I keep returning to the same old question...WHO knows what the temperature of the Earth SHOULD be? No one seems to be able to answer this.
Let me give it a shot, based again on the Voshtok Ice Core data, which reaches back 420 000 years.
There is not a "normal" temperature, in the sense that there is an average with a lot of small deviations and a few large ones. The temperature distribution is more like a gamma distribution (roughly speaking), for those of you statisticians out there.
The average (mean) temperature over the past 420 000 years is 4.5 degrees C (8.1 degrees F) colder than today. The median temperature (half the time it is warmer, half the time it is colder) is 5.1 degrees C (9.2 degrees F) colder than today. The mode (most likely temperature) is 7.6 degrees C (13.7 degrees F) colder than today. The minimum temperature was 9.4 degrees C (16.9 degrees F) colder than today, and the maximum was 3.2 degrees C (5.8 degrees F) warmer than today.
In other words, almost all of the last 420 000 years have been A LOT colder than it is today.
During that time, there have been five episodes of "Global Warming"; 420 000 years ago, 330 000 years ago, 240 000 years ago, 140 000 years ago, and the current one which began about 10 000 years ago. During each of these periods, the temperature increased (from the low point) somewhere in the neighborhood of 8 - 10 degrees C (14 - 18 degrees F)over a relatively short time (maybe a couple thousand years).