Song, Ziyao2025-06-262025-06-262025-06-26http://hdl.handle.net/10138/598164Since the 17th century, global wetland area has decreased by approximately 20%, with human activities widely recognized as the primary cause of wetland loss. As the critical role of wetlands within the climate system becomes increasingly evident, the climate factors driving wetland changes have attracted growing research attention. There are pronounced regional differences in how meteorological factors influence wetland dynamics, underscoring the urgent need for systematic investigation. Based on the latest Wetland Area and Dynamics for Methane Modeling version 2 (WAD2Mv2) global observational dataset and historical wetland area reconstructed using a Random Forest (RF) model, this study systematically analyzes the spatial and temporal distribution patterns of global wetlands and their long-term trends. The study further reveals the potential influence of sea surface temperatures (SSTs) in the tropical central-eastern Pacific and the North Atlantic on wetland extent in North America and Europe, and explores the underlying physical mechanisms. The findings provide scientific evidence for understanding wetland changes and their regional disparities under the context of global warming, and offer theoretical support for formulating more precise wetland conservation and climate adaptation policies. The main conclusions of this study are as follows: (1) This study systematically analyzes the trends in wetland area changes during the historical period and the past two decades. It finds that since 1901, wetlands in eastern North America, Europe, and East Asia have significantly declined, mainly due to agricultural expansion and urbanization. After 2000, the most significant wetland changes have occurred in the tropics, characterized by wetland loss near the equator and wetland expansion in regions farther from the equator. Climatological analysis shows that global wetlands are mainly concentrated in equatorial regions and the mid-to-high latitudes between 40ºN and 60ºN. In equatorial regions, wetlands are primarily distributed as inundated areas within the Amazon rainforest in South America and the Congo rainforest in Africa, with wetland extent peaking in spring and summer due to tropical seasonal precipitation. In the mid-to-high latitudes, wetlands mainly appear as peatlands in the Hudson Bay Lowlands of North America and the Western Siberian Lowlands of northern Asia, with maximum extent in winter and spring as a result of reduced evapotranspiration under low-temperature conditions. Since 1901, significant wetland loss has occurred in eastern North America, Europe, and East Asia, where wetlands have been largely converted to urban areas, forestry, and rice paddies, respectively. During winter and spring, wetland extent has significantly increased in northern North America and northern Asia, mainly due to the impact of global warming on the freeze-thaw cycle. After 2000, wetland changes have been primarily characterized by a significant reduction in tropical wetlands. In the Amazon Basin, this decline is associated with decreasing precipitation and increasing evaporation. In the Congo rainforest region, wetland loss is likely influenced by human activities. On the Indian Peninsula, under the influence of tropical monsoon rainfall, the declining trend is most prominent during summer. In subtropical regions, wetlands show a significant increasing trend, with expansion most evident during winter and spring in the central-western North America and a gradual increase occurring during summer in northern Eurasia. (2) The study systematically analyzes the spatiotemporal evolution of wetland area in North America on the interannual scale and reveals its connection with tropical SSTs and atmospheric circulation. The results indicate that the tropical La Niña SST pattern in autumn and the North Atlantic tripolar pattern in winter are associated with significant increases in wetland area over the North American mid-to-high latitudes. North American wetlands are primarily concentrated in the Hudson Bay Lowlands around 55ºN, the Prairie Pothole Region (PPR) near 45ºN, and the Rio Grande Basin around 30ºN, with wetlands existing as peatlands, seasonal freshwater marshes, and floodplains, respectively. The general trend of wetland change is characterized by increases in the north and decreases in the south. Northern expansion is mainly contributed by the PPR during summer and autumn, driven by increased precipitation in northern regions, while the decline in the south is centered in the Rio Grande Basin, primarily due to enhanced evaporation. Based on the first leading mode of Empirical Orthogonal Function (EOF) analysis, two primary seasonal patterns of wetland variability are identified in North America, both mainly driven by precipitation. In winter and spring, the dominant signal appears in the PPR, explaining 24.5% and 15.6% of the total variance, respectively. Taking winter as an example, positive anomalies in wetland coverage in the PPR are jointly influenced by enhanced precipitation, resulting from moisture transported from the warm North Atlantic, and reduced evaporation caused by low temperatures under the influence of cyclonic circulation. The North Atlantic exhibits a positive-negative-positive tripolar pattern of sea surface temperatures (SSTs), while a La Niña signal appears in the southeastern South Pacific. In summer and autumn, the wetland signal displays a north-south dipole pattern, with key anomalies located in the Hudson Bay Lowlands and the Rio Grande Basin, accounting for 23.0% and 20.4% of the total variance, respectively. In autumn, for example, positive anomalies in northern wetlands are caused by a La Niña signal in the central Pacific, which leads to increased precipitation through warm and moist air transport under the influence of a negative-phase Pacific-North American (PNA)-like teleconnection pattern. Correlation analysis between key climate variables and the Niño3.4 winter index shows that El Niño events produce a positive-phase PNA pattern. This pattern directs warm and moist air toward the southern part of North America through cyclonic circulation, resulting in warmer and drier conditions and reduced wetland extent in the north, while cooler and wetter conditions in the south promote wetland expansion. (3) The study finds that European wetlands are primarily distributed across the Eastern European Plain, with the wetland area peaking during winter and spring. During the negative (positive) phase of the North Atlantic Oscillation (NAO) in these seasons, temperatures over the Eastern European Plain are significantly lower (higher), evaporation is weaker (stronger), and wetland extent is larger (smaller). European wetlands are mainly distributed across the Eastern European Plain in the form of floodplains, with their extent peaking in winter and spring due to snowmelt and snow retention as the primary hydrological sources. The reduction in European wetlands primarily occurs in winter, especially in central Europe, as rising temperatures enhance evapotranspiration and reduce snow retention from the previous season. Influenced by increased precipitation, wetland expansion in the Eastern European Plain occurs in winter and extends further in spring. Based on the first leading mode of Empirical Orthogonal Function (EOF) analysis, seasonal wetland variability in Europe shows two dominant patterns, with evaporation and precipitation serving as the primary drivers for each. During winter and spring, the dominant signal appears in the Eastern European Plain, explaining 31.9% and 24.2% of the total variance, respectively. Taking spring as an example, positive anomalies in wetland coverage in the Eastern European Plain are primarily driven by reduced evaporation due to low temperatures. Upstream conditions in Europe are influenced by the negative phase of the North Atlantic Oscillation (NAO), accompanied by a positive-negative-positive tripolar pattern of SSTs in North Atlantic. In autumn, the dominant signal is located around 50ºN, explaining 21.0% of the total variance. Positive anomalies in this region are mainly driven by significantly increased precipitation under cyclonic low-pressure conditions. Correlation analysis between key climate variables and the winter NAO index indicates that, during the positive phase of the NAO, SSTs over the North Atlantic exhibit a negative-positive-negative tripolar structure. Most parts of Europe are influenced by the eastern flank of a large anticyclonic system. Moisture is transported from the ocean along this circulation toward northern Europe, resulting in warmer and wetter conditions in the north and colder and drier conditions in the south, leading to a significant reduction in overall wetland extent across the region.engIn Copyright 1.0Wetland changesClimate driversEl Niño-Southern Oscillation (ENSO)North Atlantic Oscillation (NAO)URN:NBN:fi:hulib-202506263030pro gradu -tutkielmaMaster’s Programme in Atmospheric SciencesMaster’s Programme in Atmospheric SciencesMaster’s Programme in Atmospheric Sciences