What has happened to average global temperatures during the 20th century?

Answer 1

When we started this century, the world was in a period climatologists refer to as the mini ice age. Temperatures were already 0.1 degrees below normal as we enetered the period and dropped to almosy 0.5 degrees cooler then normal. After 1913, temperatures gradually rose until 1944. From 1944 until the late 70's we saw a decline in temperatures globally (on average). After 1979 we saw an increase in temperature once again. Temps finally reached the 1944 high points again in 1998 (and surpassed that temp by 0.1 degrees.) Since this time we have seen temps stay at this higher temp and vary slightly (as of late slight declines). 2010 tied, or possibly exceed 1998 by 0.01 of a degree (accuracy 0.16). This makes the past decade the warmest decade on record despite the lack of additional long term warming this decade. We should never view single years when referring to climate. Five or ten year averages make far more sense. The longer periods show that the planet has, on average warmed an additional 0.25 since the temps of 1944. The current trend is downward, although we are still near peak temps for the past hundred years.

If we view the 1901 temps and compare to 2001 (the twentieth century), we see about 0.414 degrees of total warming from the global average for the century. This compares with the past ten thousand years of warming (11 degrees total or 0.11 degrees per century average).

It is important to remember though that accurate temperatures on a global scale were impossible until 1979. The error factor gets much larger as we go back in time and before 1979 temperatures carry higher inaccuracies. The averages also ignore periods of warmer and cooler climate then present, such as the medieval warm period. Another significant factor is that we had a mini ice age period in the late 1800's. This caused our 1900 temp to be lower then the averages of the historical past. There is attached a great NOAA link showing temperatures from 1880 until present which graphically shows the temps. Notice how they show a very regular sine wave pattern and direction of current trends. (see attached)

Another answer:

The best way to view climate detail is not day to day but more of a five year running average. With this view, global temperatures declined 0.2°C from 1900 to 1910, then climbed 0.4°C until 1940. From 1940 to 1980 they staggered a tenth of a degree, but have risen another 0.4°C since. All indications are the temperature should continue rising at least as dramatically through the 21st century.