Baby it's humid outside

Not everything scientifically useful or informative is found in research journals. When scientists have to explain themselves, its sometimes better to find content where they explain what they do to audiences unfamiliar with subject-specific jargon and symbology.

ON the subject of climate, there's a lot of wondering about what warming does to weather (and a huge amount of misunderstanding thereon). The key interaction is what warming does to water vapor. That's because (and I actually figured this out myself!) water vapor makes clouds and clouds make rain. Usually.

Now, here's what's interesting. For years there has been lots of talk and analysis and reanalysis and reanalyses of satellite temperature data. For those who haven't been following this potent and exciting issue -- and don't think you'll be forgiven if you haven't been -- there's an instrument called the Microwave Sounding Unit (MSU) on satellites orbited by NASA and operated by NOAA -- called polar-orbiting satellites to distinguish them from other kinds, like geostationary satellites. Polar orbiters are basically duplicates, with the same instruments, launched every couple of years to keep the instruments fresh and working. They look at clouds, sea surface temperatures, water vapor, etc. (different instruments do different things, the MSU doesn't do everything).

OK. For awhile, the MSU data analyzed by one particular group was a bastion of skeptics; because it didn't show any warming when there should have been. Then there was a big El Nino. Then there were some corrections. Then other groups -- notably Remote Sensing Systems, led by Dr. Frank Wentz -- did the analysis themselves. There were differences. There were more reanalyses. Finally, in general, there was synthesis. The MSU data shows warming of the lower troposphere. Now there is still some back-and-forth going on about this, but that's peripheral to what I'm writing about here.

It turns out that the instruments measure water vapor a whole lot more accurately and reliably than temperature! One of the first clear indications that temperature trends which were flat or showed cooling were wrong was the fact that water vapor was clearly increasing -- and that don't happen if temperatures are flat or cooling. So now there's not much discrepancy remaining on these salient points.

But what does increasing relative humidity mean to climate? More rain, less rain, where will it rain? So I got the foregoing and the upcoming from this:

Water Vapor, Precipitation, and Evaporation: The View from Satellites (this is a Powerpoint presentation transformed to a PDF)

in which Wentz and company explain themselves to the American Meteorological Society.

So what's been happening to rainfall? Well, if you live in western South Carolina or northern Georgia you might not agree, but rainfall has been increasing. Mainly in places where it rains a lot anyway, very much likely the tropical eastern Pacific. And the central Indian Ocean. And the tropical Atlantic. And the Sargasso Sea. HOW MUCH? Well, between 1 and 2% a decade.

What the intrepid Remote Sensing Systematicists did was to check both evaporation and precipitation trends. See, if it gets warmer, there's more evaporation. More evaporation means more water vapor. More water vapor means more rain. Usually.

And also, it looks like maybe that winds over the oceans are blowing a bit harder too -- which also means more evaporation.

So now Wentz and Co. compared models to data. As they note, there has been tome after tome written about temperature sensitivity to CO2 and warming. But what about precipitation and evaporation? The models say -- looking at the slide -- that precip and evap increase 1-3% per degree Centigrade of global warming, and water vapor increases 6.5%.

But the data show something different: a 6+ % increase for precip, evap, and water vapor. All together. Whilst the models seem to underestimate the precipitation increase significantly. The data say also -- and this is probably where there's a lot of meteorological interest, because the models tend to agree -- that areas where it rains a lot while get more rain, and areas where it doesn't rain a lot will get less. On that last point, 8 out of 10 Australians probably agree that this is getting to be more than a little annoying.

So, right now, the data say that there will be a larger increase in rainfall than the models say there will be. This is also supported by rain gauges. But there's a different bottom line here, I note: water vapor is definitely increasing (remember that part about water vapor being easier to measure than temperature)? As long as that's happening, it's warming up. And as long as that's happening, weather patterns and cloud patterns and rainfall patterns will change. I can pretty much guarantee that. Some people apparently don't understand that warmer oceans and stronger winds over the oceans are a long-term effect that determines the dynamics of the atmosphere, even if there's a lot of short-term variability. So if you think Al Gore's wrong on this, talking to Frank Wentz would probably help improve understanding considerably.