Water vapor and global warming
Water vapor, a potent heat-trapping gas, absorbs sunlight and re-emits heat into Earth’s atmosphere. Its concentrations in the stratosphere, the second of three layers in the atmosphere, appear to have decreased in the last 10 years, according to the study.
–New York Times
I read two articles discussing the relationship between water vapor and global warming on Friday: one in the Guardian and another in the New York Times. The Guardian piece reports that a recent study suggests that a third of global warming during the 1990s was caused by water vapor in the upper atmosphere and not by man made emissions. Yet the New York Times refers to the same study, stating that the increase in water vapor which sped the warming during the period of 1980-2000 was ‘the result of an increase in emissions of methane, another greenhouse gas’. Both articles quote the study’s leader, Susan Solomon of the US National Oceanic and Atmospheric Administration who was also co-chair of 2007 IPCC report on the science of global warming.
For some reason as yet unknown, water vapor levels in the stratosphere have dropped 10% since 2000. This is thought to have contributed to the ‘flattening’ of temperatures since that time. To add to the confusion, NASA data shows that the decade ending in 2009 was the warmest on record.
[Solomon] said it was not clear if the water vapour decrease after 2000 reflects a natural shift, or if it was a consequence of a warming world. If the latter is true, then more warming could see greater decreases in water vapour, acting as a negative feedback to apply the brakes on future temperature rise.
Yes, it’s confusing and yes the story is always changing. That is the way science is and it’s not very convenient for those who wish to see things in black and white, unless they only seek to exploit it for political means. Unfortunately there are plenty of people who do. Nevertheless, the scientists who authored the study say it ‘does not undermine the scientific consensus that emissions of greenhouse gases from human activity drive global warming’, but rather how water vapor is factored in.
The IPCC has estimated that one way in which air travel contributes to climate change is through airplanes’ emission of water vapor – as well as nitrous oxides, sulfate, and soot – high up in the atmosphere.
by Graham Land