  A black-body radiation (BBR) shifts of (nsnp ^3P_0) - (ns^2 ^1S_0) clock
transition in divalent atoms Mg, Ca, Sr, and Yb are evaluated. A theory of
multipolar BBR shifts is developed and its implications are discussed. At room
temperatures, the resulting uncertainties in the BBR shifts are relatively
large and substantially affect the projected 10^{-18} fractional accuracy of
the optical-lattice-based clocks.
