Global Climate Change: Human Influences

Global Climate Change: Human Influences




In his book "Biogeochemistry: An Analysis of Global Change," William  Schlesinger states, "Humans affect the global system by creating a large biogeochemical flux where none existed before". This means that while nature has its own way of keeping all of its systems in balance, human activities have created an imbalance--a flux--among the systems.

One such imbalance that our species may be causing is in the earth's climate. Many human activities produce gases that contribute to the greenhouse effect, a process that warms the earth's atmosphere. The gases that contribute to this effect are called greenhouse gases. At natural concentrations, greenhouse gases benefit the planet by maintaining temperatures required for life. However, human activities are increasing the concentrations of these gases in the atmosphere. This is increasing the greenhouse effect and thus increasing the global temperature of the planet. Major greenhouse gases include chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O), and tropospheric ozone (O₃). The biggest influence humans have on the greenhouse effect is the production of carbon dioxide (CO₂). This is a very powerful greenhouse gas that comes from many human activities.
Find out more about the chemistry of human influences on global climate change.

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Global Climate Change: Overview

Global Climate Change: Overview
Global climate change is a change in the long-term weather



patterns that characterize the regions of the world. The term "weather" refers to the short-term (daily) changes in temperature, wind, and/or precipitation of a region (Merritts et al. 1998). Weather is influenced by the sun. The sun heats the earth's atmosphere and its surface causing air and water to move around the planet. The result can be as simple as a slight breeze or as complex as the formation of a tornado (see above). Photo: The first tornado captured by the National Severe Storms Laboratory Doppler radar in Union City, Oklahoma on May 24, 1973. Photo courtesy of National Oceanic and Atmospheric Administration.

Some of the sun's incoming long wave radiation is reflected back to space by aerosols. Aerosols are very small particles of dust, water vapor, and chemicals in Earth's atmosphere. In addition, some of the sun's energy that has entered Earth's atmosphere is reflected into space by the planet's surface. The reflectivity of Earth's surface is called albedo. Both of these reflective processes have a cooling affect on the planet.
The greenhouse effect is a warming process that balances Earth's cooling processes. During this process, sunlight passes through Earth's atmosphere as short-wave radiation. Some of the radiation is absorbed by the planet's surface. As Earth's surface is heated, it emits long wave radiation toward the atmosphere. In the atmosphere, some of the long wave radiation is absorbed by certain gases called greenhouse gases. Greenhouse gases include carbon dioxide (CO₂), Chlorofluorocarbons (CFC's), methane (CH₄), nitrous oxide (N₂0), tropospheric ozone (O₃), and water vapor. Each molecule of greenhouse gas becomes energized by the long wave radiation. The energized molecules of gas then emit heat energy in all directions. By emitting heat energy toward Earth, greenhouse gases increase Earth's temperature. Note that the warning mechanism for the "greenhouse effect" is NOT exactly the same as the warning mechanism of greenhouse walls. While greenhouse gases absorb long wave radiation then emit heat energy in all directions, greenhouse walls physically trap heat inside of greenhouses and prevent it from escaping to the atmosphere.
The greenhouse effect is a natural occurrence that maintains Earth's average temperature at approximately 60 degrees Fahrenheit. The greenhouse effect is a necessary phenomenon that keeps all Earth's heat from escaping to the outer atmosphere. Without the greenhouse effect, temperatures on Earth would be much lower than they are now, and the existence of life on this planet would not be possible. However, too many greenhouse gases in Earth's atmosphere could increase the greenhouse effect. This could result in an increase in mean global temperatures as well as changes in precipitation patterns.  
When weather patterns for an area change in one direction over long periods of time, they can result in a net climate change for that area. The key concept in climate change is time. Natural changes in climate usually occur over; that is to say they occur over such long periods of time that they are often not noticed within several human lifetimes. This gradual nature of the changes in climate enables the plants, animals, and microorganisms on earth to evolve and adapt to the new temperatures, precipitation patterns, etc.
The real threat of climate change lies in how rapidly the change occurs. For example, over the past 130 years, the 7mean global temperature appears to have risen 0.6 to 1.2 degrees Fahrenheit (0.3 to 0.7 degrees Celsius). These temperatures changes are depicted in the graph below from the EPA's Global Warming site. The increasing steepness of the curve suggests that changes in mean global temperature have occurred at greater rates over time. Further evidence suggests that future increases in mean global temperature may occur at a rate of 0.4 degrees Fahrenheit (0.2 degrees Celsius) each decade. Figure: Changes in global temperature (degrees Fahrenheit) from 1861 to 1996. Graph adapted from image courtesy of the U.S. EPA. 
The geological record--the physical evidence of the results of processes that have occurred on Earth since it was formed--provides evidence of climate changes similar in magnitude to those in the the above graph. This means during the history of the earth, there have been changes in global temperatures similar in size to these changes. However, the past changes occurred at much slower rates, and thus they were spread out over long periods of time. The slow rate of change allowed most species enough time to adapt to the new climate. The current and predicted rates of temperature change, on the other hand, may be harmful to ecosystems. This is because these rates of temperature change are much faster than those of Earth's past. Many species of plants, animals, and microorganisms may not have enough time to adapt to the new climate. These organisms may become extinct. Figure: Global temperature (degrees F) changes from 1861 to 1996. Graph adapted from image courtesy of the U.S. EPA.

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what We Know: Underlying Processes

The balance between energy absorbed by the earth and energy reflected back into space is fundamental in determining how warm or cool the planet becomes. The proportion of radiation reflected away by a surface is called its albedo. Albedo can range between 0 (no reflectance) and 1 (complete reflectance—like a mirror). The earth’s average albedo is .31, which means that, overall, the planet reflects about 31% of incoming solar radiation back into space. But forests and deserts, oceans, clouds, snow, and ice all have different albedos—and changes in these types of ground cover can therefore affect how much solar radiation the earth receives. For example, the albedos of forests lie in the range 0.07–0.15, while deserts have an albedo of around 0.3.

This satellite image shows Antarctica’s Scott Coastline on January 4, 2002. The large, coke-bottle-shaped iceberg in the lower right broke off the Ross Ice Shelf in December 2001. (See “The Collapse of the Larsen B Ice Shelf” on this site for more information.)

The Dry Valleys are one of the few areas of Antarctica not covered by ice. Unlike much of the rest of the earth, the Dry Valleys have cooled over the last 100 years. (See “The Collapse of the Larsen B Ice Shelf” on this site for more information on the implications of this cooling trend.) The albedo of the earth’s surface varies from about 0.1 for the oceans to 0.6–0.9 for ice and clouds, which means that clouds, snow, and ice are good radiation reflectors while liquid water is not. (This is because clouds, snow, and ice have multiple layers that reflect radiation, whereas a body of water reflects only from its surface. A calm ocean is a poor reflector, but when it foams up in the surfline, producing many reflecting surfaces, it becomes white— reflecting most of the light hitting it.) In fact, snow and ice have the highest albedos of any parts of the earth’s surface: Some parts of the Antarctic reflect up to 90% of incoming solar radiation.

Continued global warming will have one obvious effect on the world’s polar ice, sea ice, glaciers, and permanent snow cover: Warmer temperatures will melt some of this frozen water. Melting of land-based ice sheets and glaciers could contribute to sea-level changes. (Melting sea ice would not contribute to rising sea levels: When ice floating in water melts, the level of the water doesn’t change. You can prove this to yourself by watching the ice melt in a glass of water.) (See “Global Glacier Volume Change” on this site for more on melting glacier.)

These photographs, taken in 1928 and 2000, show how South Cascade Glacier in the Washington Cascade Mountains has retreated over time.

Evidences and Uncertainties

Melting ice could change ocean temperatures. This, in turn, could change the course and speed of ocean currents, significantly change the habitats of sea organisms, and affect rainfall by altering the rate of evaporation of seawater.

Increases in sea levels and temperatures are not the only possible outcomes. When ice and snow melt, they generally expose a much darker underlying surface. Dark surfaces absorb more heat (have a lower albedo) than light surfaces. This suggests the possibility that a small amount of melting could lead to a warmer surface, which could melt more ice, warming the surface still further—initiating the positive feedback loop of a “runaway” warming trend. There is some evidence of such an albedo-reducing effect in the Cretaceous Period (120–65 million years ago): Fossil and other evidence suggests that there was little or no snow and ice cover during this time, and global temperatures then were at least 8° to 10°C higher than they are now. (See “Northern Hemisphere Snow and Ice Chart” and “South Pole/Ice Concentration” on this site to see the extent of current snow and ice cover.)

The cryosphere also provides a way to study past climatic conditions. If snow falls in a region of the earth where melting rarely occurs, it leaves a layered record as it deposits contemporary molecules and aerosols. As each layer is pushed deeper and deeper under increasing pressure, the snow turns to ice, capturing small bubbles of air. By examining ice cores taken from these areas, we can determine associations between past temperature and carbon dioxide levels. But one of the biggest problems in any ice core study is determining the age-depth relationship. Many different approaches have been used, and it’s now clear that fairly accurate time scales can be developed for at least the last 10,000 years. (See “Climate records from the Vostok Ice Core Covering the Last 420,000 years” on this site to learn more about the Antartica's Vostok ice core.)

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what We Know: Underlying Processes.

The earth receives a tremendous amount of energy from the sun. The land, sea, and air absorb some of this energy and reflect some of it back into space. The overall description of this process is called the earth’s energy budget. (See “Global Reflected Shortwave Solar Radiation” and “Global Outgoing Longwave Heat Radiation” on this site to learn more.)
The “greenhouse effect” is one aspect of this energy budget. Just as the glass walls of a greenhouse keep the interior temperature higher than that outside, the earth’s atmosphere traps some of the energy radiated from the earth near the planets surface. The presence of “greenhouse gases” (like water vapor and carbon dioxide), keeps the planet’s average temperature at a hospitable 15°C. (With no greenhouse effect, the earth’s average temperature would stabilize at about -18°C). Not all components of the atmosphere are greenhouse gases, however; in fact, oxygen and nitrogen, which together make up more than 95% of our atmosphere, are not greenhouse gases.

This diagram shows the processes that make up the planet’s energy budget. The earth’s surface absorbs shortwave radiation (red arrows) and re-radiates longwave infrared radiation (blue arrow). The numbers are percentages: For example, 30% of the solar radiation shining on the earth is reflected away.

The greenhouse effect is not in dispute—but it lies at the heart of the study of global climate change.


There’s no doubt that increases in the atmospheric concentration of carbon dioxide and other greenhouse gases strengthens the greenhouse effect and contributes to global warming.
What remains uncertain are the precise effects of a strengthened greenhouse effect on global temperatures. Because there is still much to be learned about how the world’s climate will react to increased greenhouse gas concentrations, the range of possible climate futures projected by the IPCC is an indication of uncertainty about how much the world will warm over the coming century—not of whether that warming is happening.


Aerosols are another key component of the earth’s atmosphere. These are suspended liquid and solid particles, including things like soot from fires and volcanic eruptions, sea salt, bacteria, and viruses. Aerosols affect the earth’s energy budget by scattering and absorbing radiation:


Overall, aerosols likely exert a cooling effect, because many of these particles tend to prevent radiation from reaching the planet’s surface (although due to their size and shape, some aerosols may help trap heat near the ground).

This satellite image shows a dust plume from the Sahara Desert blowing across the Atlantic Ocean. The green to red colors in the dust plume image represent increasing densities of tiny airborne particles known as aerosols.

Evidences and Uncertainties

Measurements from a variety of sources have suggested that the earth’s average atmospheric temperature has risen over the last several hundred years—but by how much? Taking the average temperature of the earth’s atmosphere is a very difficult measurement problem. First, measurements must be taken in a large and diverse enough range of locations to ensure that their average is truly a measure of global temperature and is not biased toward one region or another. Second, those locations must be chosen so that individual measurements are not thrown off by sources of unusually high or low temperatures, such as cities (which tend to be “heat islands” warmer than the surrounding landscape). Third, no measuring device is perfect—all measurements include some amount of error, or “noise.” Understanding the kinds of errors associated with different measurement techniques is a key element in evaluating the accuracy of a given temperature value. In addition, the study of climate requires measurements over very long time periods, so sources of paleoclimate data (data on climate from the distant past) are key to understanding climate change. (See “Global Stratospheric and Tropospheric Temperature Anomalies (1979–2001)” on this site to learn more about the problems of measurement.)

How much will global atmospheric temperatures change over the next century? Two kinds of problems make this an exceptionally difficult question to answer. (See “Sample Forecasts of Future Temperature Change” on this site for some possible answers.) First, the enormous complexity of the earth’s dynamic climate system—including the interacting air masses, winds and, ocean currents, and patterns of evaporation and precipitation—makes long-term climate prediction extremely problematic. Estimates drawn from reports by the Intergovernmental Panel on Climate Change (IPCC) project increases in average global temperatures ranging from 1.4 degrees to 5.8 degrees C by the year 2100. These numbers may seem small, but because average global temperatures are actually remarkably stable over long periods, this range actually represents a very significant rise in the earth’s temperature over a very short time.

A second problem complicating the picture is the unpredictability of human behavior. At what rate will the human population—and its production of carbon dioxide—grow? As formerly undeveloped countries expand their industry, often using cheaper (and more polluting) fossil-fuel technology, their contributions to greenhouse gases will rise and add to the problem—but by how much? To what extent will new, cleaner technologies (such as cars powered by hydrogen fuel cells) be developed and adopted by countries around the world? These kinds of uncertainties make the tough problem of predicting climate change all the more difficult. Even moderate increases in atmospheric temperatures could alter precipitation levels, making some areas wetter and others drier, and affecting agriculture worldwide. Warmer temperatures could increase the frequency and strength of storm systems, leading to more powerful and destructive hurricanes and subsequent flooding. Small shifts in the earth’s temperature could result in more powerful and destructive hurricanes. This satellite image shows Hurricane Isaac on the afternoon of September 29, 2000.

Slight changes in temperature may lead to higher ozone levels near the earth’s surface. This could significantly increase smog problems in large cities—bad for all of us, but especially serious for many elderly, ill, or otherwise physically vulnerable citizens.

Small increases in atmospheric temperatures could also change the way clouds form and dissipate. Warmer temperatures near the ground could cause lower clouds to evaporate, letting heat rise farther into the atmosphere.

As this heated air rises and cools, higher clouds form. But lower clouds usually reflect sunlight back into space while higher clouds tend to absorb more heat. More high clouds mean more heat trapped near the earth’s surface—so small increases in temperature could set off a cycle in which the atmosphere holds more and more heat over time.

(This is an example of a positive feedback loop—a system in which small changes in one direction may set the stage for later, larger changes in the same direction. But we don’t yet know whether positive feedback loops like this will dominate future climate, or whether other factors will prevent patterns like this from unfolding. )

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what do we know about global climate change?

We know that the earth has become warmer over the last century. The Intergovernmental Panel on Climate Change (IPCC), a group established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), reports that the average surface temperature of the earth has increased during the twentieth century by about 0.6° ± 0.2°C. (The ± 0.2°C means that the increase might be as small as 0.4°C or as great as 0.8°C.) This may seem like a small shift, but although regional and short-term temperatures do fluctuate over a wide range, global temperatures are generally quite stable. In fact, the difference between today’s average global temperature and the average global temperature during the last Ice Age is only about 5 degrees C. Indeed, it’s warmer today around the world than at any time during the past 1000 years, and the warmest years of the previous century have occurred within the past decade.



We also know that human activities—primarily the burning of fossil fuels—have increased the greenhouse gas content of the earth’s atmosphere significantly over the same period. Carbon dioxide is one of the most important greenhouse gases, which trap heat near the planet’s surface.

Red, orange, and brown coloring indicate areas where temperatures measured in 2000 are warmer than the average temperature from 1951 to 1980. The scale represents degrees in Celsius. Negative numbers represent cooling, and positive numbers depict warming.

The vast majority of climate researchers agree with these overall findings. The scientific disagreements that do still exist primarily concern detailed aspects of the processes that make up these largely accepted general themes.
You can think of this web site as a window into the world of scientific research. In this primer, you’ll find a general discussion of the physical processes underlying the earth’s climate, an outline of the kinds of data that may shed light on how the climate is changing—and the role of human activity in these changes —and a description of some of the questions and uncertainties that researchers continue to explore. This primer is organized into four interconnected sections: the Atmosphere; the Hydrosphere (the earth’s oceans and water); the Cryosphere (the areas of the planet covered by snow and ice); and the Biosphere (the living organisms inhabiting all these domains).

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Nepalese ministers meet in mountains.

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Article published on the 2009-12-04 Latest update 2009-12-04 13:09 TU

(Photo: Reuters)

Nepalese Prime Minister Madhav Kumar Nepal and 22 other government ministers held a high-attitude meeting on the Kalapattar plateau on Friday to highlight the affects of global warming. Scientists say the Himalayan glaciers are melting, threatening mountain communities.

Members of the Nepalese government arrived by helicopter, equipped with oxygen, for a meeting at the world’s highest mountain range, in the shadow of Mount Everest, 5,262 metres above sea level.
Environment Minister Thakur Prasad Sharma denied that the meeting was a publicity stunt ahead of the Copenhagen meeting, and instead he drew attention to the melting of the glaciers.
Kumar told the Nepalese media that climate change was affecting people in the region and their, “socio-economic development”.
Scientists believe that thawing of the ice is creating glacial lakes that threaten mountain communities if they burst and flood downstream.
The disappearance of the Himalayan glaciers would also bring drought to large regions of Asia, where around 1.3 billion people depend on rivers stemming from the mountain region.
According to a study by the Asian Development Bank, climate change will, “cause faster melt and retreat among Nepal’s 3,252 glaciers”, “affect agricultural production and yield”, and increase the, “risk of malaria and encephalitis”.
Officials were expected to spend around 20 minutes at the meeting place and take part in the traditional Sherpa prayer ceremony, before approving the speech to be delivered by Kumar at Copenhagen.

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