One of the biggest uncertainties is clouds: some clouds act to cool the climate, while others have a net warming effect, and we don’t yet know how our present mix of clouds will evolve. It’s important to keep in mind that uncertainties like these aren’t necessarily an escape hatch. They could help counteract climate change, or they could help make it even worse. Another source of uncertainty is the many subtle interactions between plants, soil, sea creatures, and other climate components that we don’t understand and therefore can’t take into account. Another wild card is the contribution of melting and disintegrating ice to climate change, which is currently not well represented in models. However, looking at how the climate actually reacted under similar circumstances in the past gives us confidence that even though we may not understand all the details, we have the general idea of what will happen. Other uncertainties are such things as the rate of future economic development, how long it will take to switch to alternative energy sources, or whether future technology will enable us to remove the CO₂ in the atmosphere.

Yes, science always has to leave room for unknowns. But since the models based on the factors we do understand can replicate the actual outcome of past climates, it seems scientists have a good grasp of the essential elements. Further work on the unknowns will probably help refine the results, not drastically change them.

Scientists did find that the Earth cooled slightly from the end of World War II until the 1970s, and some suggested that a "snow blitz" could accelerate the cooling and bring on the next ice age. However, the majority of the scientific community was not part of the global cooling buzz of the 70s. The peer-reviewed literature had many more articles about the planet’s potential to warm due to the effects of CO₂. The possibility of an ice age was more attractive to the media, so the cooling theory briefly received more press. As it turns out, sooty pollution from post-war factories blocked sunlight like volcanic particles, and along with possible changes in ocean currents, probably caused the short-lived cooling trend.

Other evidence besides that from climate models leads us to believe that warming is likely to continue. The well-established physics and chemistry of carbon dioxide, our measurements of past climate change, and the signs of change in the environment all supplement the evidence from models and give us confidence that we should take action. However, there is a lot of debate about what those actions should be. Those are largely policy questions, not scientific ones.