How long are the fluctuations in sunspot numbers ?
In 1989 , Landscheidt forecast a period of sunspot minima after 1990 , accompanied by increased cold , with a stronger minimum and more intense cold which should peak in 2030 His work on solar cycles has been cited by global warming skeptics to argue that observed warming is not anthropogenic and will soon be reversed , based on an assumption that fluctuations in climate are controlled by solar activity .