hence lower average temperatures and not as an interval of unremitting cold. Possible causes Scientists have tentatively identified seven possible causes of the Little Ice Age: orbital cycles, decreased solar activity, increased volcanic activity, altered ocean current flows, fluctuations in the human population in different parts of the world causing reforestation or deforestation, and the inherent variability of global climate. Orbital cycles Orbital forcing from cycles in the earth's orbit around the sun has for the past 2,000 years caused a long-term northern hemisphere cooling trend, which continued through the Middle Ages and the Little Ice Age. The rate of Arctic cooling is roughly 0.02 °C per century. That trend could be extrapolated to continue into the future and possibly lead to a full ice age, but the 20th-century instrumental temperature record shows a sudden reversal of that trend, with a rise in global temperatures attributed to greenhouse gas emissions. Solar activity Solar activity includes any disturbances on the Sun such as sunspots and solar flares associated with the variable magnetic field of the solar surface and solar atmosphere (corona). Because Alfvén's theorem applies, the coronal magnetic field is dragged out into the heliosphere by the solar wind. Irregularities in this heliospheric magnetic field shield Earth from galactic cosmic rays by scattering them, which allows scientists to track solar activity in the past by analyzing both the carbon-14 or beryllium-10 isotopes generated by cosmic rays hitting the atmosphere and which are deposited in terrestrial reservoirs such as tree rings and ice sheets. In the intervals 1400–1550 (the Spörer Minimum) and 1645–1715 (the Maunder Minimum) there were very low recorded levels of solar activity and they are both within, or at least overlapped with, the LIA for most definitions. However, solar activity deduced from cosmogenic isotopes was as high between the Spörer Minimum and the Maunder Minimum as it was in about 1940, yet this interval is also within the LIA. Hence any relationship between solar activity and the LIA is far from a simple one. A study by Dmitri Mauquoy et al. confirmed that at the beginning of the Spörer Minimum, the carbon-14 production rate rose rapidly. These authors argued this rise coincided with a sharp drop in temperatures deduced from European peat bogs. This temperature drop is also seen in mean northern hemisphere temperatures deduced from a wide variety of paleoclimate indicators but the timing of the onset of