These findings show that stratospheric water vapor represents an important driver of decadal global surface climate change.

—Solomon et al.

Over the last century, global average surface temperatures have warmed by about 0.75 °C (0.42 °C), with much of it occuring the last half. However, note the authors, the trend in global surface temperatures has been nearly flat since the late 1990s despite continuing increases in the forcing due to all the greenhouse gases.

Previous studies suggested that stratospheric water vapor might contribute significantly to climate change. The new study is the first to relate water vapor in the stratosphere to the specific variations in warming of the past few decades.

Stratospheric water vapor and radiative processes. Source: NOAA. Click to enlarge.

The stratosphere is the region of the atmosphere from about eight to 30 miles above the Earth’s surface. Water vapor enters the stratosphere mainly as air rises in the tropics.

Stratospheric water vapor changes affect the fluxes of longwave (infrared) and—to a lesser extent—shortwave (solar) radiation, and can thereby influence the temperature in the stratosphere and troposphere. In general, increases in stratospheric water vapor cool the stratosphere, but warm the troposphere; the reverse is true for decreases in stratospheric water vapor.

The researchers used data from the HALogen Occultation Experiment (HALOE) that flew on the Upper Atmosphere Research Satellite (UARS) from late 1991 through November 2005, with coverage from the troposphere to the stratosphere overs 65°S to 65 °N. Combined with two additional and independent sets of satellite data, the data provide evidence for “a sharp and persistent drop of about 0.4 parts per million by volume (ppmv)” in stratospheric water vapor after 2000.

Balloon data suggest a gradual mid-latitude increase in lower stratospheric water vapor of more than 1 ppmv from about 1980 to 2000. Other data also support increased in lower stratospheric water vapor during the 1990s of about 0.5 ppmv.

...recent observations have suggested a correlation of the post-2000 stratospheric water vapor decrease to sea surface temperature [SST] changes near the tropical warm pool region and associated cooling of the cold point that governs water vapor input to the stratosphere in the tropics. However, the relationship between SSTs in the warm pool region and stratospheric water vapor changes character (from negative to positive short-term correlations) from 1980–2009, suggesting that other processes may also be important, or that the correlation may be a transient feature linked to the specific pattern of SSTs at a given time rather than to the average warming of SSTs around the globe.

It is therefore not clear whether the stratospheric water vapor changes represent a feedback to global average climate change or a source of decadal variability. Current global climate models suggest that the water vapor feedback to global warming due to carbon dioxide increases is weak but these models do not fully resolve the tropopause or the cold point, nor do they completely represent the QBO, deep convective transport and its linkages to SSTs, or the impact of aerosol heating on water input to the stratosphere.

This work highlights the importance of stratospheric water vapor for decadal rates of warming based directly upon observations, illuminating the need for further observations and a closer examination of the representation of stratospheric water vapor changes in climate models aimed at interpreting decadal changes and for future projections.

—Solomon et al.

Authors of the study are Susan Solomon, Karen Rosenlof, Robert Portmann, and John Daniel, all of the NOAA Earth System Research Laboratory (ESRL) in Boulder, Colo.; Sean Davis and Todd Sanford, NOAA/ESRL and the Cooperative Institute for Research in Environmental Sciences, University of Colorado; and Gian-Kasper Plattner, University of Bern, Switzerland.

Resources

• Susan Solomon, Karen Rosenlof, Robert Portmann, John Daniel, Sean Davis, Todd Sanford, Gian-Kasper Plattner (2010) Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming. Science Express doi: 10.1126/science.1182488