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Detection of a direct carbon dioxide effect
in continental river runoff records
N. Gedney, P. M. Cox, R. A. Betts, O. Boucher,
C. Huntingford & P. A. Stott
Possible reasons for river runoff increase
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climate change and variability
Deforestation (land cover change)
solar dimming (aerosol effect)
direct atmospheric CO2 effects on plant transpiration
Runoff is the movement of landwater to the
oceans, chiefly in the form of rivers, lakes,
and streams.
Transpiration is the evaporation of
water into the atmosphere from the
leaves and stems of plants.
 On annual and longer timescales,
runoff (R) = precipitation (P) – evapotranspiration (ET)
 Evapotranspiration is a function of energy and water availability,
near-surface atmospheric conditions and the control of transpiration
by plants.
 Climate change and variability modify precipitation patterns and
near-surface meteorology.
 Increased aerosol concentration reduces surface solar radiation,
leading to reductions in open-pan evaporation.
 Landcover changes modify the depth of the soil from which plants
can extract water and the available energy by changing the landsurface albedo.
 Stomatal apertures on plant leaves are observed to close partially
under increased CO2, leading to suppression of transpiration
 Met Office Surface Exchange Scheme (MOSES),
using a monthly observational data set of the
twentieth-century climate.
 Five simulations of twentieth-century surface
hydrology:
All factors vary
Fixed climate
Fixed aerosol
Fixed atmospheric CO2
Fixed land use
 post-1960 observed runoff trends for
all regions tend to be more positive
 a clear disparity between the CLIM
modelled runoff and the observed runoff
trends
 ALL appears closer to the
observations than the CLIM response
except Europe
 modelled response from climate
forcing, aerosols and direct CO2 effects
appear to be important over some
regions
Land use (excluding irrigation) has a
small effect on the modelled continental
water balance
precipitation and modelled and observed runoff
trends over the century
 Use a standard optimal fingerprinting technique to see which factors
are likely to be driving the long-term changes in continental runoff by
comparing the modelled and observed annual anomalies relative to
the long-term mean
Individual region regression results:
β scale factors obtained from the optimal
fingerprinting technique. The 5 to 95
percentile ranges are shown for CLIM, AER,
CO2 and LUSE.
>0: detected at the 5% significance level
>1:modelled component is consistent with the
observed response
CLIM effect is detected over all regions at
the 5% significance level, and in South
America and Asia it is consistent with
observation.
Direct CO2 effect is detected over Africa at
the 5% significance level.
 Aerosols and the CO2 effect are both
consistent with the observed trends over Asia.
β(CO2)>0, β(CO2)> β(AER)> β(LUSE)
All regions regression results:
Only climate and the direct CO2 effect are
detected at the 5% significance
level. The simulation of CO2 effect is
consistent with that observed, whereas the
model slightly over-estimates the climate effect.
 Runoff enhancement due to suppression of
transpiration is detected in the observational records.
 Other mechanisms like expanding irrigation and
human water consumption could also contribute to
changes in observed runoff, but cannot explain the
difference between observed and climatedriven
simulated runoff.
Conclusion
 Raised CO2 levels are already having a direct influence
on the water balance at the land surface.
 As the direct CO2 effect reduces surface energy loss
due to evaporation, it is likely to add to surface warming
as well as increasing freshwater availability.
 The existence of a direct CO2 signal in river runoff
records also opens up the intriguing possibility of using
long-term river records to monitor CO2 effects on the
land carbon sink.
 Huntington(2008): CO2-induced suppression of
transpiration cannot explain increasing runoff
 Research findings from five major areas are consistent
with an increase in the rate of annual actual
evapotranspiration (AET):
 river-basin water-balance
 massive weighing lysimeters
 pan evaporation
 length of the growing season
 plant responses to climate change
 Plant responses to elevated CO2 that stimulate stomatal closure
may induce an increase in leaf and canopy temperature.
 Increases in plant canopy temperatures will result in an increase in
the vapour pressure deficit between the plant canopy and the
surrounding atmosphere, thereby increasing transpiration.
 As temperature increases in conjunction with the increase in CO2,
the increased heat burden in the canopy can result in the need for
increased stomatal opening to provide evaporative cooling. Such
adaptation in the canopy could offset some of the effect of CO2 on
stomatal closure.
 The evidence supporting an increasing rate of AET over many
temperate regions during the 20th century indicates that the
attribution of increasing runoff to decreasing transpiration fluxes is
unlikely.
 It is more likely that the increases in runoff observed for many
basins are a result of an intensification of the hydrologic cycle
whereby increases in precipitation, and in some cases net melting of
ice and snow, have more than offset increases in AET.
 Finding the causes of trends towards increasing runoff is important
for understanding hydrologic responses to ongoing climate warming.
 Discussion:
 Is it reliable to use optimal fingerprinting technique to estimate CO2
effect in the increasing river runoff trend?
 Using long-term river records to monitor CO2 effects on the land
carbon sink, is it possible?