Part 1 of Essay
The importance of human influence on global climate change was again highlighted in a groundbreaking study of Earth's temperature over the past 600 years. Reconstructing the global temperature record, climatologists have determinined that three recent years were the warmest years since AD 1400 -- 1997, 1995, and 1990. The ....

Part 2 of Essay
.... -- factors that can affect the climate significantly but which are not part of the climate system itself. Based on statistical comparisons of reconstructed northern hemisphere temperatures, the best estimates indicate that natural changes in the brightness of the sun and volcanic emissions both played an important role in governing climate variations over the period studied. But over the past few decades, greenhouse gases produced by human activities appear to have had an increasing influence on temperatures. "T ....