Sam Atwood & Janet Fang Fall 2006, EESC W4400 The Greenhouse Effect The greenhouse effect is the natural mechanism by which earth’s surface is warmed by the infrared-absorbing gases in its atmosphere. Earth’s energy balance is affected by the amount of incident sunlight, reflectivity, and the greenhouse effect. With thermal equilibrium and energy conservation, we should expect: EnergyIn = EnergyOut. In the radiative transfer processes, earth is warmed through visible (incoming, solar) radiation, and earth is cooled through infrared (outgoing, terrestrial) radiation. The Stefan-Boltzmann Law states that the energy flux emitted by a blackbody (something that emits and/or absorbs radiation with 100% efficiency) is related to the fourth power of the body’s absolute temperature: F = σT4, where T is the temperature (in kelvins) and σ is a constant valuing 5.67 x 10-8 W/m2/K4. The planetary energy balance between outgoing infrared energy and incoming solar energy is: σTe4 = S/4(1-A), where Te is effective temperature, S is solar flux, and A represents albedo. Given that earth’s surface temperature is about 15oC and its effective temperature is -18oC, the magnitude of earth’s greenhouse effect is 33oC. A secondary definition of greenhouse effect is the difference between earth’s surface temperature/radiation and earth’s effective temperature/radiation: Tg = Ts – Te. Incoming solar radiation Reflection Emission from atmos. Emission from atmos. Transmission Emission from surface Suppose the atmosphere is a singlelayer of gases, then radiation is absorbed and reflected by the atmosphere, which then reradiates it to surface. Greenhouse gases warm the surface by absorbing infrared radiation and reradiating some of it back toward the surface. Atmospheric greenhouse gas concentration changes the energy absorbed and reradiated to the surface, causing a surface temperature change. Greenhouse gases in the atmosphere have a warming effect on surface temperatures. Important atmospheric greenhouse gases include (in order of descending concentration): water vapor, carbon dioxide, methane, nitrous oxide, ozone, and CFCs. A strong absorption feature in earth’s atmosphere is the 15-μm CO2 band; this feature is especially important to climate because it occurs near the peak of earth’s outgoing radiation. Earth’s surface emits strongly in this particular wavelength region, but very little of this radiation can escape directly to space because it will be absorbed by CO2 molecules in the atmosphere. CO2 levels in the past have not been higher than 300 ppm; our CO2 levels are now at 380 ppm. The present planetary imbalance is large, relative to earth’s history (Hansen et al., 2005). Greenhouse gases exist naturally in our atmosphere, and the incoming and outgoing radiation was approximately in balance. But humans are acting to increase greenhouse gas concentrations. 2 According to IPCC-TAR, “Human activities—primarily burning of fossil fuels and changes in land cover—are modifying the concentration of atmospheric constituents or properties of the Earth’s surface that absorb or scatter radiant energy. In particular, increases in the concentrations of greenhouse gases (GHGs) and aerosols are strongly implicated as contributors to climatic changes observed during the 20th century and are expected to contribute to further changes in climate in the 21st century and beyond.” (As with most graphs, be careful not to ignore the error bars.) This anthropogenic increase in greenhouse gas concentration results in the enhanced greenhouse effect, with global warming implications. The pairing of surface temperatures with the greenhouse effect will lead to a strong positive feeback on any perturbations to our present climate (Raval & Ramanathan, 1989). 3 With the enhanced greenhouse effect, climate sensitivity (the equilibrium global temperature change due to a doubling of CO2) is between 1.5 and 4.5oC. This estimate of sensitivity is based on the contributions of the greenhouse effect as well as numerous feedback mechanisms present in the planetary system. Uncertainty in these numbers comes from the complex nature of these interactions and is the best estimate of theory and projections from global general circulation models. Models and observations agree that humans are changing greenhouse gas concentrations, which can lead to global warming. This warming which occurs as a result of a change in the greenhouse gas concentration will not occur immediately. The planet has considerable thermal inertia, which describes the process by which the Earth system transitions between one state of equilibrium and another. An equilibrium state in this context describes a situation which satisfies the planetary energy balance, namely, that energy flux into the planet equals energy flux out of the planet. If this is not the case, the earth is not in equilibrium and according to the Stefan-Boltzmann Law, its temperature must change to compensate. When the temperature changes to the point where the outgoing flux equals the incoming flux, the system is in balance and another equilibrium point is reached. 4 The shift from one equilibrium point to another takes time in a real world situation. This is due to the process which must occur before the temperature of an object emitting radiation can change. When the incident flux on an object increases, its temperature does not change immediately, as it first begins to store energy. The heat capacity of this object is the measure of how much energy it can absorb from this increased flux for a given change in temperature. If its heat capacity is high, a lot of energy is required to change the temperature. In order to return to an equilibrium state, the object must increase in temperature (thereby emitting more radiation), to the point at which the outgoing flux is equal to the incoming flux. Since the flux (Watts [energy per time period] / square meter) describes how much energy enters the object (here, of constant area) over a period of time, the increase in the flux and heat capacity of the object will determine how long it takes to transition to this new state of equilibrium. In the case of the Earth, the increase in greenhouse gases has led to an increase in the forcing (flux) for the surface of the earth. The oceans take up the majority of area over the surface of the earth and are comprised of water, which has a high heat capacity. Thus the surface and upper layers of the ocean act as a reservoir which can absorb large amounts of energy before changing temperatures. In other words, the Earth is described as having a large thermal inertia. Using current estimates of climate sensitivity (0.75 C +/- 0.25 C per W/m2) and knowledge of the ocean’s size, layers and heat capacity, models suggest that it will take between 25 and 50 years for the earth to reach 60% of the transition between equilibrium states (Hansen et al., 2005). This means that once a change in the carbon dioxide in the atmosphere takes place, only 60% of the resulting temperature change will take place in the first 25 to 50 years. The IPCC TAR’s estimate of the impact of carbon dioxide emissions on temperature through time is shown below. 5 FINAL TAKE HOME POINTS (1) Greenhouse gases in the atmosphere have a warming effect on surface temperatures. The greenhouse effect is a natural mechanism that warms our planet to its inhabitable temperature, but anthropogenic emissions are creating an enhanced greenhouse effect. (2) Rapid changes in greenhouse gases take time to alter surface temperatures. The oceans must absorb large amounts of energy before they change in temperature. This “thermal inertia” means that the rapid change in greenhouse gases over the 20th Century will not raise temperatures immediately. It will take 25 to 50 years for 60% of the temperature change to take place as the climate transitions between equilibrium states. (3) Humans are changing greenhouse gas concentrations, which can lead to global warming. Previous CO2 levels (since 400,000 years before present) have not averaged over 300 ppm. CO2 levels began its exponential incline during the Industrial Revolution of the 19th Century. Our current level is approximately 380 ppm, and while it is hoped that we will stabilize at 500 ppm, more realistic projections suggestion 700 ppm. References and Sources for Further Information Tim P. Barnett et al. (2005) Penetration of Human-Induced Warming into the World’s Oceans. Science 309:284-287. James Hansen et at. (2005) Earth’s Energy Imbalance: Confirmation and Implications. Science 308:1431-1435. Lee R. Kump, James F. Kasting, and Robert G. Crane (2004) The Earth System, 2nd Edition. Pearson Prentice-Hall, Upper Saddle River, New Jersey. A. Raval and V. Ramanathan (1989) Observational determination of the greenhouse effect. Nature 342:758-761. IPCC Third Assessment Report, accessed at http://www.grida.no/climate/ipcc%5Ftar/. Wikipedia’s entry on Greenhouse Gases, accessed at http://en.wikipedia.org/wiki/Greenhouse_gasses. 6