Inter-annual variation refers to the natural fluctuations or changes in a particular variable or phenomenon that occur from one year to the next. This can include changes in climate patterns, population sizes, economic indicators, and many other factors that can vary annually. Researchers study inter-annual variation to better understand the underlying processes and dynamics that drive these changes, and to predict future patterns and trends. By analyzing these variations, researchers can also identify potential impacts on ecosystems, economies, and societies, and develop strategies to mitigate any negative effects.