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Global warming: a stratospheric surprise

03 Apr 2024 | Science Notes

From the CO2Science archive: The authors of this week’s paper (published in 2010) write that “the trend in global surface temperatures has been nearly flat since the late 1990s despite continuing increases in the forcing due to the sum of the well-mixed greenhouse gases (CO2, CH4, halocarbons, and N2O), raising questions regarding the understanding of forced climate change, its drivers, the parameters that define natural internal variability, and how fully these terms are represented in climate models.” Solomon et al. used observations of stratospheric water vapor concentration obtained over the period 1980-2008, together with detailed radiative transfer and modeling information, in order to calculate the global climatic impact of this important greenhouse gas and compare it with trends in mean global near-surface air temperature that were observed over the same time period.

Paper reviewed: Solomon, S., Rosenlof, K., Portmann, R., Daniel, J., Davis, S., Sanford, T. and Plattner, G.-K. 2010. Contributions of stratospheric water vapor to decadal changes in the rate of global warming. Sciencexpress: 10.1126/science.1182488.

What was learned
The seven scientists report that stratospheric water vapor concentrations decreased by about 10% after the year 2000; and their analysis indicates that this decrease should have slowed the rate of increase in global near-surface air temperature between 2000 and 2009 by about 25% compared to what would have been expected (on the basis of climate model calculations) due to measured increases in carbon dioxide and other greenhouse gases over the same time period. In addition, they found that “more limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% [above what it would have been without the stratospheric water vapor increase].”

What it means
In their concluding paragraph, Solomon et al. write that it is “not clear whether the stratospheric water vapor changes represent a feedback to global average climate change or a source of decadal variability.” In either case, their findings elucidate a hugely important phenomenon that was not previously included in any prior analyses of global climate change. They also write that current climate models do not “completely represent the Quasi Biennial Oscillation [which has a significant impact on stratospheric water vapor content], deep convective transport [of water vapor] and its linkages to sea surface temperatures, or the impact of aerosol heating on water input to the stratosphere.”

Consequently, in light of (1) Solomon et al.’s specific findings, (2) their listing of what current climate models do not do (which they should do), and (3) the questions they say are raised by the flat-lining of mean global near-surface air temperature since the late 1990s, it is premature in the extreme to think that we know enough about the intricate workings of Earth’s climate regulatory system to drastically reduce our CO2 emissions, especially in ways that would radically alter – and in a negative manner – the way we obtain the energy that sustains our modern societies.

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