There's evidence from Arctic ocean sediments that during the last interglacial period (roughly 80,000 to 130,000 years ago) of the current Ice Age, the amount of sea ice fluctuated wildly and at times was greatly reduced (see, for example,
http://www.tos.org/oceanography/archive/24-3_polyak.pdf ). This was long before humans dumped a lot of greenhouse gases into the atmosphere. To quote the paper, "During some interglacial periods, climatic conditions were warmer than at present. These intervals are not yet sufficiently investigated, but some data, such as the abundance of paleobiota, especially the presence of subarctic species, indicate the possibility of considerably reduced sea ice cover."
While the authors point out that this time period was not exactly the same as today, based upon scientific evidence the popular idea that only greenhouse gases can be responsible for the current warming has yet to be proved. Whatever caused the warming 130,000 years ago could be happening now, and until climate scientists have eliminated this possibility, heroic efforts to reduce greenhouse gases may be in vain.
