What trend has been observed in global temperature change over the past 70 million years?

The observed trend in global temperature change over the past 70 million years indicates a significant cooling of the Earth's climate. The correct answer reflects the approximate average of 15° cooler temperatures compared to earlier geological periods. This substantial long-term trend is illustrated through paleoclimate data gathered from various sources, including ocean floor sediments, which provide a record of past temperatures and climatic conditions.

During this lengthy period, the Earth has undergone various climate phases, including warm periods known as "greenhouse" conditions, characterized by much higher average temperatures, and cooler "icehouse" periods, where the planet experienced glaciation. Geological data suggest that, over millions of years, the transition towards cooler climates has led to the current balance where global temperatures average significantly lower than they were in the past.

Understanding this trend is vital in the study of climate change, as it provides context for both the natural variability of the Earth's climate system and the recent anthropogenic impacts. It illustrates how the planet's climate has transitioned through different states over geological timeframes, which is crucial for accurately forecasting future climatic conditions amidst current warming trends.