What forces or phenomena generate global changes in weather?
In any one location, weather can change very rapidly from day to day and from year to year, even within an unchanging climate. These changes involve shifts in, for example, temperatures, precipitation, winds, and clouds. Fundamentally, climate is controlled by the long-term balance of energy of the Earth and its atmosphere. Incoming radiation from the sun, mainly in the form of visible light, is absorbed at the Earth’s surface and in the atmosphere above. Human activities can also change the climate. The atmospheric amounts of many greenhouse gases are increasing, especially that of carbon dioxide, which has increased by 30% over the last 200 years, primarily as a result of changes in land use (e.g., deforestation) and of burning coal, oil, and natural gas (e.g., in automobiles, industry, and electricity generation).
“Weather” is defined as short-term and local, thus the weather in one place can be hotter than normal (30-year decadal average = climate) while the weather somewhere else is colder than normal. There are few things that can change the weather globally. One of those is volcanic eruptions. The eruption of Tambora (Indonesia) in 1815 resulted in “the year without a summer”, with colder weather than expected by about 2 degrees F for two years throughout much of the world. Even larger eruptions (Toba, Yellowstone) would have an even larger and longer effect because of the aerosols (dust, sulphuric acid droplets) injected into the stratosphere. And, of course, there is always the possibility of a meteorite impact!
Global changes in weather are also caused by plate tectonics. For example the Indian sub continent crashing into the Asian continent caused the Himalayas to be formed. This disrupted the continental wind patterns that at the time moved wet air over northern Africa and dry air over parts of China and Eastern Asia. In Africa this change in weather has lead to the formation of the Sahara desert where once there were tropical rain forests. This change has also been attributed to the evolution of humans. As it is thought we moved out of the trees as a result of the changing climate and in a harsher environment we developed larger brains to cope with the challenges.
