# Long-Term Analysis of Wave Climate and Shoreline Change along the Gulf of California
## 1 Introduction  
Climate Change (CC) is the greatest environmental threat faced by humanity. The effects of CC will be particularly noticeable on the coast, where humans and oceans meet [1]. In the coastal environment, the effects of CC lead to variations in river flows and ocean/atmosphere interactions, leading to some sea level rises, increased wind-generated waves and storm surges [2], which can cause extreme coastal erosion and flooding [3]. Currently, the global mean sea level and wave energy are increasing at rates of 3.26 mm [4] and 0.4% [1] per year, respectively. However, a precise quantification of regional rates is much harder and depends on factors such as the local wind energy, the sea water temperature and tectonic uplift/subsidence motions, among others [1, 4]. In addition, it is estimated that 24 to 70% of the sandy beaches around the world are being eroded [5; 6; 7], and considering the projected variations in sea levels and wave energy, the erosion rates and the percentage of threatened sandy beaches are likely to increase. This highlights the relevance of integrating the effects of CC into coastal impact assessments, in order to generate more accurate adaptation plans. According to [8], the recent sea level rise (1973-2015) in the Gulf of California (GC), Mexico, has a geographical gradient, i.e., the upper Gulf presents a higher mean trend, while it is lower in the middle part of the Gulf. In the lower Gulf, the mean trend shows a transition from the entrance of the Gulf to the middle Gulf (see Figure 1). Quantitatively, the regional mean sea level trend, estimated on one side with satellite altimetry data and on the other side with measurements from tide gauges located inside the Gulf, is \(0.8\pm 0.8\) mm and \(2.5\pm 1.1\) mm per year, respectively. Nevertheless, the possible inaccuracy of these sea level trend values, obtained from both methods, and the observed discrepancies highlight the need for further efforts to corroborate the results and better understand the non-uniform sea level change across the GC [9]. In general, wave climate has received less attention in the GC, particularly in the context of climate change. Previous studies (e.g., [10; 11; 12]), using wave data via the reanalysis of different databases, have focused on analyzing the seasonal to long-term variability of the directional wave power at a regional scale, while defining the mean and extreme conditions of wave climate at a local scale. However, none of these studies have covered the long-term trend of wave climate at a regional scale, despite its significance for the development of efficient plans for adaptation to CC [12]. Moreover, global-scale studies (e.g., [1; 13]) on the long-term trend in wave energy and wave height do not cover the GC, or they do it only partially. This knowledge gap makes difficult the integration of the effects of CC into regional- and local-scale coastal impact assessments, required for adaptation plans. To address this, the present work investigated whether or not there was a trend in the wave period, wave height and wave power, using the ERA5 dataset [14], which provides a 40-year (1980-2019) time series of wave data. Furthermore, given the importance of knowing the shoreline evolution in the design of those CC adaptation plans and priorities [15; 16; 17; 18], a long-term analysis of the shoreline change all along the GC coast was performed using the Digital Shoreline Analysis System (DSAS) (version 5.0) [19]. Figure 1: Study area: (**a**) macro-location; (**b**) micro-location, indicating the high population density, as well as the main economic activities including tourism, agriculture, aquaculture and all sorts of fisheries (yellow shading), the river network (undammed river in blue and dammed river in red), and the location of the three wave data sources (C1, C2 and C3). The paper is divided into six sections. Following the introduction in Section 1, Section 2 presents the study area, and Section 3, the materials and methods.