The evidence seemed clear: human activities linked to farming had taken control of the trends of two major greenhouse gases thousands of years ago, forcing their concentrations to rise when nature would have driven them lower. The net impact of humans through time (fig. 10.1) was a long slow rise in greenhouse-gas concentrations prior to the industrial era, and then much more rapid increases during the last 200 years of industrialization.
Scientists use a convenient standard to evaluate the climatic impact of greenhouse gases: the amount by which Earth's climate would warm or cool if the CO2 concentration were either doubled or halved from the preindustrial concentration of 280 parts per million. This number, called the 2 x CO2 ("doubled CO2") sensitivity of the climate system, averages 2.5°C (4.5°F) for the planet as a whole. Changes in concentrations of other greenhouse gases can be converted to a form that is expressed as an equivalent change in CO2. The exact size of this 2 x CO2 sensitivity is uncertain; it lies somewhere between 1.5°C and 4.5°C, with 2.5°C the current best estimate.
For a climate sensitivity of 2.5°C, the slow buildup of CO2 and methane caused by human activities before the start of the industrial era would have warmed global climate by about 0.8°C (just under 1.5°F). This number sounds small but is far from trivial; it is roughly 15% as large as the cooling that occurred at the last glacial maximum, the time when most of northern North America and northern Europe was buried under ice sheets.
Climate scientists have long viewed the last 8,000 years as a time of naturally stable climate, a brief interlude between the previous glaciation and the next one. But the story presented here suggests this warm and stable climate of the last 8,000 years may have been an accident. It may actually reflect a coincidental near-balance between a natural cooling that should have begun and an offsetting warming effect caused by humans. If this new view is correct, the very climate in which human civilizations formed was in part determined by human farming activities. Even thousands of years ago, we were becoming a force in the climate system.
The high latitudes are the most sensitive part of the climate system. Temperature changes there are two to three times the size of the global-average value. The main reason for this larger responsiveness is the presence of snow and ice and their ability to reflect the Sun's radiation. If for some reason the planet cools,
Was this article helpful?