Final answer:
Scientists use proxies like ice cores, tree rings, sediment cores, and fossils to understand past climate and CO₂ levels. Ice cores in particular allow direct measurement of ancient atmospheric gases. Historical data and the study of natural climate cycles also contribute to reconstructing past climates.
Step-by-step explanation:
To understand what the climate and CO₂ levels were like in the past, scientists use various proxies as indirect evidence. Ice cores are one of the most valuable proxies because they contain trapped bubbles of ancient air, providing a direct measure of past atmospheric gas concentrations. The isotopic composition and the concentration of greenhouse gases in these bubbles help scientists reconstruct past climates. Similarly, tree rings reflect growth conditions and can indicate climate variations. Tree rings, sediment cores, pollen analysis, and fossil records, such as foraminifera (microorganisms which shells change with water temperature), are all studied to provide insight into Earth's climatic history. Moreover, geological evidence and the study of Milankovitch cycles inform us about the natural drivers of climate change pre-dating industrialization.
Arrhenius contributed significantly to our understanding of the greenhouse effect and predicted the warming potential of doubling CO₂ levels, which modern science has largely confirmed. Analysis of historical climate data reveals a relatively stable global climate over the last 10,000 years, coinciding with significant human development. However, data depicted in graphs since industrialization show a steady increase in atmospheric CO₂ levels, corroborating with the rise in global temperatures.