Simplify the following text for a Grade 11 native English-speaking person.
Original: NASA defines the term as a cold period between 1550 AD and 1850 AD and notes three particularly cold intervals : one beginning about 1650 , another about 1770 , and the last in 1850 , each separated by intervals of slight warming .
Simplified: It is generally agreed that there were three minima , beginning about 1650 , about 1770 , and 1850 , each separated by slight warming intervals .