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Summary

Some of the difficulties in assessing the role of the stratosphere in climate change arise from:

• poor stratospheric resolution in climate models used for detection and attribution;
• limitations of simulations of internal climate variability in the stratosphere;
• uncertainties in solar and volcanic forcing reconstructions and responses;
• uncertainties in trends in water vapour in the upper troposphere and lower stratosphere;
• the relatively short observational record for the stratosphere;
• the absence of adequate coupling between stratospheric chemistry and circulation in climate models; and
• uncertainties about dynamical links between the stratosphere and the troposphere.

The main conclusions of the IPCC Third Assessment Report on the detection of climate change and attribution of causes include that:

• there is stronger evidence now of a human influence on global climate;
• paleoclimate data and model estimates of natural climate variations suggest that the observed warming over the last 100 years is unlikely to be solely natural in origin;
• simulations of the climate response to natural forcing alone, including volcanic eruptions and changes in solar irradiance, fail to explain the warming over the last 40 years;
• stratospheric ozone depletion has been a major contributor to the observed cooling in the lower stratosphere over the last 20 years;
• it is likely that increasing greenhouse gases have made a substantial contribution to the observed warming over the past 50 years; and
• the accuracy of predictions continues to be limited by uncertainties in internal climate variability, natural and anthropogenic forcing and the climate response to external forcing;

Hence, there has been greater understanding and appreciation of the important role that stratospheric processes play in climate variability and change.