lutzj
The Last Thing You See
Global warming can also be the result of a natural cycle as well. While it's not certain, it's possible that Earth was recently in a minor ice age. Thus any recent temperature increases observed can be accounted for by a natural cycle. Of course this argument is rather difficult to prove and doesn't quite fit in to your definition of "unnatural way."
It WAS in a minor ice age: an "ice age" is defined as any period with glaciers at the poles. Earth is simply continuing on the warming trajectory it has been on since 30,000 years ago. Our own records cannot be called very accurate for two reasons:
1. We only have about 150 years of reliable data.
2. Just before we started taking systematic measurements, much of Europe, North America, and other places warmed considerably due to the end of the Little Ice Age. Temperatures have been bouncing around for most of human history, it's just that we've only caught on recently.