Growing Season Changes over the Past Millennium in Northern High Latitudes

Abstract

Chapter 1 14 General BackgroundThe climate of planet Earth has varied throughout its history and will continue to change in the future. Earth's climate has just recently, 11,700 years ago, shifted from the last Ice Age epoch of the Pleistocene to the warmer period of the Holocene. The transition towards the Holocene marked the beginning of the modern climate era and the onset of human civilizations. Naturally occurring climatic shifts like this are attributed to changes in the global distribution of solar energy that is received by the planet, where energy differences are caused by variations in the Earth's orbit around the Sun, axis of rotation and tilt. These variations, called Milankovitch cycles, govern climatic shifts over large time scales and the varying position of the Earth relative to the Sun within one orbital revolution is also the cause of yearly seasonal changes (Mayewski et al. 2004; Wu et al. 2018). On Earth, the atmosphere, biosphere, cryosphere, oceans, and landmasses constantly interact with each other through positive and negative feedback mechanisms by distributing the energy received from the Sun. The interplay between these facets of the planet, govern our climate which is the long-term expression of the daily weather. The Earth System formed by complex atmospheric and oceanic interactions between energy, humidity, and temperature lead to vast variations in daily weather, and indeed a varying average of weather over time: the climate (Steffen et al. 2020). Climate variations are moreover on short term affected by changes in sunspot activity or incidental volcanic eruptions (Salzer and Hughes 2007;Ahmed et al. 2013). The interannual variability of climate is largely governed by circulation patterns in the atmospheric and oceanic systems, transporting heat and humidity over the planet over large distances, via so called teleconnections (Franzke and Feldstein 2005).An important example of a teleconnection system is the North Atlantic Oscillation (NAO), see figure 4.1. The NAO affects System Earth by producing important climatic effects throughout the