evidence: To learn more, go here and here. + - If Earth has warmed and cooled naturally, why are scientists so concerned about today’s global warming? Experts are concerned about today’s global warming because it is different from previous warming periods in three key ways: the reason, the rate, and the repercussions. Ice ages and warm periods over the past million years or more were triggered by variations in the amount of sunlight reaching the Northern Hemisphere during summer and then amplified by other climate feedbacks. But today’s warming is mainly due to humans increasing the abundance of heat-trapping gases in the atmosphere. [1] Today’s warming is occurring much more quickly than previous interglacial episodes. In transitions from an ice age to an interglacial, it took 5,000–10,000 years for the temperature to rise between 5 and 9° Fahrenheit (3–5° Celsius). Humans could witness the same amount of global warming within the next 80 years if we continue emitting heat-trapping gases at today’s rate.