Page 297 - Earth's Climate Past and Future

CHAPTER 15

Humans and

Preindustrial Climate

Our species arrived late in Earth’s long history. The first somewhat human-like
creatures that walked on two legs and used stone tools appeared only within
the last 4 Myr, equivalent to less than 1/1000 of Earth’s age (4.55 Byr). Subse-
quent milestones in human history were marked off at intervals that grew
shorter by factors of approximately 10: (1) the initial appearance of our species,
Homo sapiens, within the last 200,000 years; (2) the development and spread of
agriculture within the last 12,000 years; and (3) the arrival and growth of the
industrial era within the last 200 years.
Over most of their time on Earth, humans and our immediate predecessors
were affected by climate but did not have any measurable impact on the cli-
mate system. With the spread of agriculture during the current interglacial
period, however, we began to alter climate by adding greenhouse gases to the
atmosphere in amounts sufficient to offset part of a natural cooling. In this
chapter we trace the progression of humans from passive participants in cli-
mate change to active contributors to the operation of the climate system.