Kelly A. Nugent & H. Damon Matthews (2012). Drivers of Future Northern Latitude Runoff Change,

Atmosphere-Ocean, 50:2, 197-206. doi: 10.1080/07055900.2012.658505

Abstract

Identifying the drivers of changing continental runoff is key to understanding current and

predicting future hydrological responses to climate change. Potential drivers of runoff change

include changes in precipitation and evaporation due to climate warming, vegetation

physiological responses to elevated atmospheric CO2 concentrations, increases in lower-

atmospheric aerosols, and anthropogenic land cover change. In this study, we present a series of

simulations using an intermediate-complexity climate and carbon cycle model, to assess the

contribution of each of these drivers to historical and future continental runoff changes. We

present results for global runoff, in addition to northern latitude runoff that discharges into the

Arctic and North Atlantic Oceans, so as to identify any potential contribution of increased

continental freshwater discharge to changes in North Atlantic deep-water formation. Between

1800 and 2100, the model simulated a 26% increase in global runoff, and a 32% runoff increase

in the northern latitude region. This increase was driven by a combination of increased

precipitation from climate warming, and decreased evapotranspiration due to the physiological

response of vegetation to elevated CO2. When isolated, climate warming (and associated

changes in precipitation) increased runoff by 16% globally, and by 27% at northern latitudes.

Vegetation responses to elevated CO2 led to a 13% increase in global runoff, and a 12% increase

in the northern latitude region. These changes in runoff, however, did not affect the strength of

the Atlantic Meridional Overturning Circulation, which was affected by surface ocean warming

rather than by runoff-induced salinity changes. This study indicates that vegetation physiological

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responses to elevated CO2 may be contributing to changes in continental runoff at a level similar

to that of the direct effect of climate warming.

1. Introduction

Predicting runoff change is challenging given the complexity and uncertainty associated

with the climate’s response to anthropogenic forcings and, particularly, the hydrological changes

associated with anthropogenic climate warming. However, quantifying a range of possible

responses is critical given the important ramifications of a modified hydrological cycle (Allen &

Ingram, 2002; Milly, Dunne & Vecchia, 2005). Analysis of historical stream discharge has

identified a trend towards increasing continental runoff at high northern latitudes (Labat et al.,

2004, Dai et al., 2009). There is some disagreement over historical records at low to mid

latitudes, with the result that estimates of global runoff changes range from an increasing trend

(Labat et al, 2004) to no statistically detectable trend (Dai et al., 2009). Climate model

predictions of future runoff change are also highly variable; though there is general agreement

that continental runoff is expected to change on the order of 5-15% per degree of global

warming. These projected changes include an anticipated decrease in runoff in many temperate

and subtropical regions, and a corresponding increase at high latitudes as a result of an overall

intensification of the global hydrological cycle (Solomon et al., 2010).

Though these model projections account for changes in precipitation and evaporation

associated with anthropogenic climate warming, they generally do not include other potentially

important drivers of continental runoff change. In particular, several recent studies have

highlighted the potential role of terrestrial vegetation responses to elevated atmospheric CO2

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concentrations as a contributor to global runoff changes. This so-called “physiological forcing”

results primarily from decreased stomatal conductance as plants are able to use available soil

moisture more efficiently in higher ambient CO2 concentrations. The effect of vegetation

physiological forcing has been shown in some regions to amplify the response of runoff to

climate warming (Betts et al., 1997; Levis, Foley & Pollard, 2000; Medlyn et al., 2001; Gedney

et al., 2006; Betts et al., 2007; Bernacchi et al., 2007; Cao et al., 2009; 2010).

In general, vegetation physiological forcing tends to decrease transpiration, leading to

increased soil moisture and a corresponding increase in continental runoff (Levis et al., 2000;

Piao et al., 2007). This effect can be damped somewhat by plant structural responses to CO2

changes whereby enhanced leaf growth can lead to an increase in available surface area for water

transpiration and a corresponding increase in moisture recirculation to the atmosphere. In so

doing, plant structural responses are thought to at least partially offset the effect of the

physiological forcing, though the outcome is also highly dependent on regional landscape

characteristics (Betts et al., 1997; Levis et al., 2000; Piao et al., 2007).

Anthropogenic land-cover change has also been identified as a potential contributor to

historical runoff change through its influence on vegetation and land surface characteristics

associated with cropland establishment and abandonment (Piao et al., 2007). For example,

deforestation and/or agricultural expansion might be expected to affect runoff as a result of

changes in evapotranspiration and soil water retention. In addition, the differing albedos of

forests and crops could modify regional radiative forcing and could therefore affect precipitation

patterns via altered surface temperatures. At a global scale, however, studies have found

relatively little impact of land cover change on historical runoff changes (Gedney et al., 2006).

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Finally, the growing knowledge of the effect of anthropogenic aerosols on climate

processes has revealed several potential means by which increased atmospheric aerosols may

affect continental runoff. Aerosols in the atmosphere absorb and scatter shortwave radiation

resulting in both a cooling of the atmosphere, and a decrease of energy received at Earth’s

surface. This has the potential to decrease precipitation (as a result of atmospheric cooling) or to

decrease surface evaporation (due to less surface energy availability), either of which could

affect overall continental runoff. In addition, anthropogenic aerosols can act as cloud

condensation nuclei, leading to changes in their physical and chemical properties, potentially

further altering precipitation patterns (Boucher, Myhre & Myhre, 2004; Solomon et al., 2007).

Some studies have found a reduction in precipitation and evaporation due to lowered surface

solar radiation from aerosol forcing (e.g. Liepert et al., 2004). However, the results of Gedney et

al.’s (2006) analysis of 20th century runoff observations showed little evidence of aerosol impacts

over northern latitude continents.

In this study, we present transient model simulations of historical and future climate

change using an intermediate complexity global climate and carbon cycle model. In a series of

sensitivity experiments, we assess the relative contributions of physical climate changes due to

greenhouse gas increases, vegetation physiological forcing and structural changes, land-use and

land-cover change, and anthropogenic aerosols as potential drivers of observed and projected

future continental runoff changes. We focus in particular on high northern latitudes, which have

seen increased historical runoff, due to the possibility that changes in continental runoff could

have implications for ocean circulation and deep water formation in the North Atlantic. Our use

of transient simulations with a coupled climate and carbon cycle model enables an examination

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of these potential links between land surface processes and ocean circulation, and in addition,

advances previous research focused on equilibrium-only climate simulations.

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2. Methodology

We use here the University of Victoria’s Earth System Climate Model (UVic ESCM)

version 2.9 (Eby et al., 2009, Weaver et al., 2001), a coupled climate-carbon model of

intermediate complexity. Version 2.7 of the UVic ESCM was a contributing model to the C4MIP

model inter-comparison project (Friedlingstein et al., 2006) and to the Fourth Assessment Report

of the Intergovernmental Panel on Climate Change (Meehl et al., 2007). Version 2.9 has an

equilibrium climate sensitivity of 3.5 °C and a transient climate response of about 2 °C, which

falls approximately within the mid-range of more comprehensive climate models (Meehl et al.,

2007).

The atmospheric component of the UVic ESCM is a reduced complexity (1 vertical level)

energy-moisture balance model, with a spherical grid resolution of 3.6° longitude by 1.8°

latitude. The atmosphere model controls heat and moisture transport through Fickian diffusion,

as well as moisture transport via advection. Precipitation in the model occurs when relative

humidity exceeds 85% (Weaver et al., 2001). This simple atmosphere is coupled to a three-

dimensional ocean general circulation model, the Modular Ocean Model version 2.2 (MOM2.2,

Pacanowski, 1996) which has 19 vertical levels (Weaver et al, 2001). MOM2.2 is driven by

surface boundary conditions from the atmosphere and land surface; in particular, the freshwater

flux (as a function of precipitation, evaporation and runoff) is added to the surface ocean in the

form of a salinity flux. The maximum Meridional Overturning Circulation (MOC) is also

calculated in the ocean model, whose value corresponds to the strength of deep-water formation

in the North Atlantic (Weaver et al., 2001).

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The land surface scheme used is the Met Office Surface Exchange Scheme (MOSES)

(Cox et al., 1999, Essery et al., 2003), coupled to a biochemical vegetation model (TRIFFID:

Top-down Representation of Interactive Foliage and Flora Including Dynamics) which together

simulate biophysical processes for five plant functional types found in the model (Cox et al,

2001). TRIFFID is a dynamic vegetation model, which simulates the structure and spatial

distribution of vegetation in response to changing climate conditions. MOSES simulates land-

atmosphere carbon fluxes and calculates various components of the hydrological cycle, including

evapotranspiration, soil moisture and runoff. Evapotranspiration in the model is calculated using

the Penman-Monteith formulation and varies as a function of both local climate conditions and

atmospheric CO2 concentration (Cox et al., 1999, Essery et al., 2003). The version of MOSES

implemented in the UVic ESCM is a simplified version of MOSES2 as described in Essery et al

(2003); in particular, soil moisture here is stored in a one-meter soil layer, as opposed to the four

soil layers in the original version. Excess moisture is diverted to the ocean as runoff via 32

realistic river basins, according to a river-routing scheme as described (Weaver et al., 2001). We

note that version 2.9 of the UVic ESCM does not represent permafrost, wetlands or frozen soils;

this is an area of active model development (Avis et al., 2011), but represents a limitation of the

current model in representing Arctic continental runoff responses to climate warming.

In our study, we performed five transient model simulations following a 10,000 year

spin-up period under pre-industrial forcing. For the reference simulation (ALL) we included all

potential drivers of runoff change: the SRES A2 business-as-usual CO2 concentration scenario,

including additional prescribed forcing from non-CO2 greenhouse gases (Nakicenovic et al.,

2000); physiological and structural responses of vegetation to CO2 changes; historical land-cover

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change from Ramankutty and Foley (1999); and, the direct effect of anthropogenic sulphate

aerosols, represented as a prescribed aerosol optical depth following historical and future A2

aerosol emissions (see Matthews et al., 2004 and http://climate.uvic.ca/EMICAR5/forcing for

more information about aerosol forcing in the UVic model). Subsequently, we performed

additional simulations wherein each of the four runoff drivers was turned off. For aerosol effects

and land-use change, the effect was removed from each respective simulation by holding the

forcing constant at pre-industrial conditions. We isolated the effect of vegetation-CO2

physiological forcing by holding CO2 concentrations constant at pre-industrial levels with respect

to the vegetation component of the model while allowing CO2 radiative forcing to affect the

physical climate model. Conversely, the climate effect of greenhouse gases was isolated by

masking the radiative forcing from increased greenhouse gases in the climate model, while

allowing CO2 increases alone to affect vegetation growth.

Model simulations are summarized in Table 1. In the results presented here, we isolated

the effect of individual drivers by calculating the difference between each of the four simulations

and the reference simulation ALL. We extracted and analysed runoff changes both globally and

for land areas that discharge into the Arctic and North Atlantic Oceans north of 45°N latitude,

and between 105°W and 90°E longitude (see grey area marked on Figure 3). Within this region,

we calculated runoff, land temperature, precipitation and evaporation over the river basins whose

discharge areas were within the specified latitude and longitude ranges. Ocean fields

(temperature and salinity) were calculated for all ocean surface areas within the region.

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3. Results

Under present-day conditions, globally-averaged runoff in the model was 1.1 Sv (where 1

Sv = 106 kg/s). In our selected northern latitude region, which includes river drainage into the

North Atlantic and part of the Arctic Oceans, total runoff was 0.13 Sv at present-day. These

values compare well to recent estimates by Dai and Trenberth (2009), who reported a total

continental discharge of about 1.15 Sv for the year 2004, and an Arctic Ocean discharge of 0.14

Sv.

Simulated trends in globally-averaged runoff, global land surface air temperature, land

precipitation and land evaporation are shown in Figures 1 (global) and 2 (northern latitude

region). Over the 20th century, global runoff in ALL increased by 7.5 mm/year, or 3.4% relative

to pre-industrial, accompanying a 0.6 ºC increase in global temperature (Fig. 1A and 1B). This

represents a 5.6% increase per degree of global warming, which is consistent with the 4%/ºC

reported by Labat et al. (2004) based on historical runoff and temperature observations, though

not with Dai and Trenberth (2009) who did not detect a significant trend in global runoff over the

past 50 years. Over the specified northern latitude land region, runoff increased by 9.3 mm/yr

(Figure 2A), corresponding to a total discharge increase of 0.006 Sv. By way of comparison, Dai

and Trenberth (2009) reported a significant increase in continental discharge to the Arctic Ocean

of 0.014 Sv between 1948 and 2004, with no significant trend in the Atlantic as a whole; this is

broadly consistent with the simulated results in our northern latitude region, which covers a

portion of both the Atlantic and Arctic Oceans (see area marked on Figure 3). Over the 21st

century, changes in global runoff per degree of global warming were slightly larger (6.3 %/ºC),

which is generally consistent with the range of 5-15 %/ºC reported from a range of different

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models by Solomon et al. (2010). In our selected northern latitude region (Fig. 2A), runoff

increased by 32% over the 21st century alongside a 4.1 ºC temperature rise – a slightly higher

regional sensitivity of 7.8 %/ºC.

As can also be seen in Figure 1, greenhouse gas increases (line GG) were the primary

driver of global runoff changes as a result of increased precipitation over all land areas at a rate

of 1.2 % per degree Celsius of land temperature increase. In the northern latitude region,

warming from greenhouse gas increases led to both an increase in precipitation, and a decrease in

evaporation, resulting in a relatively larger increase in continental runoff (Figure 2). Vegetation

physiological forcing was the second largest contributor to simulated runoff changes. At the

global scale, the effect of vegetation physiological forcing was only slightly smaller than the

effect of greenhouse gases, though was closer to half the effect of greenhouse gases in the

northern latitude region.

The mechanism for runoff changes due to physiological forcing was fundamentally

different from that of greenhouse gases. In the case of V-CO2, physiological forcing resulted in a

large decrease in precipitation, though unlike the case of GG, this was not a function of

temperature changes (which were small); rather, precipitation in V-CO2 was driven by a large

decrease in evapotranspiration (Fig. 1D and 2D) as a result of increased water-use efficiency at

higher CO2. Consequently, there was a clear division between the two mechanisms of

influencing changes in continental runoff. Climate warming tended to increase precipitation and

drive a parallel increase in runoff (line GG), whereas elevated CO2 tended to induce decreased

transpiration, leading to decreased water re-circulation to the atmosphere, increased soil moisture

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and consequent further increase in runoff. The runoff change in the ALL simulation was driven

in approximately equal measure by each of these two processes

Aerosols (AER) and land cover changes (LCC) were clearly of secondary importance in

driving both historical and future runoff changes. Aerosols tended to have the opposite effect as

GG, leading to a cooling of land temperatures, and an associated decrease in precipitation and

runoff (with little change in evaporation). The effect of aerosols was relatively larger in the

northern latitude region, owing to the higher concentration of anthropogenic aerosols over

northern continental areas; even here, however, aerosols had less than half the effect of

vegetation physiological forcing by the year 2100. Land cover changes had an almost negligible

effect on historical runoff changes, and were not relevant to future changes given that areas of

anthropogenic land use did not change in the simulations after the year 2000.

Land temperatures (Fig. 1B and 2B) increased in both the GG and ALL simulations,

though warming in GG was greater (4.8 °C globally and 5.8 °C in the northern region at 2100)

than in ALL (4.2 °C globally and 4.8 °C in the northern region) due to the absence of the effect

of reflective aerosols in GG. Globally, aerosols accounted for about 0.9 °C cooling at the year

2000, with a larger cooling of 1.3 °C in the northern latitude region. Temperatures continued

cooling slightly for the first half of the 21st century, then warmed in the latter half of the century

due to a decreased aerosol forcing after ~ 2040. V-CO2 and LCC did not play a prominent role in

affecting air temperature change over the course of the two centuries. Vegetation structural

feedbacks did result in a small amount of warming (~ 0.25 °C globally and in the northern

region) as a result of increased leaf area and spatial coverage of vegetation in response to

elevated CO2 concentrations, leading to decreased surface albedo. Land-use change led to an

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even smaller amount of cooling (~ 0.1 °C) due to the higher surface albedo associated with

agricultural vegetation types compared to forest vegetation.

Figure 3 shows the spatial distribution of precipitation minus evaporation (P–E) changes

from 1800 to 2100 for each simulation. The spatial patterns seen here reflect the contrasting

mechanisms behind simulated runoff changes as discussed in Figure 2. For instance, in GG, a

greater P–E increase is seen along coastal regions and in areas of higher elevation; this reflects

an increase in precipitation under climate warming that occurs predominantly over the oceans

and over orography. Similarly, cooling in AER generated a similar pattern of P–E changes

reflecting decreased precipitation. By contrast, P–E changes in V-CO2 are more representative of

global vegetation distributions, which reflects the pattern of decreased evapotranspiration driven

by vegetation responses to elevated CO2 (CO2 physiological forcing). In general, the spatial

pattern of P–E in ALL reflects the combined P–E changes in GG and V-CO2, with smaller

contributions from AER and LCC.

Finally, we assessed to what extent the simulated runoff increases shown here may have

affected deep-water formation in the North Atlantic. Figure 4 shows the simulated change in the

Atlantic Meridional Overturning Circulation (AMOC) (Fig. 4A) in response to changes in

surface water temperature (Fig. 4B) and salinity (Fig. 4C). For all simulations, AMOC changes

appear to correlate with both salinity and temperature variations, with a consistent pattern of

decreased (increased) strength of the AMOC associated with increased (decreased) temperature

and decreased (increased) salinity. However, it is clear that North Atlantic salinity changes were

not driven by changes in continental runoff, but rather by changes in P–E over this ocean region.

In particular, the V-CO2 simulation, which showed a large increase in continental runoff, did not

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simulate decreased North Atlantic salinity nor a change in the strength of the AMOC. Instead,

ocean circulation appears to be governed primarily by changes in high-latitude temperatures (and

the associated change in the latitudinal temperature gradient): climate warming in ALL and GG

led to a slowing of the AMOC, whereas climate cooling in AER led to a slightly stronger

AMOC. Changes in continental runoff did not have a noticeable effect.

4. Discussion

In this study we investigated the variation in and explanation for future continental runoff

changes using transient climate simulations driven by a suite of anthropogenic forcings. Climate

change (including climate warming and changes in precipitation) and vegetation physiological

responses to elevated CO2 are projected to be the predominant drivers of runoff change for the

21st century, contributing in approximately equal measure to simulated global runoff increases.

By contrast, land cover change and anthropogenic aerosols had secondary effects on continental

runoff trends. We further assessed to what extent simulated changes in continental runoff could

affect the strength of North Atlantic deep water formation; while deep water formation did

decrease in response to climate change, this occurred as a result of temperature and P–E changes

rather than due to runoff-induced salinity changes.

The effect of vegetation-CO2 interactions that we have shown here represents the net

effect of both physiological and structural responses of vegetation to CO2, though we have also

identified the specific climatic outcome of each of these two vegetation processes. The effect of

the vegetative structural feedback was primarily to generate a small increase in surface

temperature; this occurred as a result of CO2 fertilization, in which forest expansion over

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grasslands led to decreased local surface albedo, consequently increasing absorption of

shortwave radiation (see also Matthews, 2007). In contrast, the effect of CO2 physiological

forcing was seen predominantly in hydrological cycle changes; elevated CO2 led to increased

vegetation water-use efficiency, decreased evapotranspiration and a consequent increase in

continental runoff.

Our model results contribute to the growing number of studies suggesting that vegetation

physiological forcing from increasing atmospheric CO2 is an important contributor to both runoff

changes as well as to the overall climate response to CO2 emissions (Betts et al., 2007; Boucher

et al., 2009 Cao et al., 2010). The effect of vegetation physiological forcing shown here supports

the findings Betts et al. (2007) and Boucher et al. (2009), who use a similar version of the

vegetation model employed here, though coupled in their case to the Hadley Centre global

climate model. In our study, the effect of physiological forcing on the hydrological cycle was

also substantially larger than the direct effect of vegetation changes on surface temperature; we

simulated a relatively small temperature increase due to CO2 fertilization of vegetation (less than

0.25 °C increase in the northern latitude region and only 0.1 °C increase averaged over the

globe). By contrast, other studies have found a somewhat larger effect; for example, Cao et al.

(2010) simulated a ~0.42°C increase in global surface temperature due to vegetation

physiological responses to elevated CO2. This difference reflects inter-model uncertainty, as well

as differences in methodology between our studies; in particular, we have carried out transient

model simulations here, whereas Cao et al. (2010) used equilibrium time-slice simulations. This

has implications for the timescale of vegetation responses: while hydrological responses to

elevated CO2 occur quickly, the structural response and particularly the lateral expansion of

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vegetation is limited by the fairly slow growth of vegetation in the model. Consequently, we

suggest that the transient methodology we have adopted provides a more realistic representation

of the relative balance of hydrological and temperature responses to CO2-induced vegetation

changes.

Historical land cover change did not contribute in any large measure to simulated runoff

changes in our study. This is consistent with the conclusions of Gedney et al. (2006), who found

that land cover change did not have a significant influence on observed continental runoff

changes. There is some evidence that deforestation can lead to local-scale changes in runoff due

to decreased evapotranspiration and soil water retention (e.g. Costa et al., 2003, Bradshaw et al.,

2007); however, these studies have focused on tropical forests, which have experienced a much

larger extent of land cover change compared to Northern latitudes. There is also the potential for

interactions between future land-use change and vegetation-CO2 responses. For example,

agricultural vegetation is likely to respond differently to CO2 changes as compared to natural

vegetation types – a phenomenon which we have not explicitly considered here. In addition, we

have considered changes in cropland areas only as a representation of land cover change, and

have not included pastures as an additional source of land-conversion, which would likely

increase slightly the total effect of land-use change shown here.

In the case of aerosols, we did simulate decreased precipitation in response to the cooling

induced by increased anthropogenic aerosols, which is consistent with other models, as well as

the climate response to aerosols resulting from recent volcanic eruptions (Trenberth and Dai,

2009). This change in precipitation led to a small decrease in runoff, both globally and in the

northern latitude region considered here. We note, however, that we have considered only the

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direct effect of sulphate aerosols, and not a full representation of the possible direct and indirect

effect of a range of anthropogenic aerosols; this could also lead to an underestimate of the effect

of aerosols on the global hydrological cycle.

We note also, that none of the models which have been used thus far to assess the effect

of vegetation physiological forcing on continental runoff dynamics include the impacts of

permafrost and changes in frozen soils on high-latitude soil water retention in response to climate

warming. This is potentially an important additional driver of runoff changes (see e.g. Rawlins

et al., 2003, Slater et al., 2007) which would alter the response of high-latitude runoff to

continued climate warming. The response of permafrost to climate warming remains a critical

open question regarding both the potential for greenhouse gas feedbacks to climate warming, as

well as the possible implications for continental runoff changes.

We did not find that simulated runoff changes were sufficient to induce a reduction of the

strength of North Atlantic deep water formation. Our model did simulate a decrease in the

meridional overturning circulation of 4.5 Sv between 1800 and 2100, from a pre-industrial state

of 21.5 Sv (about a 20% decrease). This general trend agrees with previous research; for

example, Bryan et al. (2006) showed a 22-26% circulation decline per century in response to a

1% per year increase in CO2 forcing. This simulated decrease was driven by temperature

increases in the North Atlantic (leading to associated changes in precipitation and sea-surface

salinity), as well as by a decreased equator to pole temperature gradient brought about by climate

warming. By contrast, simulated runoff changes did not affect the strength of the overturning

simulation, as evidenced by the lack of AMOC response to runoff changes from vegetation

physiological forcing. Our results suggest that vegetation responses to elevated CO2, while

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important for projections of hydrological cycle changes, are not likely to influence projections of

the AMOC response to anthropogenic climate warming. However, we note that a significant

potential source of freshwater input to the North Atlantic from melting land ice is not included in

this study; this has been identified previously as a more important factor influencing salinity in

the North Atlantic than stream discharge (Bryan et al., 2006). Clark et al. (2002) found that only

an ~0.1 Sv change to the hydrological cycle was sufficient to induce a thermohaline circulation

response during the last glaciation, though we note that recent research has questioned the ability

of coarse-resolution ocean models to simulate AMOC responses to continental freshwater

discharge (Condron and Winsor, 2011). Nevertheless, our findings represent an ~0.04 Sv

increase between 1800 and 2100 in freshwater discharge to the North Atlantic, which if

combined with significantly increased land ice melt, may be sufficient to affect a more dramatic

deep-water formation response than that simulated here.

Considering our findings more broadly, they do suggest the potential for major regional

alterations in the hydrological cycle, which could have significant implications for both human

and ecological communities. One possible positive implication may be that increased runoff

could result in increased water availability for agriculture and other human uses. However, such

increases will likely be concentrated in areas that already have ample water supply, whereas

more arid regions may be negatively impacted by regional declines in precipitation. Ultimately,

the combination of greater soil moisture availability and a warmer climate will require adaptation

in how humans make use of the water resources available to us in a changing climate.

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5. Conclusion

Projecting runoff change for the 21st century is an important step in understanding potential

future climate change as anthropogenic activities continue to be the dominant driver of changes

in the Earth system. Understanding the complexities of the climate system components will help

us prevent harmful climate impacts that have begun as a result of unmitigated temperature

increases and changes in the spatial patterns of precipitation. This study concludes that plant

responses to atmospheric CO2 concentrations will play an important role, in addition to the direct

effect of climate warming, in driving future changes in the global hydrological cycle. Vegetation

changes will likely also influence regional patterns of temperature changes, though this effect is

secondary to the hydrological effects of vegetation physiological responses to elevated CO2. The

relationship between increased freshwater discharge and ocean circulation response was shown

to be less significant, with changes in North Atlantic deep water formation responding primarily

to temperature changes rather than changes in continental runoff. The use of transient

simulations with our coupled climate-carbon cycle model has been a useful approach as it has

enabled us to investigate potential links between land surface processes and ocean circulation,

while advancing previous runoff change research that, thus far, has focused largely on

equilibrium-only climate simulations.

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7. Figure Legends

Figure 1:

Simulated global trend in A) continental runoff, B) land temperature, C) precipitation over land,

and D) evaporation over land, in response to the drivers ALL, GG, V- CO2 , LCC and AER.

Figure 2:

Simulated northern latitude trend in A) continental runoff, B) land temperature, C) precipitation

over land, and D) evaporation over land, in response to the drivers ALL, GG, V- CO2 , LCC and

AER. The region shown here covers all river basins that drain into the North Atlantic and Arctic

Oceans north of 45 °N latitude, and between 105°W and 90°E longitude, as indicated by the

shaded box in Figure 3.

Figure 3:

Spatial distribution of precipitation – evapotranspiration (P–E) change between 1800 and 2100,

in response to each driver: A) ALL simulation; B) greenhouse gas forcing; C) vegetation

physiological forcing; E) aerosol forcing; E) land cover change. The grey bounding box

represents our selected northern latitude region.

Figure 4:

Simulated trends in the Atlantic Meridional Overturning Circulation (AMOC) (A), surface ocean

temperature changes (B) and surface ocean salinity changes (C) in response to the drivers ALL,

GG, V- CO2 , LCC and AER. Surface ocean temperature and salinity values are averaged over

the North Atlantic and Arctic Oceans, north of 45 °N latitude, and between 105°W and 90°E

longitude.

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Table 1: Description of model simulations.

Simulation

ALL Reference simulation with all runoff drivers included: CO2+non-CO2 radiative

forcing; Vegetation-CO2 physiological forcing; direct effect of sulphate aerosols and

historical land cover change.

GG Effect of CO2 and non-CO2 greenhouse gas forcing alone*

V-CO2 Effect of vegetation physiological forcing alone*

AER Effect of direct sulphate aerosol forcing alone*

LCC Effect of historical land cover change alone*

* Effect of single driver calculated as the difference between a simulation without this process, and the reference simulation ALL.

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