On the assessment of aridity with changes in atmospheric <scp>CO</scp><sub>2</sub>

Roderick, Michael L.; Greve, Peter; Farquhar, Graham D.

doi:10.1002/2015wr017031

Cited by 217 publications

(245 citation statements)

References 108 publications

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“…However, studies that use plant-centric metrics (P-E, soil moisture) tend to show a reduced impact (17,(22)(23)(24)(25). Our analysis provides a conceptual framework and quantitative approach for reconciling many of these differences; divergence of these two approaches arises primarily from omission or consideration of the physiological effects of CO 2 on plant water needs.…”

Section: Resultsmentioning

confidence: 97%

“…However, even the physically based estimates of this quantity (i.e., the Penman− Monteith equation) do not account for changes in transpiration caused by the physiological response of plants to increasing CO 2 , thereby making the implicit assumption that surface conductance is invariant with changing CO 2 . Although the climate implications of the physiological effects of CO 2 on plants have been recognized in the literature (16)(17)(18), the effects have not been well integrated into studies examining impacts and risks of climate change, including flood risk, water resource stress, predictions of future species distributions, agricultural productivity, and ecosystem processes. Further, the science community uses many different drought metrics (Table S1), and the relative sensitivity of these metrics to plant physiological responses has not been systematically quantified.…”

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confidence: 99%

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Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity

Swann

Hoffman

Koven

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

478

423

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Rising atmospheric CO 2 will make Earth warmer, and many studies have inferred that this warming will cause droughts to become more widespread and severe. However, rising atmospheric CO 2 also modifies stomatal conductance and plant water use, processes that are often are overlooked in impact analysis. We find that plant physiological responses to CO 2 reduce predictions of future drought stress, and that this reduction is captured by using plant-centric rather than atmosphere-centric metrics from Earth system models (ESMs). The atmosphere-centric Palmer Drought Severity Index predicts future increases in drought stress for more than 70% of global land area. This area drops to 37% with the use of precipitation minus evapotranspiration (P-E), a measure that represents the water flux available to downstream ecosystems and humans. The two metrics yield consistent estimates of increasing stress in regions where precipitation decreases are more robust (southern North America, northeastern South America, and southern Europe). The metrics produce diverging estimates elsewhere, with P-E predicting decreasing stress across temperate Asia and central Africa. The differing sensitivity of drought metrics to radiative and physiological aspects of increasing CO 2 partly explains the divergent estimates of future drought reported in recent studies. Further, use of ESM output in offline models may double-count plant feedbacks on relative humidity and other surface variables, leading to overestimates of future stress. The use of drought metrics that account for the response of plant transpiration to changing CO 2 , including direct use of P-E and soil moisture from ESMs, is needed to reduce uncertainties in future assessment.T he demand for water by the atmosphere is widely predicted to increase due to climate change (1). It is commonly inferred that this will cause droughts to become more widespread and severe (2). Many recent studies, however, ignore the impact of rising atmospheric CO 2 on plant water use (3-11). Plants absorb CO 2 through stomates in their leaves, and simultaneously lose water to the atmosphere by means of transpiration through the same pathway. Higher atmospheric CO 2 concentrations allow plants to reduce water losses per unit of carbon gain (12), in part by reducing stomatal conductance when the gradient of CO 2 between the atmosphere and the leaf interior increases. If leaf area stays the same, this physiological response has the potential to reduce water losses from the land surface, increase soil moisture, and reduce plant water stress (13)-the opposite effect of an increase in drought stress and aridity as predicted by many drought metrics (3,14,15). A plant-centric view may therefore suggest that ecosystem-level tradeoffs between water loss and photosynthesis under increasing CO 2 are potentially large enough to reduce drought, despite the large projected increases in water demand from a warmer atmosphere.Drought indices, river routing schemes, and water balance models frequently use potential evapotr...

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Section: Resultsmentioning

confidence: 97%

mentioning

confidence: 99%

Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity

Swann

Hoffman

Koven

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

478

423

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“…The water relations of plant canopies are one of the most important determinants of future plant productivity [e.g. 60••, 78,86]. Water dynamics are also important on their own as determinants of downstream water flows [54,55], which is important for the prediction of future water availability and infrastructure needs for the regulation of water flows [e.g.…”

Section: Discussionmentioning

confidence: 99%

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?

Kirschbaum

McMillan

2018

Curr Forestry Rep

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Plant transpiration is a key component of the terrestrial water cycle, and it is important to understand whether rates are likely to increase or decrease in the future. Plant transpiration rates are affected by biophysical factors, such as air temperature, vapour pressure deficits and net radiation, and by plant factors, such as canopy leaf area and stomatal conductance. Under future climate change, global temperature increases, and associated increases in vapour pressure deficits, will act to increase canopy transpiration rates. Increasing atmospheric CO 2 concentrations, however, is likely to lead to some reduction in stomatal conductance, which will reduce canopy transpiration rates. The objective of the present paper was to quantitatively compare the importance of these opposing driving forces. First, we reviewed the existing literature and list a large range of observations of the extent of decreasing stomatal conductance with increasing CO 2 concentrations. We considered observations ranging from short-term laboratory-based experiments with plants grown under different CO 2 concentrations to studies of plants exposed to the naturally increasing atmospheric CO 2 concentrations. Using these empirical observations of plant responses, and a set of well-tested biophysical relationships, we then estimated the net effect of the opposing influences of warming and CO 2 concentration on transpiration rates. As specific cases studies, we explored expected changes in greater detail for six specific representative locations, covering the range from tropical to boreal forests. For most locations investigated, we calculated reductions in daily transpiration rates over the twenty-first century that became stronger under higher atmospheric CO 2 concentrations. It showed that the effect of CO 2 -induced reduction of stomatal conductance would have a stronger transpiration-depressing effect than the stimulatory effect of future warming. For currently cold regions, global warming would, however, lengthen the growing seasons so that annual sums of transpiration could increase in those regions despite reductions in daily transpiration rates over the summer months.

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“…Our lack of understanding in these processes is of concern, particularly as aridity is expected to increase in the future (Berg et al, 2016). Improved representation of these processes is needed, which will probably entail scrutinizing soil moisture-evapotranspiration feedbacks (Roderick et al, 2015). For the basins in Fig.…”

Section: Mean Runoffmentioning

confidence: 99%

Mapping (dis)agreement in hydrologic projections

Melsen

Addor

Mizukami

et al. 2018

Hydrol. Earth Syst. Sci.

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Abstract. Hydrologic projections are of vital socioeconomic importance. However, they are also prone to uncertainty. In order to establish a meaningful range of storylines to support water managers in decision making, we need to reveal the relevant sources of uncertainty. Here, we systematically and extensively investigate uncertainty in hydrologic projections for 605 basins throughout the contiguous US. We show that in the majority of the basins, the sign of change in average annual runoff and discharge timing for the period 2070-2100 compared to 1985-2008 differs among combinations of climate models, hydrologic models, and parameters. Mapping the results revealed that different sources of uncertainty dominate in different regions. Hydrologic model induced uncertainty in the sign of change in mean runoff was related to snow processes and aridity, whereas uncertainty in both mean runoff and discharge timing induced by the climate models was related to disagreement among the models regarding the change in precipitation. Overall, disagreement on the sign of change was more widespread for the mean runoff than for the discharge timing. The results demonstrate the need to define a wide range of quantitative hydrologic storylines, including parameter, hydrologic model, and climate model forcing uncertainty, to support water resource planning.

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On the assessment of aridity with changes in atmospheric CO₂

Cited by 217 publications

References 108 publications

Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity

Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?

Mapping (dis)agreement in hydrologic projections

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On the assessment of aridity with changes in atmospheric CO2

Cited by 217 publications

References 108 publications

Plant responses to increasing CO 2 reduce estimates of climate impacts on drought severity

Plant responses to increasing CO 2 reduce estimates of climate impacts on drought severity

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?

Mapping (dis)agreement in hydrologic projections

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Product

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On the assessment of aridity with changes in atmospheric CO₂

Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity

Plant responses to increasing CO ₂ reduce estimates of climate impacts on drought severity