confirm that the late 20th century is probably the warmest period in that time (see the preceding section -details on attribution). Solar activity Solar sunspot maximum occurs when the magnetic field of the Sun collapses and reverse as part of its average 11-year solar cycle (22 years for complete North to North restoration). The role of the Sun in recent climate change has been looked at by climate scientists. Since 1978, output from the Sun has been measured by satellites significantly more accurately than was previously possible from the surface. These measurements indicate that the Sun's total solar irradiance has not increased since 1978, so the warming during the past 30 years cannot be directly attributed to an increase in total solar energy reaching the Earth (see graph above, left). In the three decades since 1978, the combination of solar and volcanic activity probably had a slight cooling influence on the climate. Climate models have been used to examine the role of the Sun in recent climate change. Models are unable to reproduce the rapid warming observed in recent decades when they only take into account variations in total solar irradiance and volcanic activity. Models are, however, able to simulate the observed 20th century changes in temperature when they include all of the most important external forcings, including human influences and natural forcings. As has already been stated, Hegerl et al. (2007) concluded that greenhouse gas forcing had "very likely" caused most of the observed global warming since the mid-20th century. In making this conclusion, Hegerl et al. (2007) allowed for the possibility that climate models had been underestimated the effect of solar forcing. The role of solar activity in climate change has also been calculated over longer time periods using "proxy" datasets, such as tree rings. Models indicate that solar and volcanic forcings can explain periods of relative warmth and cold between AD 1000 and 1900, but human-induced forcings are needed to reproduce the late-20th century warming. Another line of evidence against the sun having caused recent climate change comes from looking at how temperatures at different levels in the Earth's atmosphere have changed. Models and observations (see figure above, middle) show that greenhouse gas results in warming of the lower atmosphere at the surface (called the troposphere) but cooling of the upper atmosphere (called the stratosphere). Depletion of the ozone layer by chemical refrigerants has also resulted in