Earth's climate may not rise as quickly as scientists currently think, according to three new studies of how clouds are formed.reports that the studies show that aerosols produced by humans may not cause the skies to get that much cloudier than they were before the Industrial Revolution.Clouds are formed by condensing around aerosols, and the thinking has been that sulfuric acid produced by humans has caused more aerosols in the atmosphere, thus creating more cloud cover. But the new studies show that trees can also produce the aerosols needed for cloud formation."It's been long thought that sulfuric acid is really the key player," atmospheric chemist Chris Cappa of the University of California, Davis, told Science. Cappa was not part of the studies, but he said they "show pretty convincingly that we don't need sulfuric acid around to allow new particles to grow."Though scientists largely agree that man-made gases are warming the climate, they do not agree on how quickly that is happening.One thing they disagree on is sulfur dioxide, which has increased dramatically in recent years. When sulfur dioxide reacts with oxygen and water, it forms sulfuric acid, which helps clouds form. But extra cloud cover could actually help cool the earth since clouds reflect sunlight away from the planet, possibly offsetting the effects of warming.Though researchers agree that sulfuric acid contributes to cloud formation in a major way today, there is no way of knowing that skies were less cloudy in the past since plants may have played a larger role.In fact, a study conducted at the Jungfraujoch high altitude research station high in the Swiss Alps found that aerosols do form more with organic materials when that is what is more available.If pre-industrial cloud cover was closer to today's than has been thought, then the warming produced by human activity also is smaller, Science notes, and that would mean that man-made greenhouse gases are not the only cause of global warming.That means that emissions in the future may not have as big an effect as has been estimated, Urs Baltensperger of the Paul Scherrer Institute told Science.Baltensperger was an author on all three studies, and says that while current best estimates of future rises in temperature are still within reason, "the highest values become improbable."