From the new National Academy of Sciences report on climate modeling:
Current climate models are calibrated during their development process to match observations within reasonable uncertainty ranges. However, the warming to date due to greenhouse gas increases has been partially compensated by an uncertain amount of cooling caused by
human-induced enhancement of light scattering by aerosols and by their effect on clouds; this compensation has been estimated to be from 20 to 70 percent (with 90 percent confidence) based on a range of observational and model-based studies (IPCC, 2007d). Over the 21st century, global aerosol emissions are expected to not increase further, but greenhouse gas emissions are likely to accelerate for at least the next few decades, so this compensation will become less significant. Because of the uncertain cooling by aerosols the current warming cannot be used to constrain the “climate sensitivity.” Thus, the simulated 21st-century global-average warming varies across the international suite of climate models with a range of approximately 30 percent as is further discussed in Chapter 4.