Anthropogenic aerosols are a potential cause for migration of the summer monsoon rain belt in China

It has been proposed that the current and future global warming caused by increased greenhouse gases can produce a northward shift of the Earth’s rain belt (1). This conclusion is supported by a recent study in PNAS by Yang et al. (2) in which the carbon isotope composition of 21 loess-soil sections from the Chinese Loess Plateau for the past 20 ka have been systematically investigated. The authors concluded that at least a 300 km northwestward migration of the monsoon rain belt had occurred from the cold Last Glacial Maximum (LGM) to the warm Holocene in China on the basis of the spatiotemporal patterns of C₄ plant biomass. The authors further imply that the observed southern drift of the summer monsoon rain belt in China during the past few decades will reverse itself and migrate north with the continuation of global warming.

However, Yang et al.’s (2) conclusion fails to consider climate effects of anthropogenic aerosols, a critically important factor for the recent southward migration of the summer monsoon rain belt in China (3–6). The climate model experiments reveal that the observed widespread decrease of summer monsoon rain over South Asia can be attributed mainly to direct and cloud-mediated forcings of anthropogenic aerosols through the slowdown of the tropics-wide meridional overturning circulation (3). The enhancements of black carbon and sulfate aerosols are believed to be responsible for the summer “southern-flood-north-drought” (SFND) pattern over recent decades in China (4, 5). Here, we report the analysis results of the observed summer mean rainfall over China for the period of 1952–2015. We show that with the continuation of global warming for the period of 1952–2015, large-scale droughts are persistently occurring in northern China (along the 34.5°N to 45°N latitudinal band) with the increasing rainfall in eastern China (along the 27.0°N to 34.5°N latitudinal band, mainly located in the Yangtze River Delta) during the summer (Fig. 1). This trend pattern is consistent with the substantial increase of anthropogenic aerosols in eastern China due to the acceleration of industrialization and urbanization in recent decades, which is well documented by measurements because of the double emissions of anthropogenic aerosols and their precursors (5, 6). The suppression of the convective precipitation in northern China and the enhancement of the large-scale precipitation in eastern China by the elevated anthropogenic aerosols are the proposed mechanisms for the SFND (5, 6).

Fig. 1.

(A) Spatial distributions of the observed summer (June–August) monthly mean rainfall (unit: millimeters per month) over China for different periods:1952–1960, 1961–1970, 1971–1980, 1981–1990, 1991–2000, 2001–2010, and 2011–2015. (B) Spatial distributions of the average anomalies (unit: millimeters per month) of the observed summer (June–August) monthly mean rainfall over China for the different periods (relative to the mean values for the whole period of 1952–2015): 1952–1960, 1961–1970, 1971–1980, 1981–1990, 1991–2000, 2001–2010, and 2011–2015. (C) Spatial distributions of linear trends of the observed summer (June–August) monthly mean rainfall amounts (unit: millimeters per month per year) over China for the whole period of 1952–2015 and for two periods: 1952–1980 and 1981–2015. The blue and red boxes outline the northern China (34.5°N to 45.0°N, 109.5°E to 122.5°E) and eastern China (27.0°N to 34.5°N, 112.5°E to 122.5°E) regions defined in this study, respectively. The summer monthly mean rainfall decreased continuously over northern China (34.5°N to 45.0°N, 109.5°E to 122.5°E) after 1981, although the main rain belts are still located in southern China (south of 34.5°N), as shown in A. The negative values of the average anomalies of the observed summer mean rainfall after 1981 also confirm the decreasing trends over northern China, as shown in B. Over eastern China (27.0°N to 34.5°N, 112.5°E to 122.5°E), the positive values of the average anomalies of the observed summer mean rainfall in almost all stations after 1991 reveal the increasing trends, especially for the periods of 1991–2000 and 2011–2015, as shown in B. The uniformly negative trends over northern China, but positive trends in eastern China, for the whole period of 1952–2015 in C clearly indicate the abnormal climate pattern of the summer SFND. This summer SFND is mainly because of linear trends over the period of 1981–2015 due to the fact that there were consistent negative trends for most of the stations for the period of 1952–1980, as shown in C. The anthropogenic aerosol concentrations over eastern China have increased dramatically from the 1950–2015 due to the double emissions of anthropogenic aerosols and their precursors because of acceleration of industrialization and urbanization, with most of the increase taking place after the 1980 when the new Open Door Policy was carried out. The trends in C generally reflect these historical developments. The substantial increase of anthropogenic aerosols in China will favor intensification of the weakening monsoon circulation in recent decades by decreasing the temperature difference between land and sea (5, 6). The data of the observed summer (June–August) monthly mean rainfall (unit: millimeters per month) at the 206 national monitoring stations over China for the whole period of 1952–2015 are obtained from the website of the China Meteorological Administration (data.cma.gov.cn/site/index.html).

Although we strongly agree with Yang et al.’s (2) conclusion that the monsoon rain belt moved north (∼300 km) slowly in China after the end of the LGM 20 ka, we do not think that this result can be applied to the current situation in China, where there are many more anthropogenic aerosols in the atmosphere than in ancient times. We believe that the significant reduction of anthropogenic aerosols is the only possible way to reverse the observed southern drift of the summer monsoon rain belt and bring rain once again to the parched northern part of China in the future. Further research with observations and modeling effort at multiple levels is urgent.