Paris Climate Agreement Beacon of Hope

6

Eventually, population rose to an extent that humans began to exert a measurable
effect on the GHG levels of the atmosphere, a time period now known as the
Anthropocene (Crutzen and Stoermer 2000 ). Figure 1.2 illustrates the time evolution
of ΔT, CO 2 , CH 4 , nitrous oxide (N 2 O), and population during the past 2000 years, the
so-called Common Era. The association of the rise in population and increased atmo-
spheric levels of CO 2 , CH 4 , and N 2 O is again irrefutable. Before delving into the
Anthropocene, we shall comment on one more controversy.
Europe experienced unusually warm temperatures from about 950–1250 AD, a
time known as the Medieval Warm Period (MWP) (Moberg et al. 2005 ). Our
Fig. 1.2, which relies on the global temperature reconstruction of Jones and Mann
( 2004 ), does not depict the MWP. The study of Jones and Mann ( 2004 ) suggests the
MWP was regional in nature, with little to no global expression. The temperature
record in Fig. 1.2, which has become known as “The Hockey Stick”, has led to
considerable controversy. One account is described in the book The Hockey Stick
and the Climate Wars: Dispatches from the Front Lines (Mann 2012 ). In 2006, the

0.

0.

1.

Common Era

Temperature

300

350

400

Carbon Dioxide

1.

1.

2.

Methane

0.

0.

Nitrous Oxide

0 500 1000 1500 2000

0.

2.

4.

6.

8.

Population

∆

T (°C)

CO

(ppm) 2

CH

(ppm) 4

N

O (ppm) 2

Billions

Fig. 1.2 Temperature, GHGs, and population, Common Era. Time series of Earth’s global mean
surface temperature anomaly (ΔT) relative to pre-industrial baseline (1850–1900 mean) (Jones
and Mann 2004 ; Jones et al. 2012 ), the atmospheric mixing ratio of CO 2 , CH 4 , and N 2 O (MacFarling
Meure et al. 2006 ; Ballantyne et al. 2012 ; Dlugokencky et al. 2009 ; Montzka et al. 2011 ) and
global population (Klein Goldewijk et al. 2010 ; United Nations 2015 ) over the Common Era. See
Methods for further information

1 Earth’s Climate System