Geoengineering: Basic science and ongoing research efforts in China

Geoengineering （also called climate engineering）, which refers to large-scale intervention in the Earth＇s climate system to counteract greenhouse gas-induced warming, has been one of the most rapidly growing areas of climate research as a potential option for tackling global warming. Here, we provide an overview of the scientific background and research progress of proposed geoengineering schemes. Geo- engineering can be broadly divided into two categories： solar geoengineering （also called solar radiation management, or SRM）, which aims to reflect more sunlight to space, and carbon dioxide removal （CDR）, which aims to reduce the CO2 content in the atmosphere. First, we review different proposed geoengineering methods involved in the solar radiation management and carbon dioxide removal schemes. Then, we discuss the fundamental science underlying the climate response to the carbon dioxide removal and solar radiation management schemes. We focus on two basic issues： 1） climate response to the reduction in solar irradiance and 2） climate response to the reduction in atmospheric COe. Next, we introduce an ongoing geoengineering research project in China that is supported by National Key Basic Research Program. This research project, being the first coordinated geoengineering research program in China, will systematically investigate the physical mechanisms, climate impacts, and risk and governance of a few targeted geoengineering schemes. It is expected that this research program will help us gain a deep understanding of the physical science underlying geoengineering schemes and the impacts of geoengineering on global climate, in particular, on the Asia monsoon region.

Hydroclimatic anomalies in China during the post-Laki years and the role ofconcurring El Nino

Arctic is warming at an alarming speed causing accelerated melting of Greenland and rising of sea level,and geoengineering by injectingaerosol into stratosphere(SAI)has been proposed as a backup approach to mitigate warming.However,studies suggest that SAI implementationmay have adverse impacts on global especially monsoon precipitation,and Northern Hemisphere high-latitude injections may have dis-proportionally high effects than tropical injections.The 1783e1784 CE Laki eruption in Iceland provides an analogy to study the climatic andthe subsequent socioecological responses to Arctic SAI,and China possesses a rich legacy of documents recording climatic disasters anddescribing their direct impacts on agriculture and society.Using the most recent summer precipitation reconstructions and the documentary data,this study presents a systemic analysis of the hydroclimatic anomalies as well as the societal and ecological consequences in China following the1783e1784 CE Laki eruption.The results from multi-proxies show severe drought conditions in eastern China during the post-Laki years,accompanied by large scale locust breakout,famine and human pestilence.The drought and associated disasters first emerged in the North ChinaPlain in 1784 CE,intensified and expanded to the middle and lower reaches of the Yangtze River in 1785 CE.The drought and famine stresses inChina are part of the very unsettled climate conditions experienced across the Northern Hemispheric world during the 1780s.By isolating theENSO-induced precipitation from the reconstructed summer precipitation changes,our results indicate that the Laki eruption did cause severedrought in monsoon China during the next three years.The drought responses in the 1783e1784 were largely counter-balanced by the wettinginduced by the concurring strong El Ni~no event.The results help to enhance our understanding of the hydroclimate consequence of NH highlatitude volcanic eruption in China,and the potential role climate internal variation such as ENSO may play in modifying volcanic-induced perturbation.