As such cases are mostly in the earlier portions of the reco

As such cases are mostly in the earlier portions of the records, this will exaggerate the long-term warming trend. That is at best imperfect. And for those—including Rick Perry, the Republican governor of Texas and would-be president —who claim to see global warming as a hoax by grant-hungry scientists, it may look like a smoking gun.

这里的 That is at best imperfect. 是什么意思