Even though researchers have been refining their climate models for more than 30 years, there is one natural phenomenon that continues to elude them. "Clouds still pose the biggest problem for us," says Marotzke. "The uncertainties are still very big. This remains the most important issue for us."
It all seems simple enough in theory. When temperatures rise, more moisture evaporates. But does that mean that more clouds form as a result? And if so, do they curb or accelerate global warming?
On their upper surfaces, clouds act like mirrors. They reflect sunlight back into space, thereby cooling the atmosphere. But on their lower surfaces they prevent the heat reflected by the Earth from escaping, and temperatures rise.
Which of the two effects predominates depends on the height and type of clouds. "You just have to look up to see how many different types there are," says US cloud expert Björn Stevens, the new director of the MPI-M. "And each cloud type behaves differently."
Until now, no one knew exactly which clouds benefit from a greenhouse climate. But the answer to this question determines whether average global temperatures will end up being one degree higher or lower than predicted by today's models, a factor which creates significant uncertainty. "The jury is still out on which direction the pendulum will take," says Stevens.
Why is all of this important? Because cloud cover is simply one of many types of feedback mechanisms that influence overall climate. From the National Research Council (via):
If there were no climate feedbacks, the response of Earth's mean temperature to a forcing of 4 W/m2 (the forcing for a doubled atmospheric CO2) would be an increase of about 1.2 °C (about 2.2 °F). However, the total climate change is affected not only by the immediate direct forcing, but also by climate “feedbacks” that come into play in response to the forcing.
However, the true climate sensitivity remains uncertain, in part because it is difficult to model the effect of feedback. In particular, the magnitude and even the sign of the feedback can differ according to the composition, thickness, and altitude of the clouds, and some studies have suggested a lesser climate sensitivity.
When Al Gore claims that "the science is settled" on CO2, he is making a typical politician's gambit. This is true in the narrowest sense, including only the direct effects of CO2-- 1.2 degrees of warming, not the 4 or 6 that he frequently cites. But, following rather non-controversial claims about the impact of CO2, he makes a cognitive leap, from the direct impact of CO2, to the net impact of CO2, two very different questions. (BUMPED)
