Question

The following passage explores the debate surrounding the 'Early Anthropocene Hypothesis.'

For decades, the consensus among geologists and climatologists was that human activity began to alter the global climate only with the advent of the Industrial Revolution in the late eighteenth century. According to this traditional view, the burning of fossil fuels and large-scale manufacturing marked the definitive boundary of the Anthropocene. However, in $2003$, climatologist William Ruddiman proposed the controversial 'Early Anthropocene Hypothesis,' shifting this timeline back by thousands of years. Ruddiman argued that early agricultural practices initiated a slow, sustained warming trend that prevented the onset of a scheduled glacial period.

To construct his argument, Ruddiman first analyzed ice core data from Antarctica, which records historical levels of greenhouse gases. Over the past several hundred thousand years, carbon dioxide ($CO_2$) and methane ($CH_4$) levels consistently peaked during warm interglacial periods and then gradually declined due to predictable variations in Earth's orbit. Yet, Ruddiman noticed a distinct departure from this natural pattern: approximately $8{}000$ years ago, atmospheric $CO_2$ levels unexpectedly began to rise, followed by a similar upward trajectory in $CH_4$ levels around $5{}000$ years ago.

Having identified these anomalies, Ruddiman sought to establish a causal link to human action. He proposed that the rising $CO_2$ resulted from massive deforestation as early European farmers cleared land for agriculture. The subsequent rise in $CH_4$ was attributed to the expansion of wet-rice cultivation in Southeast Asia, which created artificial wetlands that released large volumes of the gas.

Critics of the hypothesis argued that early human populations were too small to generate such global effects, suggesting instead that natural changes in ocean circulation and solar output drove the warming. In response, Ruddiman and his supporters refined their models, demonstrating that early agricultural techniques were highly inefficient, requiring far more land per person than modern farming. By showing that a small population could cause disproportionately high deforestation, Ruddiman successfully defended the chronological progression of his argument, establishing a new framework for understanding human-environmental history.

Based on the passage, arrange the following steps in the development and defense of William Ruddiman’s Early Anthropocene Hypothesis in their logical chronological order, from the initial scientific observations to the final counter-defense.