<<

Comparative Climatology of Terrestrial (2012) 8083.pdf

CO2 GREENHOUSE EFFECTS ON , , AND . D. Crisp, Jet Propulsion Laboratory, Cal- ifornia Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91011, David.Crisp@jpl..gov.

Introduction: (CO2) is an effi- found in the lowest of the Venus atmos- cient, long-lived greenhouse gas that has played a key phere. role in the evolution of the climates of Venus, Earth, While the CO2 concentration in the Earth’s atmos- and Mars. On Venus, where it constitutes over 96% of phere has varied substantially over time, recent in- the massive, 90-bar , CO2 is the dominant creases associated with fossil fuel emissions and other greenhouse gas. However, even there, other green- human activities have generated the greatest attention.

house agents including trace amounts of H2O, SO2, For at least 600,000 prior to the start of the in- and the enshrouding H2SO4 are needed dustrial age, the CO2 concentration rarely exceeded to maintaining the 730 K surface . On 300 parts per million (ppm) relative to dry air. Since Earth, CO2 is a trace gas whose concentration and cli- the 1750’s, fossil fuel emissions and cement produc- mate impact has varied over time. It currently makes tion have added ~1,300 billion tons of CO2 into the up ~0.04% of the atmosphere but is still the second atmosphere. Almost half of this emission has occurred most important greenhouse gas, after . On since the mid 1970’s. The atmospheric concentrations Mars, CO2 constitutes over 95% of the ~6 hPa atmos- of CO2 are now increasing by about 0.5% per , in phere and is currently the only significant greenhouse response to these emissions, but this is not the whole gas. If Mars was substantially warmer and wetter ear- story. Given the known emission rates, the CO2 lier in its history, other greenhouse agents must have should be increasing at twice this rate. Apparently, contributed to the surface thermal balance. almost half to the CO2 emitted into the atmosphere by The role of CO2 in the Venus greenhouse mecha- these human activities is being reabsorbed by the nism has been studied extensively since the beginning ocean and the biomass. Measurements of the of the . Initially, the focus of these studies concentration of dissolved carbon in the ocean water was to demonstrate that CO2, water vapor, and other indicate that about half of the “missing CO2” is being known greenhouse agents could maintain the high sur- absorbed by the ocean. The rest is being absorbed face temperatures in the presence of the H2SO4 clouds, somewhere on land, but and geographic distri- which reflect almost 80% of the incident back bution of these to space before it can be absorbed the planet. More “CO2 sinks” is currently unknown. Because of this, recent greenhouse experiments have provided a more there is no way to predict how these sinks may respond detailed assessment of the details of the vertical ab- to a changing climate. sorption profile of CO2 and their contributions to the These changes in the Earth’s atmospheric CO2 the atmospheric thermal structure. Other investigations concentration have generated substantial concern have focused on the role of these processes in the early about greenhouse gas induced climate change. Climate evolution of the Venus environment, including the run- models developed as early as the 1960’s showed that away greenhouse and subsequent loss of water from doubling the CO2 concentration would increase the the surface and atmosphere. These investigations led global average surface by ~3 ºC. These to numerous insights into the evolution of terrestrial predictions have been verified repeatedly by increas- planetary beyond the inner boundary of ingly more sophisticated climate models. They are the ’s habitable zone. now appearing as robust features in global measure- Studies of the role of CO2 in the Venus greenhouse ments of surface and atmospheric temperature. continue to place stringent demands on our understat- While the atmosphere contains about 70 ing of gas absorption at high pressures and tempera- times as CO2 as the Earth’s, this gas produces only a tures and other radiative processes essential for simu- weak . In the global average, CO2 lating planetary atmospheres. Much of what we know adds less than 18 W/m2 to the surface radiation budget. about the absorption by CO2 isotopes, hot bands, and Most studies of the Martian atmospheric greenhouse other effects including non-local thermodynamic equi- have focused on whether, or to what extent, CO2 and librium (NLTE), quantum-mechanical line mixing and other greenhouse agents (gases, aerosols) warmed the pressure-induced absorption was derived from pio- Martian surface in the past. Was the ancient Martian neering laboratory measurements designed to support climate warm and wet or cold and dry? If it was much Venus greenhouse studies. These processes still are warmer, how much CO2 was there in the atmosphere at poorly understood at the temperatures and pressures that time? What other greenhouse agents were availa- ble in with CO2, how long did they last, Comparative Climatology of Terrestrial Planets (2012) 8083.pdf

and how effective were they at trapping thermal radia- tion and warming the surface? New measurements and more innovative modeling studies are needed to convincingly answer these questions. Here, we will attempt to review the recent progress in each of these areas and define the next logical steps in the investigation of CO2 greenhouse effects on Ve- nus, Earth and Mars. Acknowledgements: The research described here was carried out at the Jet Propulsion Laboratory, Cali- fornia Institute of Technology, under a contract with the National Aeronautics and Space Administration.