piration fluxes, such as the tropics and the summer season Northern Hemisphere midlatitudes (Figs. 2, 4, 6, and 8–10). In many densely vegetated tropical regions, specifically those that exhibit high transpiration but rely on oceanic sources of moisture for a large proportion of rainfall, future CO2 physiological forcing enhances the likelihood of heavy rainfall events through a negative surface moisture feedback, whereby diminished transpiration from reduced stomatal conductance warms the surface and enhances moisture convergence from surrounding regions (Figs. 10 and 13). In these areas, annual ET decreases while annual precipitation increases. By definition, the local precipitation recycling ratio goes down, and these regions become more reliant on external sources of moisture for rainfall.
Further, in the tropics, reduced stomatal conductance and lower transpiration increases the likelihood of dry days during the transition between the dry and wet seasons, when moisture fluxes from the land surface are particularly important for initiating rainfall (Figs. 9 and 14) (Fu and Li 2004). Likewise, in vegetated areas of the midlatitudes, reduced stomatal conductance increases the frequency of dry days, particularly during the summer season when transpiration and precipitation recycling are relatively high (Figs. 9 and 14). Interestingly, the number of summer season dry days increases in midlatitude Europe, South America, and portions of North America despite increases in summer soil moisture (Fig. S4). The increase in soil moisture reduces soil moisture stress on vegetation, promotes stomatal conductance, and serves as a negative feedback on the original reduction in transpiration from physiological forcing. However, the combined influences of enhanced stomatal conductance from low soil moisture stress and the moderate increase in soil evaporation from excess soil moisture (Fig. 4f) do not balance the much larger reduction in transpiration from the CO2-induced limitation of stomatal conductance, leading to low relative humidity and an increase in dry days (Fig. 9; see also Fig. S7). In some midlatitude regions, including the central plains of the United States and southeastern Canada, drier soils from future physiological forcing (from temperature, cloud, and rainfall feedbacks) amplify the reduction in stomatal conductance and reduce soil evaporation, leading to more frequent dry day occurrence. Importantly, the changes in precipitation characteristics from future CO2–vegetation interactions identified in this study are robust across several different climate model configurations (e.g., simulations with slab and dynamic ocean models, simulations where CO2 physiological forcing and CO2 fertilization are simulated separately and in tandem, and simulations with different CO2 concentrations) (Figs. 9, 10, and 12).
Our results highlight the influential role of transpiration in shaping daily hydrologic characteristics, particularly in the tropics. Lee et al. (2012) propose that transpiration serves to reduce the variability of daily precipitation throughout tropical forests. In contrast, our findings suggest that the role of transpiration is not the same across all tropical forest regions. For example, in the eastern Amazon, reduced transpiration increases the annual number of dry days, but it also decreases the annual number of heavy rainfall days (Figs. 9 and 10). We find that the role of transpiration in shaping daily precipitation depends on a number of factors, including regional geography and time of year. In addition to its impact on precipitation, transpiration also serves as an important control on surface runoff. Specifically, reduced transpiration from future CO2 physiological forcing increases surface runoff, particularly in the tropics (Fig. S4) (Betts et al. 2007; Gedney et al. 2006; Cao et al. 2010). Our results suggest that enhanced runoff from CO2 physiological forcing may also be a consequence of more frequent heavy rainfall events.
Although our results point to the important role of CO2 physiological forcing in shaping projected future precipitation changes in climate model simulations, our findings should be interpreted cautiously. Many uncertainties regarding the interactions among vegetation, carbon dioxide, and hydrology remain (Wang et al. 2009). Most climate models, including those employed in this study, assume that plant water use strategies depend only on the plant’s photosynthetic pathway (classified as C3 or C4) (Kala et al. 2016). However, observations indicate that plants within C3 and C4 groupings exhibit large species-specific differences in stomatal behavior (Lin et al. 2015). Kala et al. (2016) find substantial differences in future climate projections between climate model simulations that use PFT-level water use strategy parameterizations and simulations that use traditional photosynthetic pathway-level parameterizations. Specifically, they find that the implementation of PFT-level water use strategies leads to a greater reduction in ET over Europe in response to future elevated CO2, suggesting that our simulations may underestimate the regional hydrological impact of future CO2 physiological forcing there (Kala et al. 2016). Further, our model experiments do not include the impacts of enhanced CO2 concentrations on down-regulation of photosynthesis, which Pu and Dickinson (2014) have shown can substantially reduce stomatal conductance and transpiration within CCSM4.
Earth system models project widespread increases in LAI under elevated greenhouse gas concentrations (Mahowald et al. 2016). However, by the end of the twenty-first century, global mean LAI changes vary by a factor of about 14 across models under the RCP8.5 scenario (Mahowald et al. 2016). Relationships between projected changes in LAI and changes in vegetation carbon or climate variables are not straightforward and the impact of projected LAI differences on the simulated spread in twenty-first-century precipitation projections across models is unclear. Although the hydroclimate impacts from projected LAI in our CCSM4 simulation are minor, future work should assess the contribution of LAI to hydrologic change within the full suite of CMIP climate models. Additionally, our simulations do not include dynamic vegetation models, and therefore do not capture the potential impacts of future changes in vegetation type or location from climate change or land use change on ET and precipitation, which may be large (Notaro et al. 2007; Pearson et al. 2013; Bathiany et al. 2014).
Last, it is important to note that climate models struggle to properly simulate the full distribution of observed precipitation rates (Lin et al. 2006; O’Gorman and Schneider 2009). In particular, models tend to overestimate the annual number of days with very light precipitation (Lin et al. 2006). A proportion of the dry day (with less than 0.1 mm of precipitation) increases found in our simulation come at the expense of days with very light rainfall (days with 0.1–2 mm of precipitation). Although we provide a mechanistic explanation for the changes in rainfall distribution in CCSM4 [and find consistent results in HadGEM2-A and CESM1(BGC)], the robustness of our results should be tested with other models, particularly those with higher-resolution, resolved convection, and different land surface schemes.
Theoretical and climate modeling studies project a robust change in global precipitation characteristics in response to future increases in CO2 (Trenberth et al. 2003; O’Gorman and Schneider 2009; Lau et al. 2013). Given the potential for acute impacts on human and natural systems, understanding the physical mechanisms that drive these hydrologic changes is critical. To date, most studies have focused on the role of radiative forcing in driving these hydrologic changes. Our modeling results suggest that consideration of the radiative impacts of CO2 alone cannot account for projected regional-scale differences in daily precipitation changes. Within our simulations with CCSM4, we find that projected CO2 physiological forcing substantially reduces future transpiration in vegetated areas such as the midlatitudes during the summer season and the tropics. The reductions in transpiration are of sufficient magnitude to drive changes in the annual frequency of dry days and heavy rainfall days in these regions. The physiologically driven changes have the potential to reinforce or greatly diminish the influence of radiative forcing on future precipitation characteristics. These results suggest that the representation of plant water use and its relationship to atmospheric CO2 within climate models likely contributes to the persistent spread in future regional-scale precipitation projections. Given the potential for future CO2 physiological forcing to shape projected changes in precipitation characteristics, future work should aim to improve the representation of the complex interactions among CO2, vegetation, and hydrology within climate models in order to refine estimates of hydrology and daily dry and wet extremes.
This work was supported by the Turner Postdoctoral Fellowship awarded to Christopher Skinner, and by National Science Foundation Award 1602956. Robin Chadwick was supported by the Newton Fund through the Met Office Climate Science for Service Partnership Brazil (CSSP Brazil). We wish to thank Hervé Douville, Justin Mankin, Bing Pu, Deepti Singh, and Abigail Swann for helpful discussions and three anonymous reviewers whose feedback greatly improved the manuscript. The CESM project is supported by the National Science Foundation and the Office of Science (BER) of the U.S. Department of Energy. We acknowledge the World Climate Research Programme and the climate modeling groups for making available their model output, and the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison for coordinating and supporting database development. We thank Stanford University and the Stanford Research Computing Center for providing computational resources and support that have contributed to these research results. Model data are available upon request.
Food trucks are being added to the cuisine scene at City Center in Newport News.
Four trucks will be serving from 5 to 8 p.m. on these Wednesday nights: Aug. 27, Sept. 10 & 24, Oct. 8 & 22.
The lineup includes The Thirsty Frog, which serves a variety of hot, cold and blended drinks; The Lucky Peach, offering brownies, cookies, salads and sandwiches; Sofrito, serving empanadas, tacos and other Hispanic foods; and Twisted Sisters Cupcakes, offering cupcakes, brownies and