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Trace-Gas Greenhouse Effect and Global Warming: Underlying Principles and Outstanding Issues Volvo Environmental Prize Lecture-1997

V. Ramanathan
Ambio
Vol. 27, No. 3 (May, 1998), pp. 187-197
Published by: Springer on behalf of Royal Swedish Academy of Sciences
Stable URL: http://www.jstor.org/stable/4314715
Page Count: 11
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Trace-Gas Greenhouse Effect and Global Warming: Underlying Principles and Outstanding Issues Volvo Environmental Prize Lecture-1997
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Abstract

This paper describes the developments that transformed the global warming problem from that arising solely from CO2 increase to the trace-gas greenhouse effect problem in which several ${\rm non}\text{-}{\rm CO}_{2}$ gases, CFCs, CH4, N2 O, O3 and others contribute as much as CO2. Observed trace-gas increases, including CO2 increase, since the mid-19th century have enhanced the atmospheric greenhouse effect, ${\rm G}_{{\rm a}}$, (≈ 130±5 W m-2) by about 2%. Without other competing factors, this heating should have committed the planet to a warming of about 1 to 1.5 K. The added radiative energy is maximum in the low latitudes and about a factor of two smaller in the polar regions. The largest effect of the warming is increased back radiation at the surface by as much as 6 to 8 W m-2 per degree warming. Not all of this increased energy is balanced by surface emission; evaporation (and hence precipitation) increases to restore surface energy balance, by as much as 2 to 4% per degree warming. The increase in evaporation along with the increase in saturation vapor pressure of the warmer troposphere, contributes through the atmospheric dynamics to an increase in water vapor. This water vapor feedback enhances ${\rm G}_{{\rm a}}$ by another 1% per degree warming. Our ability to predict regional and transient effects, depends critically on resolving a number of outstanding issues, including: i) Aerosol and stratospheric ozone effects; ii) Response of the tropical convective-cirrus clouds, the extra-tropical storm-track systems and persistent coastal stratus to both global warming and to regional emissions of aerosols; iii) The causes of excess solar absorption in clouds; and iv) Upper troposphere water vapor feedback effects.

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