Is the Greenhouse Effect Weakening?

People who've spent any time in the climate business looking at data can tell you that the atmosphere doesn't always act the way it's supposed to. Thus the need for data to verify models, since models are hell-bent on producing results at any cost.

Today we have news that the greenhouse effect is weakening. Yes, that is the opposite of what you've heard on AB/NB/BB/C news, and it's not (P) B.S. But the explanation is a tad thick.

Robert Cess of SUNY Stony Brook is an expert on clouds and climate. Some years ago, he suggested that clouds are absorbing far more radiation than we (climate "experts") had thought for, oh, the previous 30 years. His theory immediately called into question the veracity of climate model forecasts, since they had the basic radiation numbers for the planet completely wrong.

Now Cess is at it again. He and his late colleague Petra Udelhofen examined the role of clouds on climate across Earth's tropics and subtropics (40°N to 40°S). First, they determined how a particular climate model was treating the relationship between radiation, temperature, and clouds over that region. The model results have temperatures rising since 1970 (Figure 1a), along with the greenhouse effect (Figure 1c). Here, we're not talking about the amount or concentration of greenhouse gases, but the net radiative impact of those gases. With more greenhouse gases in the atmosphere, it's harder for heat (technically, infrared or outgoing longwave radiation [OLR]) to escape from the surface and atmosphere into the void of space. So, if we put instruments in space to measure OLR, we should expect them to record a long-term decline (Figure 1b). But the model also indicates that absorbed shortwave radiation (ASW—think sunlight) is declining as well (also Figure 1b). The best way to reduce sunlight is to increase cloud cover, so the results in Figure 1 are consistent with increasingly cloudier tropics and subtropics.

Figure 1. Climate model results, 1970–1999; (a) Change in global surface temperatures; (b) change in outgoing longwave radiation (olive line) and change in absorbed short wave radiation (red pimiento line)—between the latitudes 40°N and 40°S; (c) change in the greenhouse effect, between the same latitudes. These model results are consistent with a cloudier tropical and subtropical sky.

Don't panic yet. Your planned summer month in Tahiti may not be a complete washout after all. You see, there are measurements, available from satellites since 1985, to verify this model. Figure 2a shows the temperature record since 1985, which depicts a general warming, helped along by two major El Nińos and disrupted by all the sputum of 1991's Mt. Pinatubo eruption. But both OLR and ASW are increasing, not decreasing, over the same time period. Except for a few years after Pinatubo, when less sunlight reached the surface and the cooler planet emitted less OLR, the increase has been fairly steady (Figure 2b). Interestingly, this increase in ASW matches very nicely a related decline in the portion of the sky covered with clouds (Figure 2c). This makes perfect sense: Fewer clouds result in more sunlight's being absorbed by the surface.

Figure 2. Observed data, 1985 to 1999; (a) global surface temperatures; (b) outgoing longwave radiation (olive line), absorbed shortwave radiation (red pimiento line), between latitudes 40°N and 40°S; (c) amount of cloud cover (blue line) and greenhouse effect (black line). The observations are consistent with a less cloudy tropical and subtropical sky, which is the opposite of what the model projects.

If clouds are less common, it's pretty easy to raise the surface temperature without the need for greenhouse gases. In other words, the net effect of greenhouse gases is reduced (Figure 2c). These observations imply that the climate model is wrong; there's more than one way to skin a cat, or raise the Earth's temperature.

Why are the tropics and subtropics becoming less cloudy? No one is certain. You might guess that it has something to do with how clouds respond to climate change (one of the critical questions in this business), but the jury's still out on that topic. For now, Cess and Udelhofen suggest that this 15-year cloud decline is little more than natural variation. At any rate, it suggests that climate models are generating warming at least partly for the wrong reasons. Or, in the words of the authors, "If the change in cloud cover is the result of natural variability acting over decadal time scales, this could considerably hamper efforts at detecting the radiative signature of future global warming."

Hamper, indeed. Enjoy your tropical vacation!


Cess, R.D., and P.M. Udelhofen (2003), Climate change during 1985–1999: Cloud interactions determined from satellite measurements, Geophysical Research Letters, 30, 19-1–19-4.

Cess, R.D., et al. (1994) Absorption of solar radiation by clouds: observations vs. models, Science, 267, 496–499.