Once we start deliberately messing with the climate systems, we could inadvertently shift rainfall patterns (climate models have shown that rainfall in the Amazon might be particularly vulnerable), causing collapse of ecosystems, drought, famine, and more.
By burning fossil fuels, we are already dumping 30 billion tons of CO2 into the atmosphere every year, which has a profound effect on the climate. So, like it or not, we're already messing with a system we don't understand.