THE "GREENHOUSE" EFFECT AND CLIMATE CHANGE John F. B. Mitchell Meteorological Office, Brackne!l, England Abstract. The presence of radiatively active gases in the Earth's atmosphere (water vapor, carbon dioxide, and ozone) raises its global mean surface temperature by 30 K, making our planet habitable by life as we know it. There has been an increase in carbon dioxide and other trace gases since the Industrial Revolution, largely as a result of man's activities, increasing the radiative heating of the troposphere and surface by about 2 W m -2. This heating is likely to be enhanced by resulting changes in water vapor, snow and sea ice, and cloud. The associated equilibrium temperature rise is estimated to be between 1 and 2 K, there being uncertainties in the strength of climate feedbacks, particularly those due to cloud. The large thermal inertia of the oceans will slow the rate of warming, so that the expected temperature rise will be smaller than the equilibrium rise. This increases the uncertainty in the expected warming to date, with estimates ranging from less than 0.5 K to over 1 K. The observed increase of 0.5 K since 1900 is consistent with the lower range of these estimates, but the variability in the observed record is such that one cannot necessarily conclude that the observed temperature change is due to increases in trace gases. The prediction of changes in temperature over the next 50 years depends on assumptions concerning future changes in trace gas concentrations, the sensitivity of climate, and the effective thermal inertia of the oceans. On the basis of our current understanding a further warming of at least 1 K seems likely. Numerical models of climate indicate that the changes will not be uniform, nor will they be confined to temperature. The simulated warming is largest in high latitudes in winter and smallest over sea ice in summer, with little seasonal variation in the tropics. Annual mean precipitation and runoff increase in high latitudes, and most simulations indicate a drier land surface in northern mid-latitudes in summer. The agreement between different models is much better for temperature than for changes in the hydrological cycle. Priorities for future research include developing an improved representation of cloud in numerical models, obtaining a better understanding of vertical mixing in the deep ocean, and determining the inherent variability of the ocean-atmosphere system. Progress in these areas should enable detection of a man-made "greenhouse" warming within the next two decades. 1. INTRODUCTION Our planet is made habitable by the presence of certain gases which trap long-wave radiation emitted from the Earth's surface, giving a global mean temperature of 15øC, as opposed to an estimated-18øC in the absence of an atmosphere. This phenomenon is popularly known as the "greenhouse" effect. By far the most important greenhouse gas is water vapor. However, there is a substantial contribution from carbon dioxide and smaller contributions from ozone, methane, and nitrous oxide. The concentrations of carbon dioxide, methane, and nitrous oxide are all known to be increasing, and in recent years, other greenhouse gases, principally chlorofluorocar- bons (CFCs), have been added in significant quantifies to the atmosphere. There are many uncertainties in deducing the consequential climatic effects. Typically, it is esti- mated that increased concentrations of these gases since 1860 may have raised global mean surface temperatures by 0.5øC or so, and the projected concentrations could produce a warming of about 1.5øC over the next 40 years. ,.. This paper is not subject to U.S. copyright. Published in 1989 by the American Geophysical Union. Numerical climate models indicate that other changes in climate would accompany the increase in globally averaged temperature, with potentially serious effects on many societal and economic activities. This review attempts to answer several questions. What is the greenhouse effect (section 2)? Which gases are important and why (section 3)? What are the expected changes in the concentration of greenhouse gases (section 4)? What are the potential climatic effects, and how are they determined (section 5)? How will the changes in climate evolve (section 6), and when will we be able to detect them (section 7)? The aim here is to outline the physical basis of the projected changes in climate due to enhancing the greenhouse effect and to identify the main areas of uncertainty. This review will, therefore, be selective rather than comprehensive. For a more detailed and complete discussion of the greenhouse effect the reader is referred to the major reviews edited by MacCracken and Luther [1985] and Bolin et al. [1986]. Reviews of Geophysics, 27, 1 / February 1989 pages 115-139 Paper number 89RG00094 ß 115 ß116 . REVIEWS OF GEOPHYSICS / 27,1 2. THE GREENHOUSE EFFECT 2.1. Radiative Effects The Earth-atmosphere system is heated by solar (short-wave radiation at a mean rate of S O (1 - (z)/4, where S O is the solar "constant," (z is the fraction of radiation reflected by the Earth and atmosphere, and the factor 4 allows for the spherical geometry of the Earth. This must be balanced by the emission of long-wave (thermal or ,, infrared) radiation to space (Figure la). The rate of cooling is given by c•T• 4, where c• is Stefan's constant and T• is the effective radiating temperature of the system. At equilibrium So(1 - (:z)/4 = 6• (1) which assuming the current albedo of 0.30 gives a value of T e corresponding to 255 K (-18øC). In the absence of an atmosphere, T• will be the Earth's surface temperature. Solar Longwave Incoming Reflected (a) No atmosphere Figure 1. Schematic illustration of the greenhouse effect showing (a) no atmosphere, where long-wave radiation escapes directly to space, and (b) an absorbing atmosphere, where long-wave radiation from the surface is absorbed and reemitted both downward, warming the surface and lower atmosphere, and upward, maintaining radiative balance at the top of the atmosphere. • • /• •Gaseousabs. or.ption mission (b) With atmosphere A perfect emitter, or blackbody, emits radiation over a range of wavelengths. The distribution of energy emitted with wavelength is a function of the temperature of the emitter: the hotter the emitter, the shorter the wavelength of peak emission. Thus the