evidence: The answer is paleoclimate research. By using various proxies, scientists can infer climate information from thousands or even millions of years ago. Tree rings and ice cores contain physical or chemical evidence that can be reliably traced to certain time periods. For instance, scientists can observe the width of annual tree rings in temperature-sensitive (as opposed to moisture-sensitive) trees or the relative concentrations of oxygen isotopes in ice cores (at high latitudes, precipitation in colder conditions has a lower O-18/O-16 ratio as compared to warmer conditions). [...] Studying data from well beyond 2,000 years ago, paleoclimatologists have discovered several periods of unusually high warmth. The Mid-Holocene Warm Period, observed about 6,000 years ago, was generally warmer than today during summer in the Northern Hemisphere. This natural warming was caused by slow changes in the Earth’s orbit, which caused shifts in the amount of solar radiation reaching each latitudinal band of the planet. Changes in the Earth’s orbit also caused warming in the Penultimate Interglacial Period about 125,000 years ago. Based on proxy evidence, the global mean annual surface temperature appeared to be higher than pre-industrial by about 1° to 2°C, but this warming was not uniform across the globe. However, this slowly varying climate forcing mechanism cannot be responsible for warming over the last 100 years.