Global Warming: Definition, Causes, Effects, and Risks

97% of scientists agree that global warming is largely man-made.

Polar Bear, Repulse Bay, Nunavut, Canada
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Since 1880, when record keeping began, Earth's temperatures levels have been rising steadily. The pace of global warming then increased in the middle of the twentieth century, and it did again by the end of the century. As a result, Earth is now experiencing its warmest climate in modern history. So said scientists collaborating on the 2017 report of the United States Global Change Research Program.

The Cause of Global Warming

The sun is the primary source of heat throughout the solar system. Solar radiation and average global temperatures usually rise and fall together. Over at least the past 40 years, however, that hasn’t been the case.

The Physical-Meteorological Observatory of the World Radiation Center Davos in Switzerland is one of the institutes tracking solar radiation. As reported in the peer-reviewed journal Solar Variability and Planetary Climates, their instruments determined that solar energy levels go up and down constantly, but on average they dropped slightly during the period between 1978 and 2007, even while average global temperatures soared. NASA has also published a graph charting an extension through 2020 of solar radiation and global temperature data.

If the sun is not causing the rise in global temperatures, what is?

Greenhouse Gases Cause Global Warming

Dutch Steel Mill

Bruce B. Clarke / Getty Images

As explained by the United States Environmental Protection Agency (EPA), global warming is mostly caused by the greenhouse gases carbon dioxide, methane, nitrous oxide, and a small group of synthetic chemicals called hydrofluorocarbons. The gases trap close to Earth’s surface the heat resulting from solar radiation and stop it from leaving Earth’s atmosphere for space.

Global Warming From Greenhouse Gases Is Largely Man-Made

A small percentage of global warming is caused when geological events like volcanoes add carbon dioxide to Earth’s atmosphere. The amount isn’t insignificant. The United States Geological Survey (USGS) has estimated that volcanoes contribute about 260 million tons of carbon dioxide to the atmosphere each year.

However, most scientists agree that global warming is largely caused by human activity. In 2016, as reported by the peer-reviewed journal Environmental Research Letters, “anthropogenic” was the verdict of 90%-100% of publishing climate scientists.

This echoed earlier findings published in 2013 by the same journal; a team of nine climate scientists examined 11,944 peer-reviewed, published papers. Of those papers that included an opinion about the cause of global warming, 97.1% described it as caused by humans.

Aerial view of a man on the cracked ground. Global warming concept
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How Greenhouse Gases Warm the Globe

According to the EPA, most greenhouse gases are put into the atmosphere when fossil fuels are burned as part of industrial or agricultural processes, though some (the hydrofluorocarbons) are spewed into the air by and from refrigeration, air conditioning, building insulation, and fire extinguishing products.

While methane is 28 times more effective than carbon dioxide in trapping heat in Earth’s atmosphere, the EPA has called carbon dioxide the single greenhouse gas most responsible for global warming. This is largely because it is the most abundant and it persists in the atmosphere for 300-1,000 years.

Trapping solar radiation close to Earth, the greenhouse gases warm oceans, waterways, and Earth’s surface in much the same way that insulated glass panels warm the plants growing inside a human-made greenhouse—hence the popular term “greenhouse effect” in climate change lingo.


Dried up dam
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While human-driven processes create global warming by putting greenhouse gases into the atmosphere, humans also deprive Earth of its natural ability to clear greenhouse gases and regulate temperatures.

Photosynthesis is a metabolic process through which plants convert light into glucose, which they use as energy. As part of the process, plants respirate, “inhaling” atmospheric carbon dioxide and exhaling oxygen. By pulling carbon dioxide out of the air, plants serve a vital anti-global warming function.

As described by a 2020 report of the Food and Agriculture Organization of the United Nations (FAO), forests cover 31% of land area worldwide. The FAO estimated that some 420 million hectares (over 1 billion acres) of forest have been intentionally destroyed since 1990, with agricultural expansion conducted by large, for-profit companies being the main driver of that destruction.

With deforestation, Earth is losing one of its primary methods of keeping temperatures from climbing precipitously.

Key Takeaways: Causes of Global Warming

  • Global warming is mostly caused by the “greenhouse gases” carbon dioxide, methane, nitrous oxide, and a small group of synthetic chemicals called hydrofluorocarbons. 
  • For the most part, greenhouse gases are put into the atmosphere as a result of agricultural and industrial processes. 
  • While industrial and agricultural activities create global warming, deforestation deprives Earth of its natural ability to clear greenhouse gases and regulate temperatures.

The Effects of Global Warming

Global warming destroys habitats and imperils life in terrestrial waterways and on Earth’s surface. In a way, though, oceans are the primary victims of rising temperatures. 


Covering about 70% of Earth’s surface, oceans might be expected to suffer about 70% of the injury. Instead, the effect on them is surprisingly outsized. In October 2021, the U.S. National Oceanic and Atmospheric Administration (NOAA) reported that more than 90% of the excess heat trapped in and near Earth since the 1970s has been absorbed by oceans. 

Changes in ocean systems typically take long periods of time to complete. Unfortunately, as the EPA has warned, those changes may take just as long to rectify.

aerial view of whales swimming among icebergs
Monica Bertolazzi / Getty Images

Threatening Ocean Life 

In a 10-year survey concluded in 2010, more than 2,700 scientists from 80 countries contributed to 540 ocean expeditions that counted and cataloged marine species. The survey identified 156,291 species in United States waters alone. According to NOAA, that number may be as much as 91% too low.

Regardless of whether they are known or unknown, most marine life occupies a particular space in the food web upon which humans rely. By drastically stressing ocean habitat, rising temperatures profoundly imperil a broad swath of ocean life.

Creating Droughts, Floods, and Unstable Weather

Oceans create weather on sea and land. Currents power breezes, storms, trade winds, and weather fronts. Evaporation of seawater creates clouds and, ultimately rain.

NOAA has reported that, if the world continues to warm, the speed of global winds are projected to increase. Increased wind speed will cause a greater disturbance of ocean water, which then will increase the potential of hurricane development and rainfall.

Profound oceanic changes could contribute to a feedback loop of hot and cold weather, some of it extreme and much of it catastrophic and unpredictable. Increased evaporation over ocean water could create catastrophic floods and shift precipitation patterns enough to also create new desert areas.

Contributing to Sea Level Rise and Arctic Amplification

NOAA has predicted that, as a warming world melts sea ice in polar areas, sea levels all over the world will continue to rise. Unfortunately, as outlined in an article in the peer-reviewed journal Nature Communications, a destructive feedback loop called “arctic amplification” may also continue. (This is currently happening particularly strongly in areas near the North Pole.)

Normally, white sea ice is so highly reflective that about 80% of the sunlight reaching it gets immediately reflected towards the sun. This keeps oceans cold.

Unfortunately, maintaining low ocean temperatures is a bigger job than ice alone can handle. In recent summers, unusually warm air near the North Pole has been melting sea ice, exposing bare patches of dark ocean.

Dark ocean readily absorbs sunlight. When this happens, ocean temperatures rise, and adjacent areas of sea ice start to melt from underneath. This generates a feedback loop: newly vanished ice allows more sunlight to be absorbed and more ocean to warm and more ice to melt from underneath and more sunlight to be absorbed from above. And so on.

For more than four decades, temperatures in the Arctic have risen at two to three times the pace of the rest of the world. As the differential between temperatures at the poles and those in mid-latitudes gets smaller, jet streams may weaken, and weather fronts may stall.

As reported in a review article published by NASA, many scientists have already traced arctic amplification to higher temperatures and extreme weather events throughout Earth’s mid-latitudes.

Protecting the Planet but Hurting Coral and Shellfish 

As threatened as they are by global warming, oceans serve an enormous protective function against it: according to NASA, they are a carbon “sink,” storing carbon dioxide for millions of years and keeping it out of the atmosphere altogether.

There’s an unfortunate side effect, however, to oceans’ remarkable ability to sequester carbon. Carbon causes the pH balance of ocean water to drop, rendering the water more acidic. As explained by NOAA, in the years since the Industrial Revolution, the acidity of the oceans has increased by 30%. Under these conditions the exoskeletons and shells that marine animals such as coral and shellfish create become thinner, making the animals easier for predators to eat.

The Risks of Global Warming

Global warming presents risks to almost every system on Earth. Its effects on the environment can already be seen and are expected to worsen in coming decades. A few of the more salient are:

  • Sea Levels Are Rising. NASA has predicted that sea levels could rise as much as 8 feet by 2100. If they do, many coastal areas will be permanently submerged, and cities and vast areas of agricultural lands will be lost. This could cause a massive migration crisis while also devastating food supplies worldwide.
  • Extreme Weather Events. In 2020 and 2021, global warming fueled lethal hurricanes that caused both coastal and inland floods. The nonprofit First Street Foundation Research Lab, a collection of 180 collaborating research labs and commercial partners, has warned that, within 30 years, about 25% of critical infrastructure locations like police stations, airports, and hospitals will be lost to ruinous floods. 
  • Droughts. NASA has predicted that unstable weather will continue to cause the sorts of droughts that have recently plagued Russia and central Asia, southeast Asia, Africa, Australia, and the western United States.
  • Wildfires. The number and intensity of wildfires may increase. Droughts help spark wildfires. Unfortunately, combustion adds to a drought area's atmospheric carbon dioxide loads.
  • Extinction. Land and sea species will continue to go extinct. A 2015 study published in the peer-reviewed journal Science Advances showed that vertebrate species are disappearing up to 100 times faster than the rate at which they went extinct 200 years ago.

Key Takeaways: The Effect of Global Warming on Oceans

  • More than 90% of the excess heat trapped by greenhouse gases since the 1970s has been absorbed by oceans. 
  • By drastically stressing ocean habitat, rising temperatures profoundly imperil a broad swath of ocean life—and the entire global food web.
  • Oceans create weather on sea and on land. Changes in ocean temperature disrupt weather patterns and threaten the world’s food supply.
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