Trends In Global Greenhouse emissions during the first two decades of this twenty- first century are leading us to a much hotter world by 2100, perhaps 3°C– 4°C above the late- twentieth- century average temperature and hotter than at any time in the last 20– 30 million years. Further, the rate of heating would be about 30 times faster than when Earth emerged from the most recent ice age, between 17,000 and 12,000 years ago. At that speed, environmental changes may outstrip the capacity of many species to evolve and adapt. Having once relied on fires in caves, humans in the late eighteenth and nineteenth centuries increasingly began to burn fossil fuels to release vastly more energy— and, inadvertently, vastly more carbon dioxide. About 600 billion metric tons of that invisible, stable, and odorless gas have been emitted since 1750, about two- thirds of which will persist in the atmosphere for centuries. The resulting 40 percent increase in atmospheric carbon dioxide concentration is the main cause of human- driven climate change. We have wrapped another heat- retaining blanket around the planet, causing warming of Earth’s surface at a rate that far outpaces nature’s rhythms. Humans have lived in climatically congenial times for the past 11,000 years of the Holocene geological epoch compared with the rigors of the preceding ice age. Figure 1.1 shows the world’s estimated average surface temperature over that era, and the right- hand side of the graph shows the likely global warming by 2100 averaged across many published modeled projections. The difference between the peak temperature of 7,000 years ago and the nadir of the Little Ice Age 350 years ago is 0.7°C. By early in this twenty- first century, the global average temperature had edged higher than for the past 11,000 years— by 0.6°C in six decades. If the world’s temperature were to rise by 3°C– 4°C within just three generations, our descendants might struggle to remain healthy, raise families, and survive within stable societies. I am certainly not the first to say this … A 4°C temperature increase probably means a global carrying capacity below 1 billion people.
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