The global average surface temperature of the earth has increased by about 0.7°C over the 20th century, mainly due to burning fossil fuels. While this may not seem like much, a 0.7°C change per millennium is the recent average - not per century.

COMPARISON OF OBSERVED AND PREDICTED GLOBAL TEMPERATURE UNDER VARYING “FORCING” SCENARIOS. Source: IPCC, Third Assessment Report, Climate Change 2001.