Cells adapt to changing environments by sensing ligand concentrations using specific receptors. The accuracy of sensing is ultimately limited by the finite number of ligand molecules bound by receptors. Previously derived physical limits to sensing accuracy have assumed that the concentration was constant and ignored its temporal fluctuations. We formulate the problem of concentration sensing in a strongly fluctuating environment as a non-linear field-theoretic problem, for which we find an excellent approximate Gaussian solution. We derive a new physical bound on the relative error in concentration $c$ which scales as $\delta c/c \sim (Dac\tau)^{-1/4}$ with ligand diffusivity $D$, receptor cross-section $a$, and characteristic fluctuation time scale $\tau$, in stark contrast with the usual Berg and Purcell bound $\delta c/c \sim (DacT)^{-1/2}$ for a perfect receptor sensing concentration during time $T$. We show how the bound can be achieved by a simple biochemical network downstream the receptor that adapts the kinetics of signaling as a function of the square root of the sensed concentration.