The weather is a coupled, non-linear chaotic system. Chaos theory says very small changes in inputs can result in totally different outcomes. This concept is counterintuitive for most people. We intrinsically think that if you're a little off at the beginning, you should be a little off at the end. Try that on a mountain trail next to a cliff.
More importantly, say you had perfect data and a perfect model. How could we possibly know what impact that will have? Another thought experiment: How much will temperatures vary where you live today? Ten degrees? Twenty degrees? Thirty? How much does it vary in a year? For most of the U.S., that number might be 50 degrees, a 100 degrees. So plants and animals have adapted to 20-degree temperatures changes in a day or 100-degree temperature changes in a year. Somehow a 1- or 2-degree temperature change in a hundred years is going to take them out? That's ridiculous.