The Science Behind Global Warming

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Ngày cập nhật gần nhất: 15/4/2026

Are you aware of the consequences of global warming? Paul Souders / Getty Images

Once a term rarely heard outside scientific circles, global warming has become a familiar phrase used to describe the long-term effects of pollution on our climate. What was once a niche concern among a few scientists is now a common topic. It’s not uncommon to hear someone comment, "It’s global warming" when experiencing a particularly hot day or an unusual storm.

But is it really? In this article, we’ll explore what global warming is, the causes behind it, its current and potential future effects. While the scientific community largely agrees on the existence of global warming, there remains debate over how urgently it should be addressed. We’ll also look at the proposed policy changes in the U.S. aimed at combating global warming and the arguments both for and against these measures.

Global warming refers to a significant rise in the Earth's average temperature over a relatively short time, primarily driven by human activities.

Specifically, a temperature increase of 1 degree Celsius or more over a span of 100 to 200 years qualifies as global warming. Even a rise of just 0.4 degrees Celsius within a century is considered notable.

To grasp the impact of this, let’s first distinguish between weather and climate.

Weather and Climate

The Intergovernmental Panel on Climate Change (IPCC) provides a detailed analysis of recent shifts in temperature, sea levels, and snow cover. Courtesy IPC Secretariat/World Meteorological Organization

Weather refers to short-term, local atmospheric conditions. For instance, if it snows in your town next Tuesday, that's weather. Climate, on the other hand, is a long-term pattern and applies to broader regions. The climate of a place is determined by the average weather patterns over extended periods. If your region experiences cold winters with heavy snowfall year after year, that’s considered part of its climate, as this pattern has been consistent for as long as records have been kept.

It’s crucial to recognize that climate is measured over very long periods. Even a few hundred years is considered short when discussing climate. Changes in climate can span tens of thousands of years. So, if you experience a mild winter with little snow—or even two or three such winters in a row—that doesn’t signify a shift in climate. It's simply an anomaly, an event that falls outside the usual range but doesn’t indicate a lasting change.

Even small shifts in climate can lead to dramatic impacts. When we hear about the "Ice Age," we might imagine a world completely frozen with snow and icy temperatures. However, during the last ice age, which occurs roughly every 50,000 to 100,000 years, the Earth's average temperature was only about 5°C colder than today's averages [Source: NASA].

Global warming is the rapid increase in the Earth’s temperature over a short period, primarily driven by human activities.

Specifically, a rise of 1 or more degrees Celsius over 100 to 200 years would qualify as global warming. Even a 0.4°C increase over just one century would be significant. The Intergovernmental Panel on Climate Change (IPCC), composed of over 2,500 scientists from around the world, met in Paris in February 2007 to assess climate data. They found that the Earth had warmed by 0.6°C from 1901 to 2000. By extending the period from 1906 to 2006, the temperature increase rose to 0.74°C.

Additional key findings from the IPCC include:

Out of the last 12 years, 11 have ranked among the warmest years since 1850.
The warming observed in the past 50 years is nearly double that of the previous 100 years, indicating that the warming rate is accelerating.
The ocean's temperature has risen to depths of at least 3,000 meters (over 9,800 feet), absorbing more than 80 percent of the heat added to the climate system.
Glaciers and snow cover have decreased in both the Northern and Southern Hemispheres, contributing to rising sea levels.
Average Arctic temperatures have increased at nearly twice the global rate over the past century (the IPCC also notes that Arctic temperatures vary significantly from decade to decade).
The area of frozen ground in the Arctic has decreased by about 7 percent since 1900, with seasonal reductions of up to 15 percent.
Precipitation has risen in eastern parts of the Americas, northern Europe, and parts of Asia, while regions like the Mediterranean and southern Africa have faced drying trends.
Westerly winds have grown stronger.
Droughts are now more severe, longer-lasting, and cover larger areas compared to the past.
Extreme temperatures have shifted, with hot days and heatwaves becoming more frequent, while cold days and nights are less common.
While the frequency of tropical storms hasn't increased, their intensity in the Atlantic has risen, correlating with higher ocean surface temperatures.

Natural Climate Variability

When climate shifts naturally, it can take the Earth thousands of years to change by just 1 degree. Natural processes such as recurring ice-age cycles, volcanic activity, shifts in plant life, variations in solar radiation, and changes in atmospheric chemistry can all affect the planet's climate.

The Greenhouse Effect

Courtesy U.S. Global Change Research Program

Global warming results from an intensification of the greenhouse effect. While the greenhouse effect itself isn't inherently harmful, it's what enables Earth to maintain the warmth necessary for life to thrive.

While it's not an exact match, think of Earth as being like your car left in a parking lot on a sunny day. You’ve probably noticed that the inside of your car becomes much hotter than the outside temperature after it’s been parked for a while. Sunlight streams through the windows, where it is absorbed by the seats, dashboard, and floor mats. As these objects release heat, not all of it escapes. Some of it bounces back inside. The heat emitted by these objects has a different wavelength than the sunlight that entered through the windows, so more energy goes in than out, gradually raising the temperature inside your car.

The greenhouse effect is a bit more complex than your hot car analogy. When sunlight reaches Earth's atmosphere and surface, about 70% of the energy remains on the planet, absorbed by the land, oceans, plants, and other surfaces. The remaining 30% is reflected back into space by clouds, snow, and other reflective surfaces [Source: NASA]. However, even the 70% that is absorbed doesn’t stay on Earth indefinitely (otherwise, the planet would turn into a giant fireball). Earth's oceans and land masses eventually radiate heat, some of which escapes into space. The rest is absorbed by gases in the atmosphere, like carbon dioxide, methane, and water vapor. These gases then re-emit this heat, helping to keep the planet warmer than it would be without the greenhouse effect, as more energy enters than exits. This process helps maintain Earth's temperature.

What Would Earth Be Like Without the Greenhouse Effect?

Without the greenhouse effect, Earth would likely resemble Mars. Mars doesn’t have a thick enough atmosphere to trap sufficient heat, making it extremely cold. Some scientists have even proposed the idea of terraforming Mars by sending “factories” that release water vapor and carbon dioxide into the air. If enough of these gases were produced, the Martian atmosphere could thicken enough to retain more heat and possibly support plant life. As plants spread across Mars, they would eventually start producing oxygen. After a few centuries or millennia, Mars could potentially have an environment that supports human life—all due to the greenhouse effect.

Global Warming: What's Happening?

Power stations, livestock, and vehicles are key sources of greenhouse gases such as carbon dioxide and methane. Courtesy NASA

The greenhouse effect occurs due to certain naturally occurring elements in the atmosphere. However, since the Industrial Revolution, human activities have dramatically increased the levels of these substances in the air.

Carbon dioxide (CO2) is a colorless gas produced as a result of burning organic materials. It makes up less than 0.04% of Earth's atmosphere, most of which was introduced by volcanic activity in the planet's early years. Currently, human activities are releasing large amounts of CO2 into the air, leading to a significant rise in carbon dioxide levels [Source: Keeling, C.D. and T.P. Whorf]. These higher concentrations are regarded as the primary cause of global warming because CO2 absorbs infrared radiation. Most of the energy escaping Earth's atmosphere does so in this form, so additional CO2 means more energy retention and a general increase in the planet's temperature.

Carbon dioxide levels measured at Mauna Loa, Hawaii Courtesy NOAA, Dave Keeling and Tim Whorf (Scripps Institution of Oceanography)

The Worldwatch Institute reports that global carbon emissions have surged from approximately 1 billion tons in 1900 to about 7 billion tons in 1995. Additionally, the Institute points out that Earth's average surface temperature has risen from 14.5°C in 1860 to 15.3°C in 1980.

According to the IPCC, prior to industrialization, the Earth's atmosphere contained about 280 parts per million (ppm) of CO2. This means that for every million molecules of dry air, 280 were CO2. By 2005, CO2 levels had risen to 379 ppm [Source: IPCC].

Nitrous oxide (N2O) is another significant greenhouse gas. Although the amount released by human activities is not as high as that of CO2, nitrous oxide absorbs far more energy—about 270 times as much. For this reason, efforts to reduce greenhouse gas emissions also target N2O [Source: Soil Conservation Council of Canada]. The extensive use of nitrogen fertilizers in agriculture is a major source of nitrous oxide emissions, and it is also released during combustion processes.

Methane is a flammable gas and the primary component of natural gas. It is naturally produced during the decomposition of organic materials, often referred to as "swamp gas." However, human activities also generate methane in various ways:

Extracting it from coal
From livestock digestion (i.e., digestive gases)
From bacteria in rice paddies
From the breakdown of waste in landfills

Methane functions similarly to carbon dioxide in the atmosphere by absorbing infrared radiation and trapping heat energy on Earth. In 2005, methane concentration in the atmosphere was measured at 1,774 parts per billion (ppb) according to the IPCC [Source: IPCC]. Though there is less methane than CO2 in the atmosphere, it can absorb and emit twenty times more heat than CO2 [Source: Hopwood, Nick and Cohen, Jordan]. Some scientists suggest that large-scale methane releases (such as from methane ice deposits beneath the ocean) may have triggered brief periods of intense global warming, potentially contributing to some of Earth's mass extinctions in ancient times [Source: Discover Magazine].

What will happen if the global temperature rises by just a few degrees? Continue reading the next section to discover the potential outcomes.

Concentrations of Carbon Dioxide and Methane

The concentrations of carbon dioxide and methane in 2005 surpassed the natural levels observed over the past 650,000 years. A significant portion of this rise in concentration can be attributed to the burning of fossil fuels.

Impact of Global Warming: Rising Sea Levels

Some potential consequences of global warming include the flooding of low-lying islands due to rising sea levels, an increase in the frequency of extreme weather events, and the retreat of glaciers and ice caps. Photo courtesy of NASA

We know that a mere 5-degree Celsius drop over thousands of years can trigger an ice age. But what might happen if the Earth's temperature rises by just a few degrees over a few hundred years? It's impossible to say with certainty. Even short-term weather forecasts are prone to error due to the complexity of weather systems. When it comes to long-term climate predictions, we can only make educated guesses based on historical climate patterns.

Glaciers and ice shelves across the globe are melting [Source: Guardian Unlimited]. The disappearance of large ice masses from the Earth's surface could potentially accelerate global warming, as it would reflect less of the sun's energy back into space (refer to our previous discussion on the greenhouse effect). A direct consequence of the melting glaciers would be a rise in sea levels. Initially, this rise would be just an inch or two. Even such a small increase could lead to flooding in coastal areas that are already at risk. However, if the West Antarctic Ice Sheet were to collapse into the ocean, it could raise sea levels by 10 meters (over 32 feet), completely submerging many coastal regions [Source: NASA].

Research forecasts suggest that sea levels will continue to rise. Photo courtesy of Earth Observatory, NOAA

The IPCC reports that sea levels rose by 17 centimeters (roughly 6.7 inches) during the 20th century. Projections for the 21st century suggest that sea levels will continue to rise, with estimates ranging from 7 to 22 inches by 2100. These estimates do not account for changes in ice flow, as there is insufficient data to include such factors. The actual rise in sea levels is likely to exceed these estimates, but the exact extent will depend on further research into the effects of global warming on ice flow patterns.

As ocean temperatures increase, oceanic storms such as tropical storms and hurricanes, which derive their strength and destructive power from the warm waters they traverse, may grow more intense.

If the warming climate continues to affect glaciers and ice shelves, could this jeopardize the polar ice caps and lead to rising oceans? The next section will explore this potential risk.

Water vapor is the most prevalent greenhouse gas, but it is generally more of a consequence of climate changes than a result of human-made emissions. Water or moisture on the Earth's surface absorbs heat from both the sun and its surroundings. Once the liquid absorbs enough heat, some of the molecules gain enough energy to escape into the atmosphere as vapor. As the vapor rises, the surrounding air cools down. Eventually, the vapor cools sufficiently to revert to liquid form, falling back to Earth due to gravity, completing the cycle. This cycle is known as a "positive feedback loop." Water vapor is harder to measure than other greenhouse gases, and its precise role in global warming remains uncertain. However, scientists believe there is a connection between increasing carbon dioxide levels and the rising amounts of water vapor. According to the NOAA Web site:

As water vapor increases in the atmosphere, more of it will eventually condense into clouds that are more effective at reflecting solar radiation, thus reducing the amount of energy reaching Earth's surface and preventing further heating.

If the Polar Ice Caps Melt

Photo credit: NOAA

Is there a risk that the polar ice caps will melt and cause sea levels to rise? This is a possibility, but the exact timing is unknown.

The primary ice-covered landmass on Earth is Antarctica, located at the South Pole, which holds about 90 percent of the world’s ice and 70 percent of its fresh water. The average thickness of the Antarctic ice sheet is 2,133 meters (7,000 feet). If all the ice in Antarctica were to melt, global sea levels would rise by approximately 61 meters (200 feet). However, the average temperature in Antarctica is -37°C, making it highly unlikely that the ice will melt anytime soon. In fact, most areas of the continent never experience temperatures above freezing.

At the opposite side of the Earth, the North Pole, the ice is significantly thinner compared to the South Pole. It floats on the Arctic Ocean, and if it were to melt, it would not impact sea levels.

Greenland holds a substantial amount of ice, which could raise the oceans by another 7 meters (20 feet) if it melted. Due to its proximity to the equator, Greenland experiences warmer temperatures, making it more susceptible to ice melting. According to scientists from the Universities of London and Edinburgh, ice loss from both Antarctica and Greenland accounts for about 12 percent of the increase in sea levels [Source: Science Daily].

A less dramatic cause for rising ocean levels may be the warmer temperature of the water itself. Water is densest at 4°C. Above and below this temperature, water’s density decreases, meaning the same volume of water occupies more space. As the overall temperature of the water rises, it expands slightly, causing the oceans to rise.

Effects of Global Warming: Seasons and Ecosystems

Image courtesy of the U.S. Geological Survey

As global temperatures rise, the changes around the world would become less abrupt. In regions with temperate climates and four seasons, the growing season would extend with increased rainfall. This could bring numerous benefits to these areas. On the other hand, regions that are less temperate could experience a rise in temperature and a sharp decline in precipitation, leading to prolonged droughts and the potential creation of deserts.

Due to the complexity of Earth's climate, predicting how a change in one area's climate will influence others is challenging. For example, researchers at the University of Colorado suggest that the reduction of sea ice in the Arctic could lead to less intense cold fronts in Colorado, resulting in reduced snowfall. This could affect various sectors, from agriculture to the ski industry.

The most severe and hardest-to-predict impacts are those on the world’s living ecosystems. Many ecosystems are fragile, and even minor changes can lead to the extinction of several species and disrupt those that depend on them. Because ecosystems are often interconnected, the consequences could be far-reaching. For instance, a forest might slowly die off and transition into grassland, or entire coral reefs could vanish. While some species of plants and animals may adapt or relocate to cope with the climate shift, many others could become extinct.

Some ecosystems are already undergoing dramatic changes due to climate shifts. The University of Alberta reports that much of what was once tundra in northern Canada is transforming into forests. This transformation from tundra to forest is not gradual; it appears to happen in sudden, unpredictable bursts.

The human toll from global warming is difficult to measure. Every year, thousands of lives could be lost, particularly among the elderly and those with health issues, who may succumb to heat stroke and other heat-related conditions. Poorer and less developed nations would bear the brunt of the effects, lacking the financial resources to address the challenges posed by rising temperatures. Large numbers of people could die from hunger if droughts hinder crop production or from disease if coastal flooding triggers widespread waterborne illnesses.

The Carnegie Institution estimates that global warming results in crop losses worth approximately $5 billion each year. Farmers experience a reduction of around 40 million metric tons in cereal crops such as wheat, barley, and corn annually. Research shows that a 1-degree Fahrenheit rise in average temperature leads to a 3 to 5 percent decrease in crop yields [Source: Science Daily].

Now, let's explore why some individuals aren't particularly worried about global warming.

Does Not Compute

The IPCC utilized computer models to simulate climate change, finding that only those models that incorporated human contributions to global warming accurately reflected the present climate. Models that excluded human factors failed to replicate the climate we experience today.

Is Global Warming a Real Problem?

Despite broad scientific agreement, some people remain unconvinced that global warming is occurring. There are various reasons for this belief:

Some people believe the data does not show a clear upward trend in global temperatures, either due to insufficient long-term climate data or because the existing data is too ambiguous.
There are a few scientists who think the data is being misinterpreted by those already concerned about global warming. According to them, these individuals are looking for signs of global warming in the data, rather than objectively analyzing it to understand what it truly indicates.
Others contend that the rise in global temperatures could simply be a natural climate fluctuation or caused by factors other than greenhouse gases.

While most scientists agree that global warming appears to be occurring, a small group believes it is not a cause for concern. These scientists argue that the Earth is more resilient to climate changes than we realize. They believe plants and animals will adapt to gradual shifts in weather patterns, and that the consequences of global warming will likely not be catastrophic. Changes such as slightly longer growing seasons, altered precipitation levels, and more extreme weather are not seen as disastrous. Additionally, they argue that the economic damage of reducing greenhouse gas emissions may be more harmful to humanity than the impacts of global warming itself.

In some respects, the scientific consensus may not be the deciding factor. The true power to create meaningful change lies with national and global policymakers. Some U.S. lawmakers hesitate to implement policies due to concerns that the costs of doing so may exceed the risks posed by global warming. Common concerns, objections, and arguments include:

Changes in U.S. policies regarding emissions and carbon production could lead to job losses.
Countries like India and China, which heavily depend on coal for energy, will continue to cause environmental issues, even if the U.S. changes its energy policies (critics of these policymakers argue that this logic employs the tu quoque fallacy).
Since scientific evidence deals with probabilities rather than certainties, we cannot be sure that human activity is significantly contributing to global warming or that we can effectively address it.
Some believe technology will eventually resolve the global warming issue, making any policy changes unnecessary and potentially harmful in the long run.

What’s the right answer? It can be difficult to determine. Most scientists agree that global warming is real and could cause harm, but the scale of the problem and the severity of its consequences remain subjects of debate.

In the following section, we’ll explore if there are any actions we can take to help prevent global warming.

An Economist's Perspective

Dr. Peter Tsigaris, an economist at Thompson Rivers University, argues that efforts to reduce global warming make sense from both an environmental and economic viewpoint. He estimates that transitioning away from fossil fuels and altering our behaviors to address global warming would cost about one percent of the global Gross Domestic Product (GDP) annually, while doing nothing could result in a 5 percent annual loss of global GDP. Extreme climate change could incur costs of 20 percent GDP or higher. [Source: Science Daily]

Can We Halt Global Warming?

Hydrogen-powered vehicles, increased solar cell adoption, and hydro-electric power stations are potential solutions to lower greenhouse gas emissions. Courtesy BMW AG, Munich, Germany; DOE/NREL; SunLine Transit Agency

Although scientists caution that global warming may persist for centuries due to the long natural processes involved, there are still steps we can take to minimize its effects. The key is simple: reduce the usage of things that contribute to greenhouse gases. On a personal level, you can help by consuming less energy. The electricity powering many household devices comes from power plants, which typically burn fossil fuels. You can turn off lights when not in use, take shorter showers to reduce hot water usage, and opt for a fan rather than an air conditioner on warm days.

Here are a few more specific actions you can take to reduce greenhouse gas emissions:

Ensure your car is well-maintained. A well-tuned car runs more efficiently, emitting fewer harmful gases.
Whenever possible, opt to walk, bike, or carpool to work. Cars burn fossil fuels, so using smaller, fuel-efficient cars—especially hybrids—helps reduce CO2 emissions.
Turn off lights and appliances when not in use. While a light bulb itself doesn’t emit greenhouse gases, the electricity it consumes likely comes from a power plant that does. Also, consider switching to fluorescent bulbs as they use less energy and have a longer lifespan.
Recycle. Items that aren’t recycled end up in a landfill, where they generate methane. Recycling also uses less energy than producing new goods from raw materials.
Plant trees and other vegetation wherever you can. Plants absorb carbon dioxide and release oxygen into the air.
Avoid burning garbage. This releases carbon dioxide and hydrocarbons into the atmosphere.

To truly reduce greenhouse gas emissions, we must focus on developing alternative, non-fossil fuel energy sources. Technologies like hydro-electric power, solar energy, hydrogen engines, and fuel cells could significantly cut emissions if they become more widely adopted.

On a global scale, the Kyoto Protocol was created to curb CO2 and other greenhouse gas emissions worldwide. Thirty-five industrialized countries have committed to reducing their emissions by various amounts. However, the United States, the largest emitter of greenhouse gases, chose not to sign the treaty.

Al Gore’s book and documentary, "An Inconvenient Truth," sparked widespread discussion about global warming. Photo courtesy Consumer Guide Products

In March 2007, Al Gore, the former Vice President, addressed Congress and urged them to take bold action on national policy changes. These included:

Freeze carbon emissions at current levels and implement initiatives aimed at reducing them by 90 percent by 2050
Shift taxes from employment and production to a focus on pollution taxes
Negotiate an international agreement that aligns with the Kyoto Protocol, without the associated political challenges
Stop the construction of new coal-fired power plants unless they adhere to strict carbon emission standards
Raise emission standards across both the automotive and power industries
Prohibit the use of incandescent light bulbs

Gore acknowledges that enacting these proposals, along with others, may prove difficult. He further emphasizes that climate change isn't merely a crisis, but rather the most critical challenge humanity has ever faced.

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