some of the environmental conditions that can affect human health. Use the Related Links to research and report on other environmental situations that may negatively impact health. Also see Climate Change and Factors Related to Human Health.
Increases in temperature will directly affect human health. More extreme heat waves during the summer will increase the incidence of heat-related deaths. For instance, record-setting heat waves in 2010 resulted in mortalities, especially among the elderly, people with heart or respiratory problems, and the poor. In the United States, Chicago, a city which has been the site for heat-related tragedies in the past, is predicted to experience 25 percent more frequent heat waves. Los Angeles is expected to have 4-8 times as many heat wave days by the end of the century (IPCC, 2007).
Climate change may affect the risk of infectious diseases, especially those diseases that occur primarily in warmer areas and that are spread by vectors, aninsect or other organism that transmits a pathogenic fungus, virus, or bacterium.
(For more information on climate-sensitive disease, see Human Disease and Mortality in this module.)
Note that disease occurrence and mortality rates will also be affected by the health care system of the country in which the disease occurs. If improved health care, hygiene, and sanitation procedures are provided, then some infectious diseases may not increase.
Climate change is expected to significantly contribute to a decrease in air quality. Respiratory disorders may worsen because of warming-induced increases in smog, ground-level ozone, and particulate air pollution.
Ground-level ozone can damage lung tissue. People with asthma and other chronic lung diseases are particularly susceptible. Ground-level ozone is increased when sunlight and higher temperatures are combined with other pollutants. Ozone causes lung damage and respiratory disorders and is also associated with cardiovascular diseases and cardiac dysrhythmias.
Particulate pollution may also increase because of climate change. Particulate pollution consists of a mixture of small particles and liquid droplets. The particles are small enough to travel deeply into the lungs where they cause a variety of health problems.
With longer and warmer seasons the amount of pollen in the air will increase. Incidences of allergy-related illness will rise.
As extreme weather events occur more frequently, agricultural yields and crop production will be increasingly affected. Hurricanes in Earth’s poorest areas have destroyed crops critical to human survival. Increased floods, droughts, heat waves, and hail storms have destroyed crops in the U.S. Midwest and caused prices to rise.
However, increased temperatures may lengthen the growing season. A longer growing season will allow additional crops to be harvested. The balance between all factors that affect agricultural yields will be critical to producing food. It is thought that developing countries will be more adversely affected by climate change-induced malnutrition.
Increased concentrations of ground-level carbon dioxide may increase CO2 concentrations in seawater. A more acidic water environment would adversely affect the world’s tropical ecosystems and would have dramatic consequences for fisheries and food supplies in parts of the world that rely on oceans for food.
Extreme Weather Events
Studies continue to show that global warming mechanisms will increase the frequency of extreme weather events. Data cannot show that a particular hurricane, tornado, or flood was caused by increased global temperatures, but rather studies of the mechanisms involved in warming trends and greenhouse gases indicate a link.
More Severe Hurricanes
Sea surface temperatures and deepwater temperatures have a great impact on the intensity of a hurricane. Ocean waters have to be warm enough and the warmth has to extend down to a depth of at least 200 feet in order to have the energy to fuel hurricane formation.
Increased temperatures will fuel more hurricanes and more severe hurricanes. Also, hurricane season may be extended because of increased temperatures.
Hotter, dryer conditions in some regions of the world may set up conditions that are ripe for wildfires. Increased global temperatures may extend warm seasons and moderate winter weather. This not only produces longer fire seasons, but causes outbreaks of tree-eating beetles. Insects such as these add large amounts of dead, dry wood that fuels fires.
Milder winters have also allowed the beetles to invade and survive in areas where they could not have previously lived. More northern regions have been hard hit by wildfires in recent years. Alaska, Northern Canada, Scandinavia, and Siberia are examples of areas that are not susceptible to wildfires in summer months.
Steven W. Running reported in Science magazine that "longer, warmer summers have resulted in a fourfold increase of major U. S. wildfires and a six-fold increase in the area of forest burned, compared to the period from 1970 to 1986."
These wildfires put lives at risk. The Cedar wildfire in California caused 15 deaths. The East Amarillo Complex fire in Texas caused 11 deaths, and more than 200 people were killed in the Australian wildfires in 2009.
Wildfires also risk public health through air quality. Smoke, noxious gases and soot particles can be transported thousands of miles. Respiratory problems can result.
Increased Droughts and Floods
How can there be both increased droughts and floods? It depends on the region in question.
Higher temperatures makes evaporation of water from soils occur more quickly. After the initial water evaporates, dry soils can have an effect on the amount of precipitation that occurs; with less water to evaporate from dry soils, less water goes into the atmosphere and fewer rain clouds form.
However, when water vapor does build up in the atmosphere and moves into an area with dry soil, the soil cannot handle the downpour. Most of the water runs off into streams or rivers. With continued rain, flooding often results in areas that had be in drought conditions.
Climate change involves many factors that can affect human health. Factors affecting Earth’s atmosphere, waters, and soils are interrelated with the organisms that live on Earth. The interconnections must be considered when studying effects of climate change.