• EARTH’S CLIMATE• PAST and FUTURE

• SECOND EDITION

• CHAPTER 15• Humans and Preindustrial Climate

WILLIAM F. RUDDIMAN

© 2008 W. H. Freeman and Company

Main Concepts

• Major steps in Human evolution• Theories for connection between climate and

Human evolution– Limited data for Human evolution and how that limits

our ability to test theories• Agriculture and its relationship to climate

– Role of climate in development of agriculture– Role of climate in civilization collapse– Role of agriculture in greenhouse gas increases of

last 8000 to 5000 yrs• Role of climate in megafaunal extinction

Role of Climate?

• What does the figure on the right suggest about African climate changes while humans were evolving in the last 4 million years?– While planet cooling, Africa

getting dryer and less forested

– Causes of drying may be many: precession and monsoons, lower CO2, tectonic uplift

Scarcity of human boneSamples affects two things:

1) Ability to determine true age range of human ancestors

2) Ability to determine rate of evolution of human ancestors and traits

Did human evolution occur because of:

1) The change from forest to savannah?

2) Continuous variability in climate?

Impossible to tell with sparse data set

Role of Climate in development of Agriculture

• Two possibilities presented:– Deglacial warming favored agriculture in fertile

crescent generally– Younger Dryas cooling/drying concentrated

people near water sources and led to agriculture

• Impossible to tell if either is correct!

Climate and Civilization

• Several major civilization collapses coincide with periods of intense drought

• Not surprisingly, other factors likely played a role:– War, overexploitation of

soil/resources, affecting climate by cutting trees

What is the main difficulty with the idea that climate caused the megafaunal extinction?

Last 5 interglacial CO2 records from Antarctic ice cores

Last 5 interglacial CH4 records from Antarctic ice cores

Rice patties or glaciation?!

Models suggest we wouldbe in the beginnings of thenext ice age now if not for the current levels of CO2 and CH4