Study Provides Evidence That Low-Level Clouds Act as Positive Feedback to Climate Change
26 July 2009
| Time series of annual mean values of cloud and climate quantities averaged over the NE Pacific. (A) COADS cloud data. (B) ISCCP cloud data. (C) COADS SST data. (D) Hadley Centre SLP data. Clement et al. (2009), Science. Click to enlarge. |
Low-level stratiform clouds—which play an important climatic role because of their net cooling effect on the global climate—appear to dissipate as the ocean warms, thereby enhancing the warming (i.e., a positive feedback), according to a new study of the NE Pacific by researchers from the University of Miami and UC San Diego. Their paper was published in the 24 July issue of the journal Science.
The study identified decadal fluctuations in cloud cover in multiple, independent cloud data sets. Changes in cloud cover appeared to be linked to changes in both local temperature and large-scale circulation and indicated that clouds act as a positive feedback in this region on decadal time scales. The researchers also found that only one of 18 major global climate models accurately reproduced the observed cloud behavior. That model simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.
Because of inconsistencies in historical observations, trends in cloudiness have been difficult to identify. Accordingly, the response of low-level clouds to warming has been a major uncertainty in climate models. As the earth warms under increasing greenhouse gases, it is not known whether clouds will dissipate, letting in more of the sun’s heat energy and making the earth warm even faster, or whether cloud cover will increase, and actually slowing down global warming.
In their study, researchers Amy Clement and Robert Burgman from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Joel Norris from Scripps Institution of Oceanography at UC San Diego used two long-term sets of observational data for the northeast Pacific Ocean: surface-based observations of cloud cover from 1952-2007 (Comprehensive Ocean Atmosphere Data Set) and satellite data from the International Satellite Cloud Climatology Project (ISCCP) (1984-2005), the latter adjusted to account for calibration shifts from one satellite to the next.
Other climate variables used in the analysis are sea surface temperature (SST), sea-level pressure (SLP) from the Hadley center, and vertical velocity, surface winds, and lower tropospheric static stability (potential temperature at 700 mb minus surface temperature) from the ERA-40 reanalysis. ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions.
The result of their analysis was a surprising degree of agreement between two multi-decade datasets that were not only independent of each other, but that employed fundamentally different measurement methods.
...we conclude that a change in solar heating of the ocean due to a change in stratocumulus cloud cover is the principal factor maintaining decadal SST anomalies in the NE Pacific. Previous studies have shown that decreased cloud cover and warm SST additionally promote weaker circulation. This response is caused by a decrease in longwave radiative cooling of the boundary layer by clouds that reduces large-scale horizontal temperature and pressure gradients. The existence of these same relationships among SST, cloud, and circulation on decadal time scales implies that changes in subtropical stratocumulus act as a positive feedback on climate in the region.
—Clement et al. (2009)
However, the researchers found that of 18 leading global climate models from modeling centers around the world, only two reproduced the observed cloud behavior. The Hadley Centre model from the UK Met Office (HadGEM1) delivered the best reproduction of the observations.
Together, the observations and the Hadley Centre model results provide evidence that low-level stratiform clouds may dissipate in warming climates, allowing the oceans to further heat up, which would then cause more cloud dissipation.
The question of whether low-level clouds act as a positive or negative feedback to climate change has been an issue for decades. The analysis presented here provides observational evidence that this feedback is positive in the NE Pacific on decadal time scales.
The only model in the CMIP3 archive that properly simulates clouds in the NE Pacific and exhibits 2 x CO2 circulation changes that are consistent with multimodel mean produces a reduction in cloud throughout much of the Pacific in response to greenhouse gas forcing (i.e., a positive feedback). Evaluating cloud feedback with one model is, however, far from ideal. This presents a clear challenge to develop a larger number of climate models that can pass these and other tests so that we may have greater confidence in the sign of the low-cloud feedback under future changes in greenhouse gas concentrations.
—Clement et al. (2009)
Resources
Amy C. Clement, Robert Burgman, Joel R. Norris (2009) Observational and Model Evidence for Positive Low-Level Cloud Feedback. Science Vol. 325. no. 5939, pp. 460 - 464 doi: 10.1126/science.1171255
Richard A. Kerr (2009) Clouds Appear to Be Big, Bad Player in Global Warming. Science Vol. 325. no. 5939, p. 376 doi: 10.1126/science.325_376
Goracel, Henry, Reala$$, the floor is yours!
:-D
Posted by: Willy Bio | 26 July 2009 at 11:54 AM
The atmosphere and climate are such complex systems, that NCAR in Colorado is one of the largest customers for super computers. If there is more warming there can be more evaporation and more low level cloud formation that used to sit over the cooler oceans and now do not.
It is a VERY complex system created on the largest scale that we have, the planet. No amount of so called opinion spewing is going to add clarity to the issue, these people are FAR smarter than an of the opinion spewers.
Posted by: SJC | 27 July 2009 at 09:29 AM
Well, SJC, if there is more warming, the atmosphere can hold higher level of water vapor (higher vapor pressures) and hence less condensation (ie. cloud formation). This is the take-home message of this article, just as I have postulated a few weeks ago.
The general principle is quite clear. Only predicting the specifics such as exact weather in any given location would require super computer to calculate the large number of variables involved.
Likewise, predicting the exact degree of global warming at a given time in the future would be difficult due to the large number of variables and inter-related factors , that would require super computers to sort things out. Human would still have to program these super computers down to the last line of codes.
Don't forget that our brains (well, some of our brains, anyway) are also super computers, albeit analog computers doing fuzzy logic, that are juggling a large volume of data daily.
Posted by: Roger Pham | 27 July 2009 at 11:14 PM
All those factors are changing continuously on a global scale as well. We live in a digital age in an analog world. We can predict the weather with some accuracy, but never really know where a hurricane will exactly go, nor whether it will be a drought in the west for the next 10 years. They have some pretty good guesses and that will have to do for now.
When it comes to global warming causing droughts, floods, hurricanes, tornadoes and other events, we do not know, but the down side is so severe, we can not take the chance. We can have a growing economy while using more renewable energy, so why take the chance? The worst we might do is save some fossil energy in reserve.
Posted by: SJC | 28 July 2009 at 10:18 AM