  In this paper, we study online convex optimization in dynamic environments,
and aim to bound the dynamic regret with respect to any sequence of
comparators. Existing work have shown that online gradient descent enjoys an
$O(\sqrt{T}(1+P_T))$ dynamic regret, where $T$ is the number of iterations and
$P_T$ is the path-length of the comparator sequence. However, this result is
unsatisfactory, as there exists a large gap from the $\Omega(\sqrt{T(1+P_T)})$
lower bound established in our paper. To address this limitation, we develop a
novel online method, namely adaptive learning for dynamic environment (Ader),
which achieves an optimal $O(\sqrt{T(1+P_T)})$ dynamic regret. The basic idea
is to maintain a set of experts, each attaining an optimal dynamic regret for a
specific path-length, and combines them with an expert-tracking algorithm.
Furthermore, we propose an improved Ader based on the surrogate loss, and in
this way the number of gradient evaluations per round is reduced from $O(\log
T)$ to $1$. Finally, we extend Ader to the setting that a sequence of dynamical
models is available to characterize the comparators.
