Final answer:
Temperature is the key variable measured by scientists to understand past climate changes. They use proxies like ice cores and tree rings since direct measurements are not possible, and have found significant links between human activity and recent climate change.
Step-by-step explanation:
Temperature is perhaps the single most important variable that we can measure to understand past changes in climate. Since scientists cannot go back in time to directly measure climatic variables, they instead use proxies like ice cores, boreholes, tree rings, glacier lengths, pollen remains, and ocean sediments to indirectly measure past temperatures. These methods enable scientists to reconstruct Earth's climatic history and understand the factors driving climate change.
The historical record indicates that pre-Industrial climate variability can largely be explained by natural factors such as Milankovitch cycles, solar variability, volcanic activity, and natural GHG concentrations.
However, much of the warming observed since the mid-20th century is attributed to human activities, particularly the combustion of fossil fuels that increases greenhouse gas concentrations in the atmosphere. By analyzing the geological and fossil records, scientists engage in paleoclimatology to interpret how climate has changed through time.