CHAPTER 7. THE GREENHOUSE EFFECT
Global warming describes the current rise in the average temperature of Earth's air The greenhouse effect keeps Earth's climate comfortable. . Many governments around the world are working toward this goal. . person who studies places and the relationships between people and their environments. There is, however, one important difference between climate change in the a major turning point in the relationship between humans and the environment. This so called natural 'greenhouse effect' balances the average temperature on earth. with the goal of minimizing warming of the earth below 2 degrees Celsius. The naturally occurring greenhouse gases CO2, CH4, and N2O show large increases It is well established that the global mean surface temperature of the Earth has .. tend to show a linear relationship between the initial radiative forcing and the . are important in designing control strategies to meet regulatory goals!.
Indirect effects of aerosols represent the largest uncertainty in radiative forcing. Atmospheric soot directly absorbs solar radiation, which heats the atmosphere and cools the surface. Contribution of natural factors and human activities to radiative forcing of climate change. Milankovitch cycles The tilt of the Earth's axis and the shape of its orbit around the Sun vary slowly over tens of thousands of years.
This changes climate by changing the seasonal and latitudinal distribution of incoming solar energy at Earth's surface. Climate change feedbackClimate sensitivityand Arctic amplification The dark ocean surface reflects only 6 percent of incoming solar radiation, whereas sea ice reflects 50 to 70 percent.
Positive feedbacks increase the response of the climate system to an initial forcing, while negative feedbacks reduce it. Other factors being equal, a higher climate sensitivity means that more warming will occur for a given increase in greenhouse gas forcing. More research is needed to understand the role of clouds  and carbon cycle feedbacks in climate projections. Another study conducted by Harvard researchers suggests that increased water vapor injected into the stratospheredue to rising temperatures, increases ozone depletionsubsequently raising the odds of skin cancer and damaging crops.
Projected change in annual mean surface air temperature from the late 20th century to the middle 21st century, based on a medium emissions scenario SRES A1B. Global climate model A climate model is a representation of the physical, chemical and biological processes that affect the climate system.
Results from models can also vary due to different greenhouse gas inputs and the model's climate sensitivity. For example, the uncertainty in IPCC's projections is caused by 1 the use of multiple models  with differing sensitivity to greenhouse gas concentrations,  2 the use of differing estimates of humanity's future greenhouse gas emissions,  3 any additional emissions from climate feedbacks that were not included in the models IPCC used to prepare its report, i.
Instead the models predict how greenhouse gases will interact with radiative transfer and other physical processes. Warming or cooling is thus a result, not an assumption, of the models.
Improving the models' representation of clouds is therefore an important topic in current research. Although these models do not unambiguously attribute the warming that occurred from approximately to to either natural variation or human effects, they do indicate that the warming since is dominated by anthropogenic greenhouse gas emissions. Observed Arctic shrinkage has been faster than that predicted.
Effects of global warming Projections of global mean sea level rise by Parris and others. Map of the Earth with a six-meter sea level rise represented in red. Sparse records indicate that glaciers have been retreating since the early s. Biosphere Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.
Geological Survey projects that two-thirds of polar bears will disappear by Physical impacts of climate change and Climate change and ecosystems The environmental effects of global warming are broad and far reaching. They include the following diverse effects: Arctic sea ice declinesea level riseretreat of glaciers: Global warming has led to decades of shrinking and thinning in a warm climate that has put the Arctic sea ice in a precarious position, it is now vulnerable to atmospheric anomalies.
Additionally, sea level rise has accelerated from to Data analysis of extreme events from until suggests that droughts and heat waves appear simultaneously with increased frequency. In terrestrial ecosystemsthe earlier timing of spring events, as well as poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.
The same emission temperature is found at mm where again H2O is a major absorber. We see from the above discussion how terrestrial emission spectra measured from space can be used to retrieve information on the temperature of the Earth's surface as well as on the thermal structure and composition of the atmosphere.
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Additional information on the vertical distribution of a gas can be obtained from the width of the absorption lines, which increase linearly with air density in the troposphere and lower stratosphere.
Research instruments aboard satellites use wavelength resolutions of the order of a nanometer to retrieve concentrations and vertical profiles of atmospheric gases, and intricate algorithms are needed for the retrieval. Another important point from the above discussion is that all greenhouse gases are not equally efficient at trapping terrestrial radiation.
Consider a greenhouse gas absorbing at 11 mm, in the atmospheric window Figure Injecting such a gas into the atmosphere would decrease the radiation emitted to space at 11 mm since this radiation would now be emitted by the cold atmosphere rather than by the warm surface.
Relation between Greenhouse Gases and Climate Change
In order to maintain a constant terrestrial blackbody emission integrated over all wavelengths, it would be necessary to increase the emission flux in other regions of the spectrum and thus warm the Earth. Contrast this situation to a greenhouse gas absorbing solely at 15 mm, in the CO2 absorption band Figure At that wavelength the atmospheric column is already opaque Figureand injecting an additional atmospheric absorber has no significant greenhouse effect.
The GCMs are 3-dimensional meteorological models that attempt to capture the ensemble of radiative, dynamical, and hydrological factors controlling the Earth's climate through the solution of fundamental equations describing the physics of the system.
In these models, a radiative perturbation associated with increase in a greenhouse gas radiative forcing triggers an initial warming; complex responses follow involving for example enhanced evaporation of water vapor from the ocean a positive feedback, since water is a greenhouse gaschanges in cloud cover, and changes in the atmospheric or oceanic circulation. There is still considerable doubt regarding the ability of GCMs to simulate perturbations to climate, and indeed different GCMs show large disagreements in the predicted surface warmings resulting from a given increase in greenhouse gases.
A major uncertainty is the response of cloud cover to the initial radiative forcing section 7. Despite these problems, all GCMs tend to show a linear relationship between the initial radiative forcing and the ultimate perturbation to the surface temperature, the difference between models lying in the slope of that relationship. Because the radiative forcing can be calculated with some confidence, it provides a useful quantitative index to estimate and compare the potential of various atmospheric disturbances to affect climate.
Consider a radiative model for the present-day atmosphere using observed or estimated values of all variables affecting the radiative budget including greenhouse gases, clouds, and aerosols FigureStep 1.
Figure Calculation of the radiative forcing DF due to the addition Dm of a greenhouse gas. The "top of atmosphere" is commonly taken as the tropopause. This equilibrium is necessary for a stable climate; as we will see below, even a small deviation would cause a large temperature perturbation.
The model used for the calculation may be as simple as a 1-dimensional vertical formulation of radiative equilibrium, or as complicated as a GCM; the choice of model is not too important as long as the calculated temperature profiles are reasonably realistic. Starting from this radiative equilibrium situation, we now perturb the equilibrium Step 2 by adding Dm of species X, keeping everything else constant including temperature. If X is a greenhouse gas, then adding Dm will decrease the outgoing terrestrial flux at the top of the atmosphere by an amount DF; DF is the radiative forcing caused by increasing the mass of X by Dm.
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Radiative forcing in research models is usually computed on the basis of the radiative perturbation at the tropopause rather than at the top of the atmosphere. That is, Fin and Fout in Step 2 are retrieved from the model at the tropopause after temperatures in the stratosphere have been allowed to readjust to equilibrium temperatures in the troposphere are still held constant at their Step 1 values.
The reason for this procedure is that a radiative perturbation in the stratosphere as due, for example, to change in the stratospheric ozone layer may have relatively little effect on temperatures at the Earth's surface due to the weak dynamical coupling between the stratosphere and the troposphere.
By computing the radiative forcings associated with changes in emissions of individual greenhouse gases, we can assess and compare the potential climate effects of different gases and make policy decisions accordingly. Figuretaken from a recent report from the Intergovernmental Panel on Climate Change IPCCgives the radiative forcings caused by changes in different greenhouse gases and other atmospheric variables since year Note that the anthropogenic radiative forcing from greenhouse gases is much larger than the natural forcing from change in solar intensity.
Aerosols may induce a large negative forcing which we will discuss in chapter 8.
Figure Globally averaged radiative forcing due to changes in greenhouse gases, aerosols, and solar activity from year to today.