We investigated the spring frost risk over the main apple production areas in Japan under future climates using multiple sets of global circulation models and scenarios. Frost risk was judged by the ...daily minimum air temperature. Apple phenology was estimated with a phenology model under future meteorological conditions. Since spring cold hardiness of apple trees depends on the phenophase, we took the effect into consideration by decomposing the season into three phenophases. April temperatures are projected to increase by 0.5–2 °C for 2031–2050 and 1.5–6 °C for 2081–2100 relative to that of 1981–2000, depending on the climate models and scenarios. Spring phenology will advance by 10 days or more for the highest temperature increase case for 2081–2100. Frost risk will not monotonically change with the future temperature increase. For the scenario with the temperature increase by 1–2 °C in spring, corresponding to the medium temperature increase case among our simulation cases, frost risk will be maximized in the southern Tohoku region and central highlands, where frost risk is relatively high under the current climates. Frost risk will tend to decrease for the highest temperature increase case. During bud break to foliation, apple trees will be in highest frost risk under future climates.
We investigated future frost risks in the Tohoku Region of Japan under climate change. We focused on the processes governing regional variations in the future frost risks over this region. Future ...changes in dates of development stages were determined by estimating the growing degree days. The last frost date was determined by analyzing the daily minimum temperature data. We found regional differences in future changes of the development stage at the last frost date. At sites where a greater advancement in the last frost date is projected, plants are expected to experience their last frost at an earlier development stage in comparison with their last frost experience at present. We showed that a seasonal warming rate (the temperature increase in the seasonal course from winter to summer) around the last frost date is closely linked to the magnitude of advancement in the last frost date. Sites with a low seasonal warming rate tend to show greater advancement in the last frost date, and plants at this site experience the last frost at an earlier development stage. We note that geographical proximity is not necessarily a key factor of similarity for determining the future changes in frost risk. Instead, the concept of “frost risk climatology,” based on classification by similarity of frost risk among sites, could be helpful for formulating measures to reduce future frost risk under climate change on a regional scale.
We investigated the first and last frost dates using meteorological observation data from Japan. First, we identified long-term trends of retardation (mean: +0.224 day/year) in the first frost date ...in fall and advancement (−0.228 day/year) in the last frost date in spring using historical frost observation data from 1951 to 2010. Trends determined over 20-year subperiods were distinct from long-term trends and were sensitive to decadal changes in daily minimum air temperature. Second, we proposed a scheme to infer the first and last frost dates from the time series of daily minimum air temperature. After optimization, the first (last) cold date when the daily minimum temperature fell below a temperature criterion of ca. 2 °C yielded the best estimate of the first (last) frost date with an error of 2 days in most cases. However, the overall root mean square errors were 13–16 days because some cases with significant misfits deteriorated the values. Both wind speed and humidity shifted the criterion. Sites with strong winds contributed to a decrease in the temperature criterion. Because this scheme only requires the daily minimum air temperature, it is widely applicable to the quantitative evaluation of the first and last frost dates from given meteorological or climate projection data.
We investigated meteorological conditions for frost occurrence using routine observation data during 1991–2010 from manned meteorological stations in Japan. The majority of the frost occurrence was ...observed when the daily minimum air temperature was in the range of − 5 °C to 5 °C. Due to strong radiative cooling of a frosted surface, frost was observed even when the daily minimum air temperature was above the dew-point temperature at a normal meteorological observation height. We proposed an analytical scheme to evaluate the probability of frost occurrence under given meteorological conditions. This scheme was based on a probability distribution of observed frost occurrence in a three-dimensional space of meteorological variables. The obtained distribution centered on − 1.9 °C for the daily minimum air temperature and 73% for the relative humidity, with an exponential decay with wind speed. The probability was halved when the daily minimum air temperature and relative humidity change by 3.3 °C and 24 percent-points, respectively, from the distribution center. Since this scheme gives probability of frost occurrence, it is advantageous for risk assessment under given meteorological conditions. Recent popularization of gridded meteorological datasets will help to expand the potential usage of the scheme.
Humans directly change the dynamics of the water cycle through dams constructed for water storage, and through water withdrawals for industrial, agricultural, or domestic purposes. Climate change is ...expected to additionally affect water supply and demand. Here, analyses of climate change and direct human impacts on the terrestrial water cycle are presented and compared using a multimodel approach. Seven global hydrological models have been forced with multiple climate projections, and with and without taking into account impacts of human interventions such as dams and water withdrawals on the hydrological cycle. Model results are analyzed for different levels of global warming, allowing for analyses in line with temperature targets for climate change mitigation. The results indicate that direct human impacts on the water cycle in some regions, e.g., parts of Asia and in the western United States, are of the same order of magnitude, or even exceed impacts to be expected for moderate levels of global warming (+2 K). Despite some spread in model projections, irrigation water consumption is generally projected to increase with higher global mean temperatures. Irrigation water scarcity is particularly large in parts of southern and eastern Asia, and is expected to become even larger in the future.
Water scarcity severely impairs food security and economic prosperity in many countries today. Expected future population changes will, in many countries as well as globally, increase the pressure on ...available water resources. On the supply side, renewable water resources will be affected by projected changes in precipitation patterns, temperature, and other climate variables. Here we use a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways) to synthesize the current knowledge about climate change impacts on water resources. We show that climate change is likely to exacerbate regional and global water scarcity considerably. In particular, the ensemble average projects that a global warming of 2 °C above present (approximately 2.7 °C above preindustrial) will confront an additional approximate 15% of the global population with a severe decrease in water resources and will increase the number of people living under absolute water scarcity (<500 m3 per capita per year) by another 40% (according to some models, more than 100%) compared with the effect of population growth alone. For some indicators of moderate impacts, the steepest increase is seen between the present day and 2 °C, whereas indicators of very severe impacts increase unabated beyond 2 °C. At the same time, the study highlights large uncertainties associated with these estimates, with both global climate models and GHMs contributing to the spread. GHM uncertainty is particularly dominant in many regions affected by declining water resources, suggesting a high potential for improved water resource projections through hydrological model development.
By introducing two scalar quantities, namely, the Gini and Lorenz asymmetry coefficients, we examined their characteristics and applicability to the global analysis of changes in river flow regimes ...under future climate change. First, by applying these coefficients to river discharge data, we showed that various types of flow‐duration curves can be interpreted quantitatively in terms of the seasonal inequality in the discharge (i.e., the unevenness of the temporal distribution of river discharge). Their statistical characteristics, based on five theoretical distribution functions frequently used in hydrological analysis, were also shown. Next we used these coefficients to evaluate the seasonal inequality of major global rivers using the global hydrological model H08 for four 30 year time spans (1960–1989, 2010–2039, 2040–2069, and 2070–2099) under four climate‐change scenarios. We used ensembles of hydrological simulation results with five general circulation models. From the analysis of the Gini coefficient, future changes in seasonal inequality show a contrasting geographical pattern: a decreasing trend at high northern latitudes and an increasing trend in most other areas. The Lorenz asymmetry coefficient shows large changes at high northern latitudes, attributable to major shifts in the flow regime accompanied by different snow‐melting properties under different future climate scenarios. Although a flow‐duration curve is a pictorial representation of river discharge suitable for one specific site, by depicting the geographical distribution of these two coefficients along river channels, different characteristics of flow‐duration curves at different sites can be detected, even within the same river basin.
Key Points
We introduced two quantities representing the shape of a flow‐duration curve
We evaluated future changes in flow regimes of global major rivers
Different flow regimes were found at different sites even within the same river
Crop irrigation is responsible for 70% of humanity's water demand. Since the late 1990s, the expansion of irrigated areas has been tapering off, and this trend is expected to continue in the future. ...Future irrigation water demand (IWD) is, however, subject to large uncertainties due to anticipated climate change. Here, we use a set of seven global hydrological models (GHMs) to quantify the impact of projected global climate change on IWD on currently irrigated areas by the end of this century, and to assess the resulting uncertainties arising from both the GHMs and climate projections. The resulting ensemble projections generally show an increasing trend in future IWD, but the increase varies substantially depending on the degree of global warming and associated regional precipitation changes. Under the highest greenhouse gas emission scenario (RCP8.5), IWD will considerably increase during the summer in the Northern Hemisphere (>20% by 2100), and the present peak IWD is projected to shift one month or more over regions where ≥80% of the global irrigated areas exist and 4 billion people currently live. Uncertainties arising from GHMs and global climate models (GCMs) are large, with GHM uncertainty dominating throughout the century and with GCM uncertainty substantially increasing from the midcentury, indicating the choice of GHM outweighing by far the uncertainty arising from the choice of GCM and associated emission scenario.
Key Points
IWD will considerably increase during the summer in the Northern Hemisphere
Peak demand is projected to shift over 80% of the present irrigated areas
Global hydrological models dominate the uncertainty in projected IWD
We compare ensembles of water supply and demand projections from 10 global hydrological models and six global gridded crop models. These are produced as part of the Inter-Sectoral Impacts Model ...Intercomparison Project, with coordination from the Agricultural Model Intercomparison and Improvement Project, and driven by outputs of general circulation models run under representative concentration pathway 8.5 as part of the Fifth Coupled Model Intercomparison Project. Models project that direct climate impacts to maize, soybean, wheat, and rice involve losses of 400–1,400 Pcal (8–24% of present-day total) when CO2 fertilization effects are accounted for or 1,400–2,600 Pcal (24–43%) otherwise. Freshwater limitations in some irrigated regions (western United States; China; and West, South, and Central Asia) could necessitate the reversion of 20–60 Mha of cropland from irrigated to rainfed management by end-of-century, and a further loss of 600–2,900 Pcal of food production. In other regions (northern/eastern United States, parts of South America, much of Europe, and South East Asia) surplus water supply could in principle support a net increase in irrigation, although substantial investments in irrigation infrastructure would be required.
Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate ...change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by bias-corrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO ₂ and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty.
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