水汽变化在东亚区域增暖中的气候效应及其归因研究

Water vapor is the most abundant and single most important greenhouse gas (GHGs) in Earth's climate system. It plays a key role in the atmospheric branch of the global hydrologic and energy cycles, thus affecting our weather and climate significantly. Because atmospheric water vapor content increases with air temperature, changes in water vapor provide one of the strongest feedbacks of the climate system, and it greatly amplifies the warming effects due to increases of other greenhouse gases (GHGs) such as atmospheric CO2. The enhanced warming and increased water vapor can lead to more frequent extreme weather and climate events. Therefore, it is essential to study the spatio-temporal variations and long-term changes of atmospheric humidity and water vapor, and to quantify the effect of the water vapor feedback on regional and global climate. This can help us understand the mechanisms of regional climate change under global warming. Our project will firstly construct an integrated long-term time series for atmospheric humidity and water vapor over East Asia based on the homogenized radiosonde humidity, reanalysis and satellite observations, and then focus on the analysis for the spatio-temporal variations and long-term trends of the atmospheric humidity and water vapor over East Asia since the early 1970s based on observations and model simulations. Future changes of the atmospheric humidity and water vapor over East Asia projected by CMIP5 and CMIP6 models will also be analyzed. The relationship between the long-term changes of the atmospheric humidity and water vapor and the regional warming over East Asia will be examined to help quantify the water vapor feedback in both observations and models. In addition, the relationship between the atmospheric water vapor and extreme climate events will be investigated using the observations and model simulations. Furthermore, we will use the CMIP5 as well as the CMIP6 multi-model historical simulations to estimate and characterize the response of atmospheric humidity and water vapor over East Asia to historical anthropogenic and natural external forcings. We will then remove this forced component from the observations and analyze the residual to quantify and characterize the tempo-spatial variations of the leading modes of the unforced internal variability in humidity and water vapor over East Asia since the early 1970s, such as those associated with ENSO, PDO/IPO, and AMO. The separation of the forced changes and the unforced variations will allow us to compare their magnitudes and spatial and temporal patterns, and provide a basis for proper comparison between models and observations. In particular, the unusual behavior in atmospheric humidity (e.g., decreases in relative humidity) during the recent warming hiatus period since about 2000 will be examined together with the causes of the warming hiatus. This project is expected to enhance our understanding of the externally forced changes and internally unforced variations in atmospheric humidity and water vapor over East Asia, and to provide scientific basis for the research of climate change impacts and adaptation over East Asia.

水汽是地球系统中最丰富、也是最重要的温室气体之一，在全球水分和能量循环中扮演着重要角色。水汽的反馈作用能够放大其他温室气体的增暖效应，对天气和气候变化有着非常重要的影响。因此，深入研究水汽变化的规律成因与区域气候效应有助于我们深入认识全球变暖背景下区域气候变化及其响应机理。本项目拟充分利用均一化观测资料、再分析资料以及卫星遥感信息等构建近50 年东亚区域大气水汽和湿度集成观测时间序列，并结合多模式模拟结果，深入分析东亚区域水汽和湿度变化的时空分布规律并预估其未来变化情景，估算水汽和湿度变化在东亚区域增暖中的气候效应及其区域差异，特别关注2000年以来水汽和湿度变化在“变暖停滞”中的作用，揭示水汽变化与干旱、洪涝等极端气候变化之间的联系，检测水汽和湿度变化中人为和自然因素外强迫的影响，区分不同外强迫和气候系统内部变率的相对贡献，预期为东亚区域气候变化的影响评估及其适应研究提供科学依据。

水汽是大气中最丰富、最重要的温室气体之一,在全球能量和水循环过程中扮演了至关重要的角色。水汽变化引起的正反馈作用能够放大其他温室气体的增暖效应，对天气和气候变化有着极其重要的影响。因此，深入研究大气水汽变化的时空分布格局及其成因，评估水汽反馈的区域气候效应，对于我们深入认识和理解全球变暖背景下区域气候变化响应机理具有重要意义。围绕这一关键科学问题和科学目标，本项目基于最新研制的高空湿度均一化观测数据集并结合第五次国际耦合模式比较计划（CMIP5）多模式数值试验结果，深入研究了东亚区域大气水汽长期演变规律的观测事实及其与数值模拟结果之间的差异，预估了未来不同气候变化情景下东亚区域水汽变化响应强度及其不确定性，揭示了大气水汽变化与区域气候变暖之间的定量关系，检测了气候系统内部自然变率与外强迫因素的影响，归因了自然因素和人类活动在中国区域大气水汽长期变化中的相对贡献及其区域差异。研究的主要结论是：近40年东亚区域大气水汽含量增加的变化主要受气候变暖所调控，区域平均气温每升高1℃大气水汽含量约增加6.7%（即6.7%/℃），略低于Clausius-Clapeyron方程（简称C-C方程）的理论结果（约7%/℃）；在未来中高端排放情景下，水汽的响应速率则超过了C-C方程的理论估算结果（7%/℃），特别在中国东部地区具有较强的响应强度；人为排放的温室气体是近40年东亚区域水汽含量增加最主要的外强迫因子（可解释观测趋势的90%以上），而自然因素外强迫的影响则不显著。本项目研究的科学贡献主要体现在两个方面：在基础理论上，取得了气候变暖背景下东亚区域水汽变化机制及其定量归因的新认识，厘定了区域水汽反馈效应的理论结果，为东亚区域气候变化研究及其影响评估提供科学依据；在基础数据方面，发展和构建了东亚区域高空湿度均一化长时间序列数据集，为再分析数据产品的研制与评估、国家气候评估报告的撰写提供了第一手的基础数据，并在相关研究领域以及业务部门得到有效应用。

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