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Vegetation optical depth (VOD) retrievals from three satellite‐based passive microwave instruments were merged to produce the first long‐term global microwave‐based vegetation product. The resulting VOD product spans more than two decades and shows seasonal cycles and inter‐annual variations that generally correspond with those observed in the Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI). Some notable differences exist in the long‐term trends: the NDVI, operating in the optical regime, is sensitive to chlorophyll abundance and photosynthetically active biomass of the leaves, whereas the microwave‐based VOD is an indicator of the vegetation water content in total above‐ground biomass, i.e., including wood and leaf components. Preliminary analyses indicate that the fluctuations in VOD typically correlated to precipitation variations, and that the mutually independent VOD and NDVI do not necessarily respond in identical manners. Considering both products together provides a more robust structural characterization and assessment of long‐term vegetation dynamics at the global scale. Key Points First long‐term global microwave‐based vegetation product (VOD) was developed Notable similarities and differences exist between VOD and NDVI product Mutually independent products will provide a more comprehensive analysis

Given the critical role of precipitation on hydroclimate, we quantified the contributions of moisture source regions to precipitation in the Great Lakes Region (GLR) using multiple reanalysis data sets. Results show that the Great Plains (GPs) and the GLR itself are the primary sources of moisture. The moisture sources for the double peaks in the GLR precipitation that occur in June and September are identified, which is caused by a shift in the peak timing of moisture contribution from the GLR and GPs. In particular, moisture from the GPs contributes more to the heavy precipitation, while moisture from the GLR contributes more to the light precipitation. We also found a statistically significant (p < 0.05) increasing trend in the moisture contribution from the mid‐Pacific, caused by an intensified zonal moisture transport from the mid‐Pacific through changes in atmospheric circulation. Plain Language Summary Rain and snow are critical in determining how much water is in the Great Lakes—which in turn influences socioeconomic activity in the Great Lakes Region (GLR). Therefore, we need to understand where the moisture for rain and snow comes from, especially in a changing climate. We found that rain and snow over the GLR mostly originate from water vapor that comes from the Great Plains (GPs) and the GLR. The GPs contribute more water vapor in spring and summer, and the GLR contributes more in autumn and winter. We also found that when rain or snow is strong, much of the water comes from the GPs. On the other hand, when precipitation is weak, a large fraction of the water comes from the GLR itself. We did not find that significant increasing trend in the amount of rain or snow over the GLR over the past 40 years; however, strong winds from the west bring more and more moisture from the mid‐Pacific region each spring. Key Points The Great Plains (GPs) and the Great Lakes Region (GLR) are the major sources of moisture for precipitation in the GLR Moisture evapotranspired from the GPs contributes to more than 40% of strong precipitation in the GLR Moisture contribution from the Mid‐Pacific to the Great Lakes precipitation has significantly increased in spring

We examine the relationships between the observed long‐term trends of the zonal wind in the mesopause regions at King Sejong Station (KSS), Antarctica, and wind trends in the Southern Hemisphere (SH) middle atmosphere using the 15‐year data set from KSS meteor radar, Aura MLS and MERRA‐2. During July, significant positive trends of zonal winds appear above z = 90 km and near the stratopause over the KSS, while negative trends exist between the two layers. In the SH winter, the observed mesopause winds correlate positively (negatively) with stratospheric (mesospheric) winds in the polar region, while they exhibit opposite correlations with the low‐latitude winds. The positive mesopause trends of zonal winds near KSS are connected, through the thermal wind relationship, to cooling (warming) trends induced by the upward (downward) trends of residual circulation over the high‐latitude mesosphere and low‐latitude stratosphere (high‐latitude stratosphere), which shows vertical coupling throughout the SH winter middle atmosphere. Plain Language Summary Based on 15‐year (2007–2021) observations of horizontal winds obtained at mesopause altitudes of 80–100 km by a meteor radar at King Sejong Station (KSS; 62.22°S, 58.78°W), Antarctica, we find long‐term trends of eastward winds above 90 km in July. To investigate the relationship between the observed long‐term trends of the mesopause wind over the Southern Hemisphere (SH) polar region and winds in the SH middle atmosphere, we analyze Aura MLS satellite data and MERRA‐2 reanalysis data. In the SH winter, the mesopause zonal winds over polar regions are positively (negatively) correlated with zonal winds in the polar stratosphere (polar mesosphere), while the opposite correlations are found between the polar mesopause and low latitudes. We demonstrate that the trends of temperature induced by adiabatic warming or cooling associated with meridional circulation in the stratosphere and mesosphere are connected to the observed mesopause wind trends in the polar region (KSS), which accounts for the vertical coupling between the mesopause region and stratosphere across the mesosphere. Key Points Trends of zonal winds in the Southern Hemisphere mesopause and stratosphere are investigated using meteor radar and satellite observations Significant correlations are found between the mesopause wind at high latitudes and the winds in the middle atmosphere The observed wind trends in austral winter are coupled vertically between the mesopause region and stratosphere across the mesosphere

In this paper, we first elucidate the significant seasonality in long‐term trends in the Asian monsoon on a monthly mean basis. Advanced monsoon onsets over the Bay of Bengal and the western Pacific were evident in recent decades. Increasing rainfall in May along 10°N reflected the advanced monsoon onset. Decreasing rainfall trends in June along 10°N were also detected. Because the rainfall trends in July and August showed less significance, the monsoon transition phase should be discussed in the context of climate change rather than boreal summer mean field. The advanced monsoon onset and weakening of the monsoon during early summer are most likely to be attributed to the heat contrast between the Asian landmass and the tropical Indian Ocean. The heating trend over the Asian landmass primarily contributes to the heat contrast variability with the persistent SST increase in the Indian Ocean throughout the season. Key Points Seasonality in the long‐term monsoon trend Advanced monsoon onset New sight on the interpretation of the long‐term change in seasonal mean

Tropical cyclone (TC) stalling has been widely perceived to yield a greater threat of flooding. Understanding the effect of stalling and its long‐term trends will enhance adaptation strategies to cyclone‐associated disasters. We show that stalling prolongs western North Pacific TCs to live 24 hr longer and produces 23% greater 24‐hr rainfall accumulations in a more concentrated area, which is more prominent over a 72‐hr rolling period. More importantly, we discover a northward migration of TC stalling (∼0.7°N decade−1) over 1979–2020, bringing increasingly higher flood risks to the highly‐populated East Asian coast. Further diagnoses suggest the role of binary cyclone interactions in TC stalling, whereby the second larger TC slows down the smaller one by weakening the northwestward steering flows. The northward shift of TC stalling can be explained by a similar trend in binary TC cases and environmental fields. Our findings are robust across various best track and precipitation products. Plain Language Summary Tropical cyclone (TC) stalling occurs when the cyclone resides in a small area for long. It is widely perceived to produce increased accumulated rainfall and flood risks in coastal areas nearby. However, little effort has been made to comprehensively quantify the difference in the general properties and hydrological impacts between stalled and non‐stalled TCs, as well as the spatial pattern and the possible causes of TC stalling. This study addresses the above research gaps by focusing on TC stalling over the western North Pacific (WNP) since the satellite era (1979–2020). We discover that stalling enables TCs to live significantly longer and produce greater rainfall accumulations in a more concentrated area, which is more prominent when measuring over a long rolling period (e.g., 72 hr). We highlight a northward migration of TC stalling phenomenon over the past decades, bringing increasingly greater flood risks to the highly‐populated East Asian coast, especially the Pearl River Delta. The plausible physical causes of stalling are discussed and our conclusions are validated across various sources of data. Findings here improve the understanding of TC stalling and stress the need for future adaptations against more frequent stalling events along the East Asian coasts. Key Points Stalling prolongs tropical cyclones (TCs) by 24 hr more and produces 23% greater 24‐hr rolling downpours over a more concentrated area The hydrological impacts of TC stalling are more prominent over a long rolling period (e.g., 72‐hr) A northward shift of TC stalling (∼0.7°N decade−1) brings increasingly greater threats to coastal areas in the western North Pacific

Simulations based on physical models of the thermosphere‐ionosphere system suggest that the ionosphere will sink as the thermosphere cools and contracts in response to increasing greenhouse gas concentrations. As a consequence, long‐term trends can be expected in ionospheric parameters such as: total electron content (TEC), the critical frequency of the F2 layer, foF2, and its peak height, hmF2. Since early 1990s, foF2 and hmF2, though to a lesser extent, have been widely analyzed to find these trends. This study shows long‐term TEC trends for the period 1999–2023 from available global International GNSS service TEC maps. Using F30, F10.7 or MgII as proxies to filter out the effect of solar EUV, the trends are negative, not only for the mean global value but also for most regions with very few exceptions. This would align with the greenhouse effect hypothesis, even though our results show higher negative trend values than expected theoretically. Plain Language Summary The Earth's ionosphere presents long‐term trends besides regular changes such as daily and seasonal, and irregular variations of transient character. Many studies suggest that the long‐term increase in greenhouse gas concentrations will produce a global cooling in the upper atmosphere together with the global warming in the troposphere. Therefore, long‐term trends can be expected in the ionosphere total electron content (TEC), the critical frequency of the F2 layer, foF2, and its peak height, hmF2, which are the three most important ionospheric parameters used in several applications. TEC measurements have the advantage over other parameters that characterize the upper atmosphere having 24 × 365 worldwide coverage thanks to the continuous International GNSS service (IGS). Trends in the ionosphere are much weaker than those associated with the solar cycle, thus its effects were removed using different solar EUV radiation proxies. The trends are negative, as expected, not only for the mean global value case but also for most of the regional values with very few exceptions. Key Points Trend depends on the solar EUV proxy used for filtering being most negative with MgII and non‐significant with SN Based on recommended solar proxies, noon total electron content (TEC) global average reveal a negative significant trend along 1999–2023 The negative trend of the global TEC at noon is much more pronounced than the theoretical prediction

Using model projections to study the emergence of observable climate signals presumes omniscient knowledge about the climate system. In reality, observational knowledge suffers from data quality and availability issues, for instance data gaps, changes in instrumentation, issues due to gridding and retrieval algorithms. Overlooking such deficiencies leads to misrepresentations of the time of emergence (ToE). We introduce a new definition of ToE that accounts for observational limitations, and show that significant corrections to the ToE may be necessary to achieve the same statistical confidence as would be afforded by omniscient knowledge. We also show how our method can inform future observational needs and observing systems design. Plain Language Summary Long‐term planning for climate change adaptation requires accurate forecasts of climate impacts. Such forecasts are produced using computer models, which provide omniscient knowledge of the climate states they simulate. However, real‐world knowledge is based on incomplete and sometimes flawed observational data. Ignoring these flaws yields a distorted view of the timing of observable climate impacts. We propose a method to address this issue by accounting for observational limitations such as data gaps, changes in measuring equipment, data post‐processing, etc. We also show how to use the method to plan future data collection. Key Points The degree of confidence placed in observed climate trends is misrepresented when overlooking observational limitations We provide a nonparametric method to account for such limitations The method can also inform the design of future observing platforms

Studies reporting the historical trends of SO2, NOx, CO, and nonmethane volatile organic compounds emissions in China using unified approaches and sources are limited. Here we established 66‐year emission trends of these four species in China. Six primary anthropogenic sources were included, and we made a series of improvements to the few existing inventories based on detailed statistical data, recently published emission factors, and technology renewal to reduce the uncertainties. National SO2, NOx, CO, and nonmethane volatile organic compounds emissions in 2015 were 27.1, 20.6, 188, and 28.4 Mt, with annual growth rates of 5.8%, 5.9%, 3.8%, and 4.6% since 1949, respectively. In recent years, fossil fuel combustion was the major contributor to SO2, NOx, and CO emissions, whereas industrial process contributed most to VOCs emissions. Our results revealed a 10% decrease in the SO2 emissions from 2005 to 2010 as a result of improvements in the flue gas desulfurization installation rate. NOx emissions stopped rising and started falling in 2011, with technology renewal and the penetration of end‐of‐pipe control measures in industrial boilers and cement production. Furthermore, we calculated future speciated VOCs emissions and ozone formation potential under alternative policy scenarios and projected to reduce emissions by 10% (compared with 2013 levels) with stringent control measures in 2020. A reactivity‐based control strategy was proposed to achieve greater ozone formation potential reductions while requiring less VOCs emissions reduction. Plain Language Summary Over past half century, the emissions of air pollutants generated through human activities in China have increased rapidly. The study of long‐term emissions trends will improve our understanding of human influence on climate change and air quality. Here we established 66‐year emission trends of four major species for the first time and evaluated the impact of government policy on emission. The central government initialed the most stringent ever action plan in China to cut emissions, for example, from 1 January 2018, China began to levy environmental protection tax, aimed at SO2, NOx, CO, VOCs, and other pollutants. Our result reveals a decrease in the SO2 and NOx emissions, consistent with satellite observation, while VOCs emissions continued to rise. The emission‐based reduction plan for VOCs have been implemented nationwide. In this study, we further proposed a reactivity‐based control strategy to improve the air quality more effectively. Key Points Historical emission trends were evaluated for SO2, NOx, CO, and volatile organic compounds (VOCs) in China since 1949 National SO2 and NOx emissions started to decline in 2007 and 2012, respectively, while VOCs emissions kept on rising A reactivity‐based control strategy was proposed to achieve greater OFP reduction with less VOCs emissions reduction

Ionospheric ion temperature is an excellent approximation to neutral temperature in the upper atmosphere, especially, for altitudes below 300 km. This analysis of long‐term ionospheric ion temperature changes between 100 and 550 km at noon is based on a database of incoherent scatter radar observations spanning more than three solar cycles during 1968–2006 at Millstone Hill and provides direct evidence of long‐term changes and their height dependency in the upper atmospheric temperature. A cooling trend at altitudes above 200 km and an apparent warming trend below 200 km are found. The cooling increases with height and shows variability with solar activity. The apparent warming varies with season and solar activity. It may result from the thermal subsidence caused by atmospheric contraction and pressure level change and from the ion temperature overestimation in the F1 region due to ion composition long‐term changes. These long‐term changes in ion temperature are accompanied by changes in electron density, being lower above the F2 peak and higher below the F2 peak. Electron temperature is accordingly enhanced. All these changes appear to be suggestive of a long‐term greenhouse gas effect. Key Points Upper atmospheric long‐term trend based on incoherent scatter radar data Direct evidence of the upper atmospheric temperature trend Height dependence of the trends

While invasive plant distributions are relatively well known in the eastern United States, temporal changes in species distributions and interactions among species have received little attention. Managers are therefore left to make management decisions without knowing which species pose the greatest threats based on their ability to spread, persist and outcompete other invasive species. To fill this gap, we used the U.S. National Park Service’s Inventory and Monitoring Program data collected from over 1,400 permanent forest plots spanning 12 yr and covering 39 eastern national parks to analyze invasive plant trends. We analyzed trends in abundance at multiple scales, including plot frequency, quadrat frequency, and average quadrat cover. We examined trends overall, by functional group, and by species. We detected considerably more increasing than decreasing trends in invasive plant abundance. In fact, 80% of the parks in our study had at least one significant increasing trend in invasive abundance over time. Where detected, significant negative trends tended to be herbaceous or graminoid species. However, these declines were often countered by roughly equivalent increases in invasive shrubs over the same time period, and we only detected overall declines in invasive abundance in two parks in our study. Present in over 30% of plots and responsible for the steepest and greatest number of significant increases, Japanese stiltgrass (Microstegium vimineum) was the most aggressive invader in our study and is a high management priority. Invasive shrubs, especially Japanese barberry (Berberis thunbergii), Japanese honeysuckle (Lonicera japonica), multiflora rose (Rosa multiflora), and wineberry (Rubus phoenicolasius), also increased across multiple parks, and sometimes at the expense of Japanese stiltgrass. Given the added risks to human health from tick-borne diseases, invasive shrubs are a high management priority. While these findings provide critical information to managers for species prioritization, they also demonstrate the incredible management challenge that invasive plants pose in protected areas, particularly since we documented few overall declines in invasive abundance. As parks work to overcome deferred maintenance of infrastructure, our findings suggest that deferred management of natural resources, particularly invasive species, requires similar attention and long-term commitment to reverse these widespread increasing invasive trends.