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Single and double cropping regions over the NCP.

After detecting croplands over the entire NCP region using the land-cover classification from the Global Land Cover Facility²¹, SCRs and DCRs within the total cropland are identified. To identify these two different cropping practices, we use the LAI from the Advanced Very High Resolution Radiometer for the period 1985–2005 (ref. 22). The satellite LAI data starts from 1985, at which point double cropping practices were well established in the NCP region. That is, double cropping in the NCP started before the starting measurement date of the satellite data used for identifying the region. Because no record of exact double cropping areas is available, the satellite data is the only means of identifying cropping practices in the NCP. Seasonal variations in the growth of wheat and maize are well demonstrated in the LAI time series. The double and single LAI peaks in the DCRs and SCRs, respectively, can be identified from the LAI annual cycle (Supplementary Fig. 1). For the double LAI peak, the early maximum occurs in April–May, and the late maximum occurs in August. The single LAI peak occurs in August. More details are described in a previous study⁹. Based on these different characteristics of the LAI annual cycle, out of a total of 51 World Meteorological Organization stations on the NCP, 25 stations and 26 stations are grouped into DCRs and SCRs, respectively. The 21-year climatology of the surface air temperature variables T_mean, T_min, T_max and DTR over the DCRs and SCRs in Fig. 2 are defined by averaging the corresponding variables over the stations identified in this way for the entire study period.

Changes in crop area and yield.

The changes in crop area over NCP are calculated using the historical changes in global land cover data²³ (Fig. 1a). The 16-year analysis period in this study (Fig. 1b), 1995–2010, has been selected because the official crop-yield data⁸ used to support increased crop yields in the double cropping regions is available from 1995. It is extremely difficult, if not impossible, to exactly match historical records of crop area, yield and LAI-based double cropping. Thus, we have used all available data sets, even if they cover different time periods.

Regional climate simulations.

The Advanced Research version WRF 3.3 (ref. 24) is used to investigate the impacts of the cropping practice in the NCP (20°–30° N, 112°–122° E) on the regional climate of East Asia. The one-way nested WRF model simulation is configured with a 54-km horizontal resolution and 27 vertical layers between the surface and the model top level (70 hPa). The model domain covers an East Asia region with a grid nest of 144 (East–West) ×74 (South–North) grid points on the Arakawa C-grid system. Seasonal simulations are performed over an eight-month (January–August) period for the 21 years, 1985–2005, using initial and lateral boundary conditions from the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 (ref. 25). To examine the impact of the initial conditions on the simulated climate, four simulations are performed using four different initial conditions over four subsequent days. Because the four simulations generate similar climatology (not shown), results from only one of the four simulations are used in this study. For example, the inter-member standard deviation over the NCP is 0.04 °C for temperature and 0.03 mm d⁻¹ for rainfall, whereas the differences due to the effects of double cropping are much larger than these inter-member standard deviation values at the 95% confidence level.

To isolate the impact of double cropping, we performed WRF simulations using two different annual cycles of the LAI based on satellite observations (Supplementary Fig. 1) over the NCP for the same initial and lateral boundary forcing. In particular, we tested the hypothesis obtained from the observation (Fig. 2) that the DCRs have a climatologically higher temperature than the SCRs owing to the absence of vegetative cover during the inter-cropping period. The CTR (EXP) simulation is prescribed with single (double) cropping LAI over croplands within the NCP. Thus, the difference between these two runs originates only from the impact of the prescribed vegetation seasonality differences over the NCP. The actual effect of the transition from single cropping to double cropping on the regional climate will be the difference between the results of these two simulations. For evaluation (Fig. 2), the simulated values at the grid points closest to the 51 stations are used (Fig. 3).

Analysis of East Asian summer monsoon (EASM).

The inter-cropping period corresponds to the active EASM period. To investigate the impact of double cropping on the East Asian summer monsoon, strong- or weak-monsoon years are identified using the western North Pacific monsoon index²⁶. This results in six strong EASM years (1988, 1993, 1995, 1996, 1998, and 2003) and five weak EASM years (1985, 1986, 1990, 1994, and 1997), for which strong- and weak-monsoon composites are constructed. The observed synoptic circulation and rainfall for the 21-year period during strong or weak EASM years are also investigated using reanalysis²⁵ and Global Precipitation and Climatology Project (GPCP) data²⁷. These observed data are used as the basis for analysing and evaluating the simulated climatology for strong or weak EASM years (Supplementary Fig. 2).