the Greenhouse Effect Weakening?
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.
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.
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.
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.
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.
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
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.
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.
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."
indeed. Enjoy your tropical vacation!
R.D., and P.M. Udelhofen (2003), Climate change during
1985–1999: Cloud interactions determined from satellite
Research Letters, 30,
R.D., et al. (1994) Absorption of solar radiation by clouds:
observations vs. models, Science,