Basics Of The Atmospheric Greenhouse Warming Phenomenon

This section explains the basic science of human-caused climate warming.2 It is designed to be accessible to the ''educated layperson'' who is not an expert in atmospheric or climate sciences, but who has some grounding in the physical sciences. The information contained here can provide an improved understanding of the reasons why human-caused climate change is a serious issue, why it is grounded in sound scientific principles, and why this knowledge leads to the global warming dilemma. Readers who are either well versed in the science of climate change or are not particularly interested in pursuing the more difficult parts of the basic science can skip to Section 3 without significant loss of capability to understand the key points in the remaining sections.

Surprisingly, all of the physical drivers of the global warming problem are contained within the atmosphere. Despite being a region of relatively inconsequential mass, water amount, and heat capacity, it is in the atmosphere that the temperature at the earth's surface is ultimately determined. The special properties of the atmosphere define the essence of how climate warming works.

The earth is strongly heated every day by incoming radiation from the sun. This heating is offset by an equally strong infrared radiation leaving the planet. Interestingly, if the earth were without any atmosphere, and if its surface reflectivity did not change, global mean surface temperature would be roughly 33°C colder than it is today. This large difference is due to the strong atmospheric absorption of infrared radiation leaving the earth's surface. The major atmospheric infrared absorbers are clouds, water vapor, and CO2. This strong infrared absorption (and strong re-emission) effect is extremely robust. It is readily measured in the laboratory and directly measured from earth-orbiting satellites. Simply put, adding CO2 to the atmosphere adds another ''blanket'' to the planet and thus directly changes the heat balance of the earth's atmosphere.

Individuals skeptical about the reliability of global warming have correctly noted that in terms of direct trapping of outgoing infrared radiation, water vapor is by far the dominant greenhouse gas on earth. Since water vapor dominates the current radiative balance, how can it be that CO2 is anything other than a minor contributor to earth's absorption of infrared radiation? Part of the answer comes from the well-known modeling result that net planetary radiative forcing changes roughly linearly in response to logarithmic changes in CO2.3 Thus, a quadrupling of CO2 gives another roughly 1°C direct warming over the direct 1°C warming for a CO2 doubling, valid for the extreme assumption that water vapor mixing ratios and clouds do not change.4 Interestingly, this approximate relationship also holds for a large extended range as CO2 is decreased.

It is thus hard to escape the conclusion that increasing atmospheric CO2 concentrations provides a measurable direct addition to the atmospheric trapping of the infrared radiation leaving the surface of our planet. However, a simple comparison of the relative greenhouse efficiencies of water vapor and CO2 quickly becomes problematic because water vapor enters the climate system mostly as a ''feedback'' gas.

All models and observations currently indicate that as climate warms or cools, the observed and calculated global-mean relative humidity of water vapor remains roughly constant, whereas its mixing ratio does not.5 Thus, as climate warms or cools, the holding capacity of atmospheric water vapor increases or decreases, respectively, exponentially. This is a powerful water vapor positive feedback mechanism - that is, a process that acts to amplify the original warming caused by increasing CO2 levels. With this major positive feedback, the modeled ''climate sensitivity'' increases by about a factor of 3 to roughly 3°C.6 Currently, observational evidence remains generally consistent with the modeling results that project a strong positive water vapor mixing ratio feedback under an approximate constancy of relative humidity as the climate changes (Oort & Liu, 1993; Sun & Held, 1996).

An additional, but smaller, positive feedback is the relationship between ice (or its absence) at the earth's surface and its reflectivity of solar radiation. In essence, if ice or snow cover melts, the surface left exposed (ground, vegetation, or water) is generally less reflective of incoming solar radiation. This leads to more absorption of the solar radiation, thus more warming, less ice, and so on. This feedback is expected to become important as snow lines retreat poleward and when polar ice sheets begin to melt at their lower-latitude edges.

Inclusion of this ''ice-reflectivity'' feedback process in mathematical models of the climate amplifies further the calculated warming response of the climate to increased concentrations of CO2 and infrared absorbing gases. It would also amplify any calculated cooling if ice at the earth's surface were to increase. Other kinds of feedback, both positive and negative, result from the interaction of land-surface properties (e.g., changes of vegetation that lead to reflectivity and evaporation changes) with climate warming/cooling mechanisms or from changes in CO2 uptake by the biosphere. Both the ice reflectivity and the vegetation feedbacks still remain somewhat uncertain, particularly in their details on the regional scale.

The major source of uncertainty in determining climate feedback concerns the impact of clouds on the radiative balance of the climate system.7 A CO2-induced increase in low clouds would mainly act to reflect more solar radiation and would thus act to produce a negative feedback to global warming. An increase in high clouds mainly adds to the absorption of infrared radiation trying to escape the planet and would thus provide a positive feedback. A change in cloud microphysical and optical properties could go either way. Which of these would dominate in an increasing CO2 world? We are not sure. Our inability to answer this question with confidence is the major source of uncertainty in today's projections of how the climate would respond to increasing greenhouse gases. Furthermore, it is not likely that this cloud radiation uncertainty will be sharply reduced within the next 5 years. This is because there still remain formidable barriers in obtaining the needed cloud measurements, preparing sufficiently comprehensive cloud models, and formulating accurate theories of cloud behavior and cloud properties.

Although clouds dominate the climate modeling uncertainty, other key processes are also in need of improved understanding and modeling capability. An example is the effect of human-produced airborne particulates (aerosols) composed mostly of sulfate (from oxidation of the sulfur in fossil fuels) or carbon (from open fires). Sulfate aerosols are mostly reflective of solar radiation, producing a cooling effect, whereas carbonaceous aerosols in the lower troposphere mostly absorb solar radiation, producing a net heating effect. Efforts to reduce the current uncertainty are limited by inadequate measurements of aerosol concentrations and the sensitivity of climate to their radiative effects. Even more uncertain are the so-called indirect effects that atmospheric aerosols have on the determination of cloud amounts and their radiative properties.

Another key uncertainty lies in modeling the response of the ocean to changed greenhouse gases. This affects the calculated rate of response of the climate over the next several centuries. For details, see Section 3.

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