Despite ups and downs from year to year, global average surface temperature is rising. Roughly 30 percent of incoming sunlight is reflected back into space by bright surfaces like clouds and ice. Of the remaining 70 percent, most is absorbed by the land and ocean, and the rest is absorbed by the atmosphere.
The absorbed solar energy heats our planet. From the surface, this energy travels into the atmosphere where much of it is absorbed by water vapor and long-lived greenhouse gases such as carbon dioxide and methane. They radiate in all directions. The energy that radiates back toward Earth heats both the lower atmosphere and the surface, enhancing the heating they get from direct sunlight. This absorption and radiation of heat by the atmosphere—the natural greenhouse effect—is beneficial for life on Earth.
What has scientists concerned now is that over the past years, humans have been artificially raising the concentration of greenhouse gases in the atmosphere at an ever-increasing rate, mostly by burning fossil fuels, but also from cutting down carbon-absorbing forests. Since the Industrial Revolution began in about , carbon dioxide levels have increased nearly 38 percent as of and methane levels have increased percent.
Increases in concentrations of carbon dioxide top and methane bottom coincided with the start of the Industrial Revolution in about Measurements from Antarctic ice cores green lines combined with direct atmospheric measurements blue lines show the increase of both gases over time. The atmosphere today contains more greenhouse gas molecules, so more of the infrared energy emitted by the surface ends up being absorbed by the atmosphere.
We know about past climates because of evidence left in tree rings, layers of ice in glaciers, ocean sediments, coral reefs, and layers of sedimentary rocks. The chemical make-up of the ice provides clues to the average global temperature. Earth has cycled between ice ages low points, large negative anomalies and warm interglacials peaks.
But the paleoclimate record also reveals that the current climatic warming is occurring much more rapidly than past warming events. As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5, years. In the past century alone, the temperature has climbed 0.
Temperature histories from paleoclimate data green line compared to the history based on modern instruments blue line suggest that global temperature is warmer now than it has been in the past 1, years, and possibly longer. Graph adapted from Mann et al. Models predict that Earth will warm between 2 and 6 degrees Celsius in the next century. When global warming has happened at various times in the past two million years, it has taken the planet about 5, years to warm 5 degrees.
The predicted rate of warming for the next century is at least 20 times faster. This rate of change is extremely unusual. Most often, global climate has changed because of variations in sunlight.
Variations in the Sun itself have alternately increased and decreased the amount of solar energy reaching Earth. Volcanic eruptions have generated particles that reflect sunlight, brightening the planet and cooling the climate. Volcanic activity has also, in the deep past, increased greenhouse gases over millions of years, contributing to episodes of global warming.
We know this because scientists closely monitor the natural and human activities that influence climate with a fleet of satellites and surface instruments. Remote meteorological stations left and orbiting satellites right help scientists monitor the causes and effects of global warming. On the ground, many agencies and nations support networks of weather and climate-monitoring stations that maintain temperature, rainfall, and snow depth records, and buoys that measure surface water and deep ocean temperatures.
Taken together, these measurements provide an ever-improving record of both natural events and human activity for the past years. Scientists integrate these measurements into climate models to recreate temperatures recorded over the past years.
Climate model simulations that consider only natural solar variability and volcanic aerosols since —omitting observed increases in greenhouse gases—are able to fit the observations of global temperatures only up until about After that point, the decadal trend in global surface warming cannot be explained without including the contribution of the greenhouse gases added by humans.
For example, two major volcanic eruptions, El Chichon in and Pinatubo in , pumped sulfur dioxide gas high into the atmosphere. Temperatures across the globe dipped for two to three years. Graphs adapted from Lean et al. Although volcanoes are active around the world, and continue to emit carbon dioxide as they did in the past, the amount of carbon dioxide they release is extremely small compared to human emissions.
On average, volcanoes emit between and million tonnes of carbon dioxide per year. By burning fossil fuels, people release in excess of times more, about 26 billion tonnes of carbon dioxide, into the atmosphere every year as of As a result, human activity overshadows any contribution volcanoes may have made to recent global warming.
Changes in the brightness of the Sun can influence the climate from decade to decade, but an increase in solar output falls short as an explanation for recent warming. The total energy the Sun radiates varies over an year cycle. During solar maxima, solar energy is approximately 0. The transparent halo known as the solar corona changes between solar maximum left and solar minimum right.
Each cycle exhibits subtle differences in intensity and duration. As of early , the solar brightness since has been slightly lower, not higher, than it was during the previous year minimum in solar activity, which occurred in the late s.
Satellite measurements of daily light line and monthly average dark line total solar irradiance since have not detected a clear long-term trend. Scientists theorize that there may be a multi-decadal trend in solar output, though if one exists, it has not been observed as yet. Even if the Sun were getting brighter, however, the pattern of warming observed on Earth since does not match the type of warming the Sun alone would cause. Satellite measurements show warming in the troposphere lower atmosphere, green line but cooling in the stratosphere upper atmosphere, red line.
This vertical pattern is consistent with global warming due to increasing greenhouse gases, but inconsistent with warming from natural causes.
The stratosphere gets warmer during solar maxima because the ozone layer absorbs ultraviolet light; more ultraviolet light during solar maxima means warmer temperatures.
Increased concentrations of carbon dioxide in the troposphere and stratosphere together contribute to cooling in the stratosphere. To further explore the causes and effects of global warming and to predict future warming, scientists build climate models—computer simulations of the climate system.
Climate models are designed to simulate the responses and interactions of the oceans and atmosphere, and to account for changes to the land surface, both natural and human-induced.
Though the models are complicated, rigorous tests with real-world data hone them into powerful tools that allow scientists to explore our understanding of climate in ways not otherwise possible. Model simulations by the Intergovernmental Panel on Climate Change estimate that Earth will warm between two and six degrees Celsius over the next century, depending on how fast carbon dioxide emissions grow.
Scenarios that assume that people will burn more and more fossil fuel provide the estimates in the top end of the temperature range, while scenarios that assume that greenhouse gas emissions will grow slowly give lower temperature predictions.
The orange line provides an estimate of global temperatures if greenhouse gases stayed at year levels. Greenhouse gases are only part of the story when it comes to global warming. Changes to one part of the climate system can cause additional changes to the way the planet absorbs or reflects energy. These secondary changes are called climate feedbacks, and they could more than double the amount of warming caused by carbon dioxide alone.
The primary feedbacks are due to snow and ice, water vapor, clouds, and the carbon cycle. Perhaps the most well known feedback comes from melting snow and ice in the Northern Hemisphere. Warming temperatures are already melting a growing percentage of Arctic sea ice, exposing dark ocean water during the perpetual sunlight of summer.
Snow cover on land is also dwindling in many areas. In the absence of snow and ice, these areas go from having bright, sunlight-reflecting surfaces that cool the planet to having dark, sunlight-absorbing surfaces that bring more energy into the Earth system and cause more warming. In the past years, the glacier has lost half its volume and has retreated more than 1.
As glaciers retreat, sea ice disappears, and snow melts earlier in the spring, the Earth absorbs more sunlight than it would if the reflective snow and ice remained. The largest feedback is water vapor. Water vapor is a strong greenhouse gas. In fact, because of its abundance in the atmosphere, water vapor causes about two-thirds of greenhouse warming, a key factor in keeping temperatures in the habitable range on Earth.
But as temperatures warm, more water vapor evaporates from the surface into the atmosphere, where it can cause temperatures to climb further. The question that scientists ask is, how much water vapor will be in the atmosphere in a warming world?
The atmosphere currently has an average equilibrium or balance between water vapor concentration and temperature. As temperatures warm, the atmosphere becomes capable of containing more water vapor, and so water vapor concentrations go up to regain equilibrium. Will that trend hold as temperatures continue to warm? The amount of water vapor that enters the atmosphere ultimately determines how much additional warming will occur due to the water vapor feedback.
The atmosphere responds quickly to the water vapor feedback. So far, most of the atmosphere has maintained a near constant balance between temperature and water vapor concentration as temperatures have gone up in recent decades. If this trend continues, and many models say that it will, water vapor has the capacity to double the warming caused by carbon dioxide alone. Closely related to the water vapor feedback is the cloud feedback.
Clouds cause cooling by reflecting solar energy, but they also cause warming by absorbing infrared energy like greenhouse gases from the surface when they are over areas that are warmer than they are. In our current climate, clouds have a cooling effect overall, but that could change in a warmer environment. Clouds can both cool the planet by reflecting visible light from the sun and warm the planet by absorbing heat radiation emitted by the surface. On balance, clouds slightly cool the Earth.
Clouds can become brighter if more moisture converges in a particular region or if more fine particles aerosols enter the air. If fewer bright clouds form, it will contribute to warming from the cloud feedback. See Ship Tracks South of Alaska to learn how aerosols can make clouds brighter. Clouds, like greenhouse gases, also absorb and re-emit infrared energy. Low, warm clouds emit more energy than high, cold clouds. However, in many parts of the world, energy emitted by low clouds can be absorbed by the abundant water vapor above them.
In a world without low clouds, the amount of emitted infrared energy escaping to space would not be too different from a world with low clouds. Clouds emit thermal infrared heat radiation in proportion to their temperature, which is related to altitude. This image shows the Western Hemisphere in the thermal infrared. Warm ocean and land surface areas are white and light gray; cool, low-level clouds are medium gray; and cold, high-altitude clouds are dark gray and black.
High cold clouds, however, form in a part of the atmosphere where energy-absorbing water vapor is scarce. These clouds trap absorb energy coming from the lower atmosphere, and emit little energy to space because of their frigid temperatures. In a world with high clouds, a significant amount of energy that would otherwise escape to space is captured in the atmosphere.
As a result, global temperatures are higher than in a world without high clouds. It took nearly a century of research and data to convince the vast majority of the scientific community that human activity could alter the climate of our entire planet.
In the s, experiments suggesting that human-produced carbon dioxide CO2 and other gases could collect in the atmosphere and insulate Earth were met with more curiosity than concern. By the late s, CO2 readings would offer some of the first data to corroborate the global warming theory.
Eventually an abundance of data, along with climate modeling would show not only that global warming was real, but that it also presented a host of dire consequences. Dating back to the ancient Greeks, many people had proposed that humans could change temperatures and influence rainfall by chopping down trees, plowing fields or irrigating a desert. Accurate or not, those perceived climate effects were merely local.
The idea that humans could somehow alter climate on a global scale would seem far-fetched for centuries. In the s, French mathematician and physicist Joseph Fourier proposed that energy reaching the planet as sunlight must be balanced by energy returning to space since heated surfaces emit radiation. But some of that energy, he reasoned, must be held within the atmosphere and not return to space, keeping Earth warm.
Energy enters through the glass walls, but is then trapped inside, much like a warm greenhouse. But the so-called greenhouse effect analogy stuck and some 40 years later, Irish scientist John Tyndall would start to explore exactly what kinds of gases were most likely to play a role in absorbing sunlight. He eventually demonstrated that CO2 alone acted like sponge in the way it could absorb multiple wavelengths of sunlight.
By , Swedish chemist Svante Arrhenius became curious about how decreasing levels of CO2 in the atmosphere might cool Earth. In order to explain past ice ages, he wondered if a decrease in volcanic activity might lower global CO2 levels. His calculations showed that if CO2 levels were halved, global temperatures could decrease by about 5 degrees Celsius 9 degrees Fahrenheit.
Next, Arrhenius wondered if the reverse were true. Arrhenius returned to his calculations, this time investigating what would happen if CO2 levels were doubled. The possibility seemed remote at the time, but his results suggested that global temperatures would increase by the same amount—5 degrees C or 9 degrees F. By the s, at least one scientist would start to claim that carbon emissions might already be having a warming effect. He would continue to argue into the s that a greenhouse-effect warming of the planet was underway.
That attention played a part in garnering some of the first government-funded projects to more closely monitor climate and CO2 levels. Scripps geochemist Charles Keeling was instrumental in outlining a way to record CO2 levels and in securing funding for the observatory, which was positioned in the center of the Pacific Ocean.
The dawn of advanced computer modeling in the s began to predict possible outcomes of the rise in CO2 levels made evident by the Keeling Curve. Computer models consistently showed that a doubling of CO2 could produce a warming of 2 degrees C or 3. In the early s, a different kind of climate worry took hold: global cooling. As more people became concerned about pollutants people were emitting into the atmosphere, some scientists theorized the pollution could block sunlight and cool Earth.
In fact, Earth did cool somewhat between due to a postwar boom in aerosol pollutants which reflected sunlight away from the planet. But as the brief cooling period ended and temperatures resumed their upward climb, warnings by a minority of scientists that Earth was cooling were dropped.
Part of the reasoning was that while smog could remain suspended in the air for weeks, CO2 could persist in the atmosphere for centuries.
He writes: "The temperature [of the Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in re-passing into the air when converted into non-luminous heat.
More than a century later, he is honoured by having a prominent UK climate research organisation - the Tyndall Centre - named after him. He suggests this might be beneficial for future generations. His conclusions on the likely size of the "man-made greenhouse" are in the same ballpark - a few degrees Celsius for a doubling of CO2 - as modern-day climate models.
Although he does not realise the significance, Angstrom has shown that a trace gas can produce greenhouse warming.
He also shows that CO2 concentrations had increased over the same period, and suggests this caused the warming. The "Callendar effect" is widely dismissed by meteorologists.
He concludes that doubling CO2 concentrations would increase temperatures by C. Revelle writes: "Human beings are now carrying out a large scale geophysical experiment
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