Final answer:
One hypothesis for the rise in greenhouse gases 55 Mya is extensive volcanism in what is now India, which released significant amounts of sulfur dioxide and other gases, altering global temperatures and contributing to the greenhouse effect and climate change.
Step-by-step explanation:
One of the hypotheses for the significant increase in greenhouse gases 55 million years ago (Mya) is the event of extensive volcanism. Specifically, around 66 Mya, near the end of the Cretaceous period, India experienced massive volcanic eruptions as the Deccan Traps were being formed. These eruptions released colossal amounts of volcanic gases, notably sulfur dioxide, into the atmosphere. This volcanic activity likely played a significant role in altering global temperatures and could have contributed to a natural variation of the greenhouse effect, which is crucial for maintaining Earth's warmth. Hence, the greenhouse gases from these volcanic eruptions could have been a key factor in the climate changes observed at that time, including the mass extinction at the end of the Cretaceous period.