There is evidence that the activity of the sun produces them. Assume for a moment that is is desirable to reduce carbon dioxide concentration in the atmosphere. Reducing fossil fuel usage would be one of the more expensive ways to do so. For about 2 percent of the cost of restricting fossil fuel use, ocean fertilization with minute quantities of iron would increase dramatically the uptake of carbon dioxide by photo plankton. Plants love it.
More carbon dioxide is good.
But if warming should be curtailed, there is a much cheaper way than reducing use of fossil fuels. Edward Teller described it in a letter to the Wall Street Journal published on Oct. 17, 1997. It involves injecting light-scattering particles into the stratosphere. The interesting thing about this is it could work both ways. Between the Earth and sun, we'd get cooling.