Ozone recovery will accelerate global warming

The Montreal Protocol is one of the great success stories in global environmental protection. In 1987, the world's government recognized the threat of ozone-depleting chemicals and agreed to phase them out. The treaty to that effect came into force in 1989 and now has 196 signatories. It can be done.

Unfortunately, the necessary and desired recovery of the ozone is going to slightly accelerate global warming, as ozone is itself a greenhouse gas. A recent study (h/t AGWObserver) quantifies this warming:

Tropospheric temperature response to stratospheric ozone recovery in the 21st century – Hu et al. (2011) “Recent simulations predicted that the stratospheric ozone layer will likely return to pre-1980 levels in the middle of the 21st century, as a result of the decline of ozone depleting substances under the Montreal Protocol. Since the ozone layer is an important component in determining stratospheric and tropospheric-surface energy balance, the recovery of stratospheric ozone may have significant impact on tropospheric-surface climate. Here, using multi-model results from both the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC-AR4) models and coupled chemistry-climate models, we show that as ozone recovery is considered, the troposphere is warmed more than that without considering ozone recovery, suggesting an enhancement of tropospheric warming due to ozone recovery. It is found that the enhanced tropospheric warming is mostly significant in the upper troposphere, with a global and annual mean magnitude of ~0.41 K for 2001–2050. We also find that relatively large enhanced warming occurs in the extratropics and polar regions in summer and autumn in both hemispheres, while the enhanced warming is stronger in the Northern Hemisphere than in the Southern Hemisphere. Enhanced warming is also found at the surface. The global and annual mean enhancement of surface warming is about 0.16 K for 2001–2050, with maximum enhancement in the winter Arctic.” Hu, Y., Xia, Y., and Fu, Q., Atmos. Chem. Phys., 11, 7687-7699, doi:10.5194/acp-11-7687-2011, 2011.

Full text here. At the time of the IPCC's 2007 report, the effects of ozone on warming were held to be doubtful:

Hence, the net heating or cooling is subject to large uncertainty, and available model simulations do not give a consistent picture of future development of ozone, particularly in the Arctic.

While the IPCC models do include the recovery of the ozone layer (and are among the resources used by the authors), it seems fair to say that if this result holds up, we are looking at more warming "in the pipeline" than projected in the Fourth Assessment Report; a not-insignificant 0.03C/decade, or 15% of the overall expected trend of 0.2C/decade.