But couldn't the warming still be a natural variation? Scientists have looked for alternative explanations based on what caused climate changes in Earth's past. None of them fit the bill. Computer models help cinch the case for a human cause. The solar and volcanic conditions of the last 50 years would likely have produced cooling, based on our models' best estimates. Only the addition of the extra greenhouse gases and particles to our computer models can reproduce the pattern of warming we've seen over the past century.