Yogi Berra is credited with having said that “predictions are hard to make, especially about the future.” It’s true, but climate scientists do their best anyway. Computer models can do a pretty good job of simulating real-world events, but they can’t ever replicate them exactly (you’ve probably noticed this firsthand if you’ve ever seen a computer-animated human in motion). Even well-understood phenomena, like the physics of flight, can’t be simulated perfectly, but aeronautical engineers know they can rely on flight simulations because they know how closely their models represent the real world.
Similarly, models of the planet’s climate can’t simulate today’s climate precisely, but they do a good job of approximating it (and they’re getting better, as the models are continually improved and tested). The places where models and reality disagree help point out the areas of uncertainty.
These uncertainties come from the fact that the local climate is influenced by many different things, including ocean currents, ice cover, vegetation, and cloud cover, to name just a few. As temperatures rise, each of these can change, leading the other parts of the system to react, leading to still more changes. Right now, scientists can’t reproduce all of these changes in their models, so they have to make approximations. For instance, cloud cover is likely to change as the temperature goes up. The changes could lead to extra warming, or to some cooling, or some of both. Scientists aren’t certain at this point about what the total effect will be, because cloud processes take place on too local a scale for models to handle. The balance of observational evidence so far suggests that clouds are likely to cause some extra warming. But it’s unclear how large this effect will be.
Since the models can’t yet give a dead-on representation, scientists know they can’t represent the future climate flawlessly either. That’s why projections for warming and sea-level rise and other changes are given as ranges, not specific numbers. But even if the models were perfect, they still wouldn’t be able to tell us how much the climate will change over the next century to the number. That’s because the amount of climate change will depend on human actions. For the last several decades, human greenhouse-gas emissions have been the biggest human-caused force changing global climate.
The amount of climate change we can expect, therefore, depends on just how much CO2 (and other greenhouse gases) humans will emit in the future. Since no one can predict the choices individuals, businesses, and governments will make, climate models cover their bases by running several different simulations to project rates of warming over the next century. Each simulation is based on a different scenario involving the world’s population, economy, technology, and energy use. Those possible futures all translate into different amounts of emissions, so each scenario results in a different amount of climate change later in this century. Until about 2050, it doesn’t matter much what we assume about future CO2 emissions, because much of the warming until then will be a delayed result of past emissions and because the different emissions scenarios are pretty similar early in the twenty-first century.
The results of these different scenarios probably won’t come as a huge surprise: the models show that if we end up cutting emissions by a lot, our climate will change significantly less than if we stick to business as usual. As shown in the latest IPCC report, from 2007, the scenario based on the lowest emissions estimate led to about 3.2°F of overall warming from 2000 to 2100. The scenario based on the highest estimate translated to 7.2°F of warming.
Note, however, that different climate models come up with different numbers, even with the same emissions scenario. So the 3.2°F and the 7.2°F represent an average of what the models say. In fact, some of the models say it will be less than 3.2°F on the low end and more than 7.2°F on the high end.
It’s important to understand that climate models are constantly being improved. That’s partly because computers keep getting more powerful and software keeps getting more sophisticated. It’s also partly because our observations of the actual climate are improving, so modelers can test their models better. The 2007 IPCC report was based on the best available models and information at that time, but there have been plenty of improvements since then.
The point is that projections are always a snapshot of a science that keeps evolving. Today’s projections are more reliable than earlier ones, but as models continue to improve, we’ll get new projections that will make these obsolete.