THE ANTHROPOGENIC GREENHOUSE ERA
BEGAN THOUSANDS OF YEARS AGO
WILLIAM F. RUDDIMAN
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, U.S.A.
Abstract. The anthropogenic era is generally thought to have begun 150 to 200 years ago, when
the industrial revolution began producing CO
at rates sufﬁcient to alter their compositions
in the atmosphere. A different hypothesis is posed here: anthropogenic emissions of these gases
ﬁrst altered atmospheric concentrations thousands of years ago. This hypothesis is based on three
arguments. (1) Cyclic variations in CO
driven by Earth-orbital changes during the last
350,000 years predict decreases throughout the Holocene, but the CO
trend began an anomalous
increase 8000 years ago, and the CH
trend did so 5000 years ago. (2) Published explanations for
these mid- to late-Holocene gas increases based on natural forcing can be rejected based on paleocli-
matic evidence. (3) A wide array of archeological, cultural, historical and geologic evidence points
to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia,
including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago. In
recent millennia, the estimated warming caused by these early gas emissions reached a global-mean
value of ∼0.8
C and roughly 2
C at high latitudes, large enough to have stopped a glaciation of
northeastern Canada predicted by two kinds of climatic models. CO
oscillations of ∼10 ppm in the
last 1000 years are too large to be explained by external (solar-volcanic) forcing, but they can be
explained by outbreaks of bubonic plague that caused historically documented farm abandonment
in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for
the observed CO
decreases. Plague-driven CO
changes were also a signiﬁcant causal factor in
temperature changes during the Little Ice Age (1300–1900 AD).
Crutzen and Stoermer (2000) called the time during which industrial-era human ac-
tivities have altered greenhouse gas concentrations in the atmosphere (and thereby
affected Earth’s climate) the ‘Anthropocene’. They placed its start at 1800 A.D., the
time of the ﬁrst slow increases of atmospheric CO
previous longer-term values. Implicit in this view is a negligible human inﬂuence
on gas concentrations and Earth’s climate before 1800 AD.
The hypothesis advanced here is that the Anthropocene actually began thou-
sands of years ago as a result of the discovery of agriculture and subsequent
technological innovations in the practice of farming. This alternate view draws
on two lines of evidence. First, the orbitally controlled variations in CO
concentrations that had previously prevailed for several hundred thousand years fail
to explain the anomalous gas trends that developed in the middle and late Holocene.
Climatic Change 61: 261–293, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.