Climate Change and Soil Carbon Impacts - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 237

and moisture regimes) and physical and
chemical soil factors (e.g. soil parent mater-
ial, clay content, cation exchange capacity;
Dawson and Smith, 2007). For a given soil
type, however, SOC stock can also vary, the
stock being determined by the balance of
net C inputs to the soil (as organic matter)
and net losses of C from the soil (as carbon
dioxide, dissolved organic carbon and losses
through erosion).
Examining climate impacts on cropland
and grassland soils in Europe, Smith et al .
(2005) showed that SOC stocks were pro-
jected to change little between 1990 and
2080, since increase in productivity, feeding
more C into the soil, balanced the increased
losses of SOC from faster decomposition
under climate change. Further, in some
European regions, the future climate is pro-
jected to dry so much that the decompos-
ition rate is slowed, despite large increases
in soil temperature (Smith et al ., 2005).
Ciais et al . (2010) reviewed a number of
European studies and showed that other
modelling studies confirmed this finding,
with cropland soil C stocks estimated to
change little during 1990-1999, ranging
from a small sink of 15 g C m - 2 year - 1 to a
source of over 1.3-7.6 g C m - 2 year - 1 from
the ORCHIDEE-STICS, LPJmL and RothC
models, respectively. Future changes in
cropland SOC were found to be highly de-
pendent on management and land-use change
assumptions, but the direct impact of com-
bined climate drivers were not found to be
large (Ciais et al ., 2010).
Globally, there is some uncertainty
about the impact of climate change on min-
eral soils, related to the complexity of the
factors determining the C balance of soils
(Smith and Fang, 2010) and uncertainties
in the ways models deal with interactions
among the climate drivers. Despite this un-
certainty, projections are similar. Cramer
et al . (2001) used the IS92a anthropogenic
emissions scenario (which is comparable
to the later IPCC A1b scenario) in conjunc-
tion with the HadCM2-SUL version of the
Hadley Centre (UK) climate model. Their
simulations show a c .10% increase (mean
of six Dynamic Global Vegetation Models;
DGVMs) between 2000 and 2100. Gottschalk

et al . (2012), using the RothC model, driven
by a range of climate scenarios and scaled
NPP changes from the IMAGE 2.2 model,
found a similar impact with a comparable
scenario of ~8% increase in SOC stocks.
Ito (2005) reported projected global SOC
changes for the 21st century using seven
climate model realizations of the IPCC A2
scenario, which showed lower climate for-
cing than the scenarios used by Cramer et al .
(2001) and Gottschalk et al . (2012). Unsur-
prisingly, Ito (2005) projected smaller changes
in SOC, in some cases showing a small loss of
SOC, where similar scenarios in the studies
of Cramer et al . (2001) and Gottschalk et al .
(2012) showed small gains. Lucht et al .
(2006) used the LPJ model to simulate SOC
stock changes from 2000 to 2100, and found
similar percentage increases in SOC (~ 5- 6%)
to the 3.5% increase projected by Gottschalk
et al . (2012) for similar climate-forcing scen-
arios. The bulk of the evidence from models
suggests that, at the global scale, projected
changes in SOC in mineral soils are rela-
tively small, and that SOC stocks may well
increase under future climate change.
This global finding, however, masks a
complex pattern of regional responses
(Gottschalk et al ., 2012; Arnell et al ., 2013,
2014). Whereas SOC stocks increase in most
regions, because the increase in NPP offsets
the effects of higher temperatures, there is
little change, or some loss, in high-latitude
parts of Canada and Eastern Europe (Siberia)
and parts of East Asia, where the effects of
higher temperatures outweigh changes in
rainfall and NPP. The complex regional pat-
terns of change in SOC are demonstrated in
Plate 12, which shows average trend in SOC
stock change from 1971 to 2100 across ten
climate scenarios (from Gottschalk et al .,
2012). The spatial heterogeneity in the re-
sponse of SOC to changing climate shows
how delicately balanced the competing gain
and loss processes are, with subtle changes
in temperature, moisture, soil type and land
use interacting to determine whether SOC
will increase or decrease in the future.
Given this delicate balance, we should stop
asking the general question of whether soils
will increase or decrease in SOC under fu-
ture climate, as there appears to be no single