What is the Greenhouse Effect?

The Greenhouse Effect is a naturally occurring phenomenon necessary to sustain life on earth. In a greenhouse, solar radiation passes through a mostly transparent piece of glass or plastic and warms the inside air, surface, and plants. As the temperature increases inside the greenhouse, the interior of the greenhouse radiates energy back to the outside and eventually a balance is reached.

The earth and its atmosphere simulates these greenhouse conditions. Short-wave radiation from the sun passes through the earth’s atmosphere. Some of this radiation is reflected back into space, some of it is absorbed by the atmosphere, and some of it makes it to the earth’s surface, where it is either reflected or absorbed. The earth, meanwhile, emits long-wave radiation toward space. Gases within the atmosphere absorb some of this long-wave radiation and re-radiate it back to the surface. It is because of this greenhouse-like function of the atmosphere that the average global temperature of the earth is 15°C (59°F). Without the atmosphere and these gases, the average global temperature would be a frigid -18°C (0°F), and life would not be possible on earth. These gases are called greenhouse gases and include carbon dioxide (CO2), water vapor (H2O), methane (CH4), nitrous oxide (N2O), chloroflourocarbons (CFCs), and ozone (O3).

The role of greenhouse gases in the atmosphere was first discovered during the 1800s (Kellogg, 1988). By 1896, Swedish scientist Svante Arrhenius was already calculating that the earth’s surface temperature would increase by 5-6°C (9–10.8°F) with a doubling or tripling of the atmospheric CO2 content, although he did not perceive that the greenhouse gas concentrations would increase so much in such a short period of time. Such predictions received relatively little notice until the 1950s. In 1957, Roger Revelle and Hans Suess, scientists at the Scripps Institution of Oceanography, said that by adding CO2 into the atmosphere humans were “now carrying out a large-scale geophysical experiment.” They also pointed out that the CO2 would remain within the atmosphere for a very long time because of how slowly it is absorbed by the oceans. Since then there has been a growing acknowledgment of the increasing concentrations of not only CO2 but also the other greenhouse gases and their potential impact on the global climate.

Public awareness of the Greenhouse Effect and the concern that the impact of increased emissions of CO2, CH4, N2O, and CFCs would raise global temperatures grew in the 1980s. During an intense drought and heat wave in 1988, the media and several scientists speculated that the drought and heat wave affecting much of the United States were evidence of climate change. In hindsight, the 1988 drought was likely within the range of normal climate variability, but the attention was focused on the Greenhouse Effect. Likewise, climate attribution researchers determined that the record-breaking warmth across much of the middle U.S. in March 2012 was much more extreme than climate change alone could explain.

What We Know about the Greenhouse Effect and Climate Change

We know with certainty that the concentrations of CO2, CH4, N2O, and CFCs have increased as a result of recent human activity. Carbon dioxide is responsible for more than half of the increase. Annual emissions of CO2 between 1970 and 2004 have increased by about 80% (IPCC, 2007). Concentrations of CO2 have been systematically monitored at the Mauna Loa Observatory in Hawaii since 1958. Scientists can determine the COconcentrations from before 1958 using data collected from air bubbles within ice cores around the world. In 1750, levels of CO2were around 270 parts per million (ppm). By 2005, these levels were up to 379 ppm, and the 10-year rate of growth in concentrations, 1.9 ppm per year, was greater than any previous 10-year period in the modern record (IPCC, 2007).

According to the congressional Office of Technology Assessment (OTA, 1993), 70–90% of the CO2 added to the atmosphere is due to the burning of fossil fuels, and the rest is from deforestation. Methane, N2O, and CFCs are also increasing at a similar rate. But because CFCs destroy ozone, the net warming effect from CFCs is approximately zero (but that leads to an entirely different problem: depletion of stratospheric ozone). 

Scientists can use this information within large-scale models of the atmosphere called General Circulation Models (GCMs). These models are composed of mathematical equations and relationships designed to simulate global atmospheric conditions and make projections of the future climate. Although there are differences between the GCM projections, the models are in general agreement that, as a result of increasing greenhouse gas concentrations, the current best estimate is that average global temperature will increase 1.8–4.0°C (3.24–7.2°F) by 2100 (IPCC, 2007). In the past 100 years, the global average surface temperature has increased 0.60°C (1.08°F). This increase by itself is within the normal variability and, although it may be a result of climate change, it cannot be used as definitive proof that recent human activities have caused a global warming. Between 1995 and 2006, however, 11 of the 12 years ranked among the 12 warmest years on record, with records dating back to 1850 (IPCC, 2007).

Climate Change and the Hydrological Cycle

With the projected global temperature increase, some scientists think that the global hydrological cycle will also intensify. GCMs indicate that global precipitation could increase 7–15%. Meanwhile, global evapotranspiration could increase 5–10% (OTA, 1993). Thus, the combined impacts of increased temperature, precipitation, and evapotranspiration will affect snowmelt, runoff, and soil moisture conditions. The models generally show that precipitation will increase at high latitudes and decrease at low and mid-latitudes. Therefore, in mid-continent regions, evapotranspiration will be greater than precipitation and there will exist the potential for more severe, longer-lasting droughts in these areas. In addition, the increased temperatures alone will cause the water in the oceans to expand. It has been estimated that global sea levels in the 20th century rose 17cm (6.7 in), and future predictions show that sea levels could reach 22-44 cm (8.66 - 17.3 in) above 1990 levels by 2100 (IPCC, 2007).

Observations show that precipitation amount, intensity, frequency, and type is changing across the globe. According to the IPCC, 2007:

Pronounced long-term trends from 1900 to 2005 have been observed in precipitation amount in some places: significantly wetter in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. More precipitation now falls as rain rather than snow in northern regions. Widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased. These changes are associated with increased water vapour in the atmosphere arising from the warming of the world’s oceans, especially at lower latitudes. There are also increases in some regions in the occurrences of both droughts and floods.

What We Do Not Know

One of the weaknesses of GCM climate change predictions is that they cannot adequately resolve factors that might influence regional climates, such as the local effects of mountains, coastlines, lakes, vegetation boundaries, and heterogeneous soils, or how human activities might play a role. This makes it difficult to predict the impacts on regional resources. GCMs also cannot tell us whether the occurrence of extreme events will increase. Some scientists have suggested that droughts, floods, and hurricanes will become more common with climate change. Although the losses due to natural disasters have increased worldwide and within the United States, this is more the result of increased vulnerability than an increased number of events.

No one knows who will be hurt or who will benefit from a CO2-induced climate change. Clearly, coastal regions will suffer as a result of a rise in sea levels. But for interior regions, there might be beneficial gains in agricultural production resulting from the indirect effects of a warmer climate and adequate precipitation, especially in higher latitudes across Canada and Russia. The increased CO2 might also directly increase plant growth and productivity as well. In fact, this theory, known as the CO2 Fertilization Effect, has led some scientists to suggest that the Greenhouse Effect might be a blessing in disguise. Laboratory experiments have shown that increased CO2concentrations potentially promote plant growth and ecosystem productivity by increasing the rate of photosynthesis, improving nutrient uptake and use, increasing water-use efficiency, and decreasing respiration, along with several other factors (OTA, 1993). The scientists, encouraged by these benefits, hypothesize that increased ecosystem productivity will actually help draw excess CO2 from the atmosphere, thereby diminishing concerns about global warming (OTA, 1993). Whether any benefits would result from the CO2 Fertilization Effect within the complex interactions of natural ecosystems is still unknown. Ecosystem productivity can only increase, however, in regions where supplies of light, water, and nutrients are plentiful (OTA, 1993).

Water Resources and Climate Change

Although we don’t know how climate change will affect regional water resources, it is clear that water resources are already stressed, independent of climate change, and any additional stress from climate change or increased variability will only intensify the competition for water resources. Current stresses on water resources around the globe include:

  • growing populations
  • increased competition for available water
  • poor water quality
  • environmental claims
  • uncertain reserved water rights
  • groundwater overdraft
  • outmoded institutions
  • aging urban water infrastructure

In all likelihood, the direct impacts of climate change on water resources will be hidden beneath natural climate variability. With a warmer climate, droughts and floods could become more frequent, severe, and longer-lasting. The potential increase in these hazards is a great concern given the stresses being placed on water resources and the high costs resulting from recent hazards. The drought of the late 1980s showed what the impacts might be if climate change leads to a change in the frequency and intensity of droughts across the United States. From 1987 to 1989, losses from drought in the United States totaled $39 billion (OTA, 1993). More frequent extreme events such as droughts and floods could end up being more cause for concern than the long-term change in temperature and precipitation averages.

The best advice to water resource managers regarding climate change is to start addressing current stresses on water supplies and build flexibility and robustness into any system. Flexibility helps to ensure a quick response to changing conditions, while robustness helps people prepare for and survive the worst conditions. With this approach to planning, water system managers will be better able to adapt to the impacts of climate change, whatever they may be, and will also be better equipped for the climate variability we have now.


IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. Cambridge University Press, Cambridge. 

Kellogg, W.W. 1988. Human impact on climate: The evolution of an awareness. In M.H. Glantz, ed. Societal Responses to Regional Climatic Change. Westview Press, Boulder, Colorado.

OTA (Office of Technology Assessment). 1993. Preparing for an Uncertain Climate, Vol. I. OTA–O–567. U.S. Government Printing Office, Washington, D.C.

NOAA (National Oceanic and Atmospheric Administration), Earth System Research Laboratory, Physical Sciences Division. 2012. Meteorological March Madness 2012. Hoerling, Martin.

Climate Change Links

Intergovernmental Panel on Climate Change (IPCC). The IPCC was established in 1988 and operates under the World Meteorological Organization and the United Nations Environment Program. It consists of a panel of scientists from more than 50 countries that study climate change and assess its potential impacts. Reports have been completed in 1990, 1992, 1995, 2001, and 2007.

United Nations Environment Program (UNEP). This site contains a variety of information related to the environment and sustainable development, including issues involving climate change. A section of the website is devoted to climate change. Topics in this section include information about impacts, publications, recent news, educational materials, and more.

Institute for the Study of Society and Environment (ISSE). ISSE is a multidisciplinary group of scientists working for the National Center for Atmospheric Research (NCAR) focusing on the interactions between human activities and the environment. A major research area covered by ISSE involves climate change.

Climate Impacts Group (CIG). CIG is an interdisciplinary group of researchers at the University of Washington who are focused on studying the impacts climate change and variability on the Pacific Northwest of the United States. CIG’s assessment examines climate impacts on water, forests, salmon, and coasts and the human socioeconomic and/or political systems associated with each.

Interpreting Climate Conditions, Physical Science Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration.

For more information, please contact Michael J. Hayes