1. The IPCC wants to claim that the global average temperature has unexpectedly and abruptly increased during the 20th century after a gradual cooling from the year 1000, and that this unexpected increase of the temperature is mostly man-made-the greenhouse effect of CO2.
2. For their purpose, the IPCC ignored the fact that the Earth went through a cold period called "the Little Ice Age" from 1400 to 1800.
3. The Earth has been recovering from the Little Ice Age from 1800 to the present. A recovery from a cold period is warming. It is mostly this warming that is causing the present climate change and it is not man-made. If they admit the existence of the Little Ice Age, they cannot claim that the global average temperature unexpectedly increased from 1900.
Updated below by Bob Ellis-Dakota Voice