There are a number of technically feasible proposals to deliberately alter the global climate, most at the moment directed toward cooling it if we should find ourselves in a run-away warming situation. The orbital reflectors proposed by Jim Rutt and others - which could be in place an operating within a decade for less we currently spend on major programs such as ABM systems - are one example.
Of course, such efforts might have unpredictable results - as might our current unplanned tinkering with the Earth's atmosphere.
This assumes that man can actually have the ability to "tinker" with the Earth's climate/atmosphere on any significant and permanent basis. It is part of the hubris of Man until some natural catastrophic event occurs to demonstrate how really powerless and insignificant we are. We would have a hard time just equalling the impact of Krakatoa on the global climate and even that was relatively short-lived. The Earth was here long before man came on to the scene and will survive long after man has disappeared.