A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5-percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analysesof observed interior ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthopogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late 19th century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulationmodel, it provides an independent observationally based constraint on this important parameter of the climate system.