Constraining Possibilities Versus Signal Detection

RICHARD S. LINDZEN1

ABSTRACT

Rather than concentrate on the problem of greenhouse signal detection, one can exploit the intimate relation of climate sensitivity to length of ocean delay in order to constrain possibilities. It is noted that the observed record of surface temperature is "broadly" consistent with equilibrium sensitivity to CO2 doubling of between 0°C and 4.8°C—provided that the ocean delay for the higher sensitivity is of the order of 160 years. If ocean delay can be independently established as being shorter, then one must reduce the maximum consistent sensitivity. It is suggested that volcanic responses can be used to estimate ocean delay. Preliminary results suggest very short delays.

REMARKS

As noted in the papers in this section by North and Kim (1995) and by Wigley and Raper (1995), simple box-diffusion-upwelling models yield useful insights into the issues of the seasonal and long-term delay in climate response to changes in geophysical forcing quantities that results from the ocean's heat capacity. The point, quite simply, is that the more rapidly heat is mixed downward into the ocean, the greater the ocean's effective heat capacity and the longer the delay. Also evident is the fact that the delay itself is time dependent and is different for each transient problem and for the annual cycle.

The geometry of simple energy-balance box-diffusion-upwelling models is shown in Figure 1. No attempt will be made here to repeat the mathematical development of such a model. However, certain points are worth noting. The equilibrium response to forcing, ΔS, is ΔTeqbm = ΔS/a. Also, the intensity of the coupling of the atmospheric climate system to the ocean is given by a. As noted by Hansen et al. (1985), the larger ΔTeqbm is for a given ΔS, the smaller a must be, and with the weaker coupling, the ocean delay will be longer.

Reasonable parameter choices are h = 75 m, k = 1.5 cm2 sec1, and upwelling, which limits downward diffusion to about H = 400 m. A common choice for a is 1.55 W m2 deg1 (viz., Sellers, 1969; North and Kim use a somewhat different value, but for our purposes the small differences are not of consequence). Other values involve negative or positive feedbacks in the atmospheric climate system. If we refer to the original choice of a as ao, then

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Center for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, Massachusetts



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