Final answer:
Historical climate shifts, including major cooling periods, are attributed to natural factors such as changes in solar radiation, volcanic eruptions, and variations in atmospheric CO₂ levels, along with Earth's orbital changes known as Milankovitch cycles.
Step-by-step explanation:
The question regarding the causes of major climate shifts over the last billion years is related to natural phenomena rather than human activity. The major cooling periods that lasted for millions of years throughout Earth's history can be attributed to various factors. Among these are changes in solar radiation, volcanic eruptions, and atmospheric carbon dioxide levels.
During historical periods such as the Little Ice Age, reduced solar activity known as the Maunder Minimum was partly responsible for lower temperatures. Longer geological time scales have seen climate shifts due to changes in Earth's orbit, known as Milankovitch cycles, which have had a significant impact on ice ages and interglacial periods. Additionally, the levels of atmospheric CO₂ have influenced the greenhouse effect, affecting Earth's temperature.