Past, Present, and Future Climate Change: Can Palaeoecology Help Manage a Warming World? - Biodiversity Conservation and Environmental Change - page 90

Medieaval
warm period

Roman warm
period

0.4

0.2

Little ice age

0

-0.2

-0.4

-0.6

-0.8

1 200 400 600 800 1000 1200 1400 1600 1800 2000

Year CE

Figure 5.3 Northern Hemisphere decadal mean temperature variations 1-1999 ce, relative to 1961-1990.
The past two millennia have seen various climate anomalies, including the Roman Warm Period from c.
100bce-400 ce, the Mediaeval Warm Period (MWP), from c. 900-1300 ce. Unusually cold periods
occurred in the Dark Ages c. 400-550 ce and the Little Ice Age (LIA), c. 1400-1850 ce (Crowley 2000,
Rosenthal et al. 2013). Grey shading shows 2 standard deviation error bars, black dotted line shows dec-
adal mean values 1850-1999 ce (Ljungqvist 2010). Reproduced with permission from John Wiley & Sons.

incoming solar radiation (Wild et al. 2007, Gao et al. 2008, Haigh et al. 2010). There is good
evidence that the MWP and LIA were global phenomena (Rosenthal et al. 2013), though there
were regional differences in timing, intensity, and rainfall because of the effects of regional
climate systems, topography, and climate-vegetation feedbacks (Thompson et al. 1986, Grove
and Switsur 1994, Bard et al. 1997, Holmgren et al. 1999, 2003, Harrison et al. 2007, Chase et al.
2009, Licciardi et al. 2009, Mann et al. 2009, Schaefer et al. 2009). For example, subtropical
regions north of the equator tended to be drier in the LIA, whereas south of the equator the
subtropics were wetter because of the southward displacement of the intertropical conver-
gence zone (Russell and Johnson 2007, Kirleis et al. 2011).
Land cover and vegetation also play a role in regional climate, determining the balance
between reflected and absorbed solar radiation; high albedo surfaces like snow or desert
reflect a high proportion of radiation back into space, while forests absorb heat and contrib-
ute to warming. Vegetation also affects water balance; for example the Amazon rainforest
generates its own rainfall through the evapotranspiration of water from leaves. We can learn a
lot about the impacts of warmer climates, changes in rainfall, and the interacting effects of
vegetation cover by looking back at previous warmer periods in the Earth's history.
Some climate change sceptics have argued that as the MWP is a natural phenomenon,
then present-day warming could also be natural. The picture is complicated by the fact that
the onset of the Industrial Revolution and major anthropogenic impacts on climate were
superimposed on the naturally warming conditions that marked the end of the LIA. However,