Final answer:
Natural forces that had a measurable effect on Earth's climate since 1750 include changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas concentrations. However, recent changes in climate are primarily attributed to human activities, such as the combustion of fossil fuels.
Step-by-step explanation:
Since 1750, natural forces that have had a measurable effect on Earth's climate include changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas concentrations. These factors can cause short-term variations in climate. However, recent changes in climate, especially since the mid-20th century, are primarily attributed to human activities, such as the combustion of fossil fuels, which contributes to the greenhouse effect and global warming.