Quantifying Agriculture as a Driver of Regional Climate Extremes

上个世纪的全球农业转型被正确地称为一场革命，因为化肥、杀虫剂、灌溉和杂交的使用使作物产量提高了三倍。 产量的增加意味着今天的农田是地球上最密集的植被生长的家园，伴随着地球上最大的植物用水。 植物吸收的大部分水分通过叶子释放到大气中，因此作物产量的提高应该伴随着大气湿度的增加。 此外，由于在将液态水转化为水蒸气时消耗了热能，因此增加的湿度通常伴随着冷却。 在没有灌溉的情况下，大气湿润是以土壤水分为代价的，因为从叶子释放的水蒸气首先被植物根部从土壤中吸收。 考虑到这些对湿度、温度和土壤湿度的影响，再沿着农田覆盖了世界陆地表面的三分之一，人们自然会问绿色革命对全球气候产生了什么影响。但是，这些影响很难确定，尤其是因为它们可能因地区而异。在本项目中，主要研究人员（PI）研究了农业对极端气候的影响，重点关注三种特定类型：1）热浪和湿热浪，后者由湿球温度测量，2）闪电干旱，意味着突然发生的干旱，（3）长时间的干热和湿冷条件。 PI研究了几个假设，其中一个假设是，农业集约化增加了炎热和干旱生长季节发生突发干旱的风险。 另一个原因是，农业减少了干燥热浪的风险，但增加了潮湿热浪的风险。 这项工作的一个核心考虑是，植被对极端气候的影响可能取决于该地区的干旱程度，因为在较为干旱的地区，蒸发和蒸腾作用受到土壤中水分含量的限制，而在水资源丰富的地区，限制因素是能量含量。（特别是阳光）可以将液态水转化为水蒸气。这项工作涉及分析几个数据集，包括再分析产品和气候降水数据集，社区地球系统模型（CESM 2）的模拟，包括社区土地模型（CLM，第5版）的第5版，以及土地利用模型相互比较项目（LUMIP）的模拟分析。 CLM包括农业土地覆盖和灌溉的表示，可以启用或禁用，以允许比较有和没有农业影响的模拟。 LUMIP档案包括一个历史时期（1850年至今）的模拟，在这个时期，二氧化碳或土地覆盖保持不变，农场和牧场被明确地视为农业用地，或者相反地被视为未管理的草地或灌溉固定在1850年水平的农业用地。考虑到极端气候对人类活动的影响，这项工作具有社会和科学意义。 此外，该项目还为一名研究生和一名本科半工半读学生提供支持和培训。 该奖项是与锡拉丘兹大学本科生研究和创造性参与办公室（SOURCE）和其他致力于提高科学和技术劳动力多样性的组织合作招募的。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

The transformation of global agriculture over the last century is rightly called a revolution, as the use of fertilizers, pesticides, irrigation, and hybridization has boosted crop yields by perhaps a factor of three. The increased yields mean that today's croplands are home to the most intense vegetation growth on earth, accompanied by the greatest plant water use on earth. Most of the water taken up by plants is released to the atmosphere through the leaves, thus higher crop yields should be accompanied by increases in atmospheric humidity. Moreover, since thermal energy is expended in converting liquid water into water vapor, increased humidity is typically accompanied by cooling. In the absence of irrigation, atmospheric moistening comes at the expense of soil moisture, as the water vapor released from the leaves is first taken up from the soil by the plant roots. Given these effects on humidity, temperature, and soil moisture, along with the fact that croplands cover about a third of the world's land surface, it is natural to ask what effects the Green Revolution has had on global climate. But such effects are difficult to identify, particularly as they could differ from one region to another.In this project the Principal Investigators (PIs) examine the effects of agriculture on climate extremes, focusing on three particular types: 1) heat waves and humid heat waves, the latter measured by the wet bulb temperature, 2) flash drought, meaning droughts with sudden onset, and 3) prolonged hot/dry and cool/wet conditions. The PIs examine several hypotheses, one that agricultural intensification increases the risk of flash drought during hot and dry growing seasons. Another is that agriculture reduces the risk of dry heat waves but increases the risk of humid heat waves. A central consideration for the work is that the effects of vegetation on climate extremes are likely to depend on the aridity of the region, as in more arid regions evaporation and transpiration are limited by the amount of moisture in the soil, while in regions of greater water abundance the limiting factor is the amount of energy (sunlight, in particular) available to convert liquid water into water vapor.The work involves analysis of several datasets including reanalysis products and climatic precipitation datasets, and simulations from the Community Earth System Model (CESM2) including version 5 of the Community Land Model (CLM, version 5), and analysis of simulations from the Land Use Model Intercomparison Project (LUMIP). CLM includes a representation of agricultural land cover and irrigation which can be enabled or disabled to allow comparisons of simulations with and without agricultural effects. The LUMIP archive includes simulations for a historical period (1850 to the present) in which either CO2 or land cover is held fixed, and with farm and pastureland treated explicitly as agricultural land or counterfactually as either unmanaged grassland or as agricultural land with irrigation fixed at 1850 levels.The work is of societal as well as scientific interest given the impacts of climate extremes on human activities. In addition the project provide support and training to a graduate student and undergraduate work-study students. Students are recruited in collaboration with the Syracuse Office for Undergraduate Research and Creative Engagement (SOURCE) and other organizations dedicated to enhancing the diversity of the science and technology workforce.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.