The existing temperature-aware clock embedding assumes a time-invariant temperature gradient. However, it is not solved how to find the worst-case temperature gradient leading to the worst case skew. In this paper, we develop a PErturbation based Clock Optimization (PECO) considering the timevariant temperature gradient. For a given clock topology, we minimize the worst case skew without asking for the worst case temperature map. We decide the merging point level by level based on the sensitivity of the skew with respect to the change of merging point. Such sensitivity is calculated using a parameterized model, which is compressed by a singularvalue-decomposition (SVD) and K-means based clustering considering the temperature correlation. The experimental results show that our algorithm reduces worst-case skew by up to 5X compared to the existing zero skew based ZST/DME method with small (up to 1%) wirelength overhead. Categories and Subject Descriptors: B.7.2 [Hardware]: Integrated c...