  There is a deep tension between the well-developed theory of gravitational
waves from isolated systems and the presence of a positive cosmological
constant $\Lambda$, however tiny. In particular, even the post-Newtonian
quadrupole formula, derived by Einstein in 1918, has not been generalized to
include a positive $\Lambda$. We first explain the principal difficulties and
then show that it is possible to overcome them in the weak field limit. These
results also provide concrete hints for constructing the $\Lambda >0$
generalization of the Bondi-Sachs framework for full, non-linear general
relativity.
