Posted on 03/16/2004 2:20:53 PM PST by cogitator
Satellite Finds Warming 'Relative' To Humidity
A NASA-funded study found some climate models might be overestimating the amount of water vapor entering the atmosphere as the Earth warms. Since water vapor is the most important heat-trapping greenhouse gas in our atmosphere, some climate forecasts may be overestimating future temperature increases.
(go to linked article for schematic diagram)
In response to human emissions of greenhouse gases, like carbon dioxide, the Earth warms, more water evaporates from the ocean, and the amount of water vapor in the atmosphere increases. Since water vapor is also a greenhouse gas, this leads to a further increase in the surface temperature. This effect is known as "positive water vapor feedback." Its existence and size have been contentiously argued for several years.
Ken Minschwaner, a physicist at the New Mexico Institute of Mining and Technology, Socorro, N.M., and Andrew Dessler, a researcher with the University of Maryland, College Park, and NASA's Goddard Space Flight Center, Greenbelt, Md, did the study. It is in the March 15 issue of the American Meteorological Society's Journal of Climate. The researchers used data on water vapor in the upper troposphere (10-14 km or 6-9 miles altitude) from NASA's Upper Atmosphere Research Satellite (UARS).
Their work verified water vapor is increasing in the atmosphere as the surface warms. They found the increases in water vapor were not as high as many climate-forecasting computer models have assumed. "Our study confirms the existence of a positive water vapor feedback in the atmosphere, but it may be weaker than we expected," Minschwaner said.
"One of the responsibilities of science is making good predictions of the future climate, because that's what policy makers use to make their decisions," Dessler said. "This study is another incremental step toward improving those climate predictions," he added.
According to Dessler, the size of the positive water vapor feedback is a key debate within climate science circles. Some climate scientists have claimed atmospheric water vapor will not increase in response to global warming, and may even decrease. General circulation models, the primary tool scientists use to predict the future of our climate, forecast the atmosphere will experience a significant increase in water vapor.
NASA's UARS satellite was used to measure water vapor on a global scale and with unprecedented accuracy in the upper troposphere. Humidity levels in this part of the atmosphere, especially in the tropics, are important for global climate, because this is where the water vapor has the strongest impact as a greenhouse gas.
UARS recorded both specific and relative humidity in the upper troposphere. Specific humidity refers to the actual amount of water vapor in the air. Relative humidity relates to the saturation point, the amount of water vapor in the air divided by the maximum amount of water the air is capable of holding at a given temperature. As air temperatures rise, warm air can hold more water, and the saturation point of the air also increases.
In most computer models relative humidity tends to remain fixed at current levels. Models that include water vapor feedback with constant relative humidity predict the Earth's surface will warm nearly twice as much over the next 100 years as models that contain no water vapor feedback.
Using the UARS data to actually quantify both specific humidity and relative humidity, the researchers found, while water vapor does increase with temperature in the upper troposphere, the feedback effect is not as strong as models have predicted. "The increases in water vapor with warmer temperatures are not large enough to maintain a constant relative humidity," Minschwaner said. These new findings will be useful for testing and improving global climate models.
NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth system science to improve prediction of climate, weather and natural hazards using the unique vantage point of space. NASA plans to launch the Aura satellite in June 2004. Along with the Terra and Aqua satellites already in operation, Aura will monitor changes in Earth's atmosphere.
Well, thats the last grant he's ever going to get.
I understand your cynicism, but in reality this is a very important discovery and if confirmed, it will likely result in considerable scientific recognition and reward for these scientists.
The nuts have usually been wrong, and have an agenda that is nearly detached from scientific research, so I doubt that they'll apologize. The scientists are intent on getting it right, and this is a good step in that direction.
Minorities, single mothers and children expected to come off the worse. Film at eleven.
Ramanthan (Journal of Geophysical Review, vol. 84, pp. 4949-4958) states:
"the direct radiative effects of doubled CO2 can cause a maximum surface warming [at the equator] of about 0.2 K, and hence roughly 90% of the 2.0-2.5 K surface warming obtained by the GCM is caused by atmospheric feedback processes described above."
A Lukewarm Greenhouse
"The average warming predicted by the six methods for a doubling of CO2, is only +0.2 degC."
It may be that the level of confirmation of the model in the upper troposphere data doesn't allow (at present) the quantification that you mention here. Still, this is just getting out -- we may see estimates like that soon.
Correct, but this study confirms that there is a positive water vapor feedback, and indicates that it can also be quantified. That's a major advance.
I found a really interesting figure, which it would be nice to reproduce but I can't. It's in this Powerpoint presentation:
noaasis.noaa.gov/2003/bates.ppt
Find the "Water Vapor Feedback" slide.
It confirms exactly what you say (not that I was disputing you!). Direct CO2 greenhouse heating provides 0.17 C (the slide notes that this is model dependent). Tropospheric heating by CO2 provides 0.33 C (total of 0.5 C). Water vapor feedback, and this was based on the constant relative humidity assumption now in question, was 1.7 C, for a total of 2.2 C.
It will be very interesting to find out how much weaker the researchers (or others) think that this research indicates the water feedback mechanism is, as discussed with JasonC above.
That's not what it means.
What about warming due to increases in solar radiation, or can they safely discard that due to political considerations?
Either warming due to increased solar radiation, or cooling due to decreased solar radiation -- both of which could happen -- are not predictable components of the Earth's climate system. It'd be really obliging of the Sun if it could jump into a "Maunder Minimum" mode for the next 200 years and help keep global temperatures from rising as much as they might under a constant solar input scenario, which is all that the models can assume.
What about warming due to increases in solar radiation
Conclusions of the Workshop on Ion--Aerosol--Cloud Interactions, CERN, 18--20 April 2001 (view: PDF), page 3
A.W. Wolfendale
" In the case of the current global warming, there is increasing agreement that the climate model fits to the temperature record need to amplify the solar contribution by about a factor 3. The presently-assumed solar contribution is only from the (Lean et al., 1995) direct irradiance changes."
The high feedback factor The UN/IPCC models assign to watervapor require suppressing the estimated influence of the sun in global warming models.
The assumed contribution of solar heating of Lean et al. presumes a minimal variation in solar irradiation where temperature change since the Maunder solar minimum is 25% solar. Other studies in the mid-late '90s place that solar contibution as high as 65%.
http://www1.ncdc.noaa.gov/pub/data/paleo/climate_forcing/solar_variability/bard_irradiance.txt
2. Data from Figure 3.
Reconstructed Solar Irradiance
Scaled against Maunder Minimum Total Solar Irradiance reductions of
0.25% (Lean et al. 1995),
0.40% (Zhang et al. 1994, Solanki and Fligge, 1998)
0.55% (Cliver et al. 1998), and
0.65% (Reid 1997)
In short the UN/IPCC climate models assume the most minimal contribution from Solar factors they can forcing there models into a high sensitivity to greenhouse gases presuming watervapor provides the necessary feedback factor to produce that sensitivity.
The findings of the study in the thread's article, demands solar inputs perform the dominant role in climate variation, which actual measurment and the preponderance of pre-IPCC climate studies have held all along.
Do you know of a paper that explains, "we actually measure and see so and so many watts per square meter, in excess of direct CO2 forcing, from tropospheric heating"? (Also, don't satellite and balloon measurements show precious little high altitude temperature change, as opposed to surface measurements, to begin with?)
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