From the Massachusetts Institute of Technology
The missing piece of the climate puzzle
In classrooms and everyday conversation, explanations of global warming hinge on the greenhouse gas effect. In short, climate depends on the balance between two different kinds of radiation: The Earth absorbs incoming visible light from the sun, called “shortwave radiation,” and emits infrared light, or “longwave radiation,” into space.
Upsetting that energy balance are rising levels of greenhouse gases, such as carbon dioxide (CO2), that increasingly absorb some of the outgoing longwave radiation and trap it in the atmosphere. Energy accumulates in the climate system, and warming occurs. But in a paper out this week in the Proceedings of the National Academy of Sciences, MIT researchers show that this canonical view of global warming is only half the story.
In computer modeling of Earth’s climate under elevating CO2 concentrations, the greenhouse gas effect does indeed lead to global warming. Yet something puzzling happens: While one would expect the longwave radiation that escapes into space to decline with increasing CO2, the amount actually begins to rise. At the same time, the atmosphere absorbs more and more incoming solar radiation; it’s this enhanced shortwave absorption that ultimately sustains global warming.
“The finding was a curiosity, conflicting with the basic understanding of global warming,” says lead author Aaron Donohoe, a former MIT postdoc who is now a research associate at the University of Washington’s Applied Physics Laboratory. “It made us think that there must be something really weird going in the models in the years after CO2 was added. We wanted to resolve the paradox that climate models show warming via enhanced shortwave radiation, not decreased longwave radiation.”
Donohoe, along with MIT postdoc Kyle Armour and others at Washington, spent many a late night throwing out guesses as to why climate models generate this illogical finding before realizing that it makes perfect sense — but for reasons no one had clarified and laid down in the literature.
They found the answer by drawing on both computer simulations and a simple energy-balance model. As longwave radiation gets trapped by CO2, the Earth starts to warm, impacting various parts of the climate system. Sea ice and snow cover melt, turning brilliant white reflectors of sunlight into darker spots. The atmosphere grows moister because warmer air can hold more water vapor, which absorbs more shortwave radiation. Both of these feedbacks lessen the amount of shortwave radiation that bounces back into space, and the planet warms rapidly at the surface.
Meanwhile, like any physical body experiencing warming, Earth sheds longwave radiation more effectively, canceling out the longwave-trapping effects of CO2. However, a darker Earth now absorbs more sunlight, tipping the scales to net warming from shortwave radiation.
“So there are two types of radiation important to climate, and one of them gets affected by CO2, but it’s the other one that’s directly driving global warming — that’s the surprising thing,”
…says Armour, who is a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
Out in the real world, aerosols in air pollution act to reflect a lot of sunlight, and so Earth has not experienced as much warming from shortwave solar radiation as it otherwise might have. But the authors calculate that enough warming will have occurred by midcentury to switch the main driver of global warming to increased solar radiation absorption.
The paper is not challenging the physics of climate models; its value lies in helping the community interpret their output. “While this study does not change our understanding of the fundamentals of global warming, it is always useful to have simpler models that help us understand why our more comprehensive climate models sometimes behave in superficially counterintuitive ways,” says Isaac Held, a senior scientist at NOAA’s Geophysical Fluid Dynamics Laboratory who was not involved in this research.
One way the study can be useful is in guiding what researchers look for in satellite observations of Earth’s radiation budget, as they track anthropogenic climate change in the decades to come. “I think the default assumption would be to see the outgoing longwave radiation decrease as greenhouse gases rise, but that’s probably not going to happen,” Donohoe says. “We would actually see the absorption of shortwave radiation increase. Will we actually ever see the longwave trapping effects of CO2 in future observations? I think the answer is probably no.”
The study sorts out another tricky climate-modeling issue — namely, the substantial disagreement between different models in when shortwave radiation takes over the heavy lifting in global warming. The authors demonstrate that the source of the differences lies in the way in which a model represents changes in cloud cover with global warming, another big factor in how well Earth can reflect shortwave solar energy.
The paper: http://www.pnas.org/content/111/47/16700 (open access)
Shortwave and longwave radiative contributions to global warming under increasing CO2
The greenhouse effect is well-established. Increased concentrations of greenhouse gases, such as CO2, reduce the amount of outgoing longwave radiation (OLR) to space; thus, energy accumulates in the climate system, and the planet warms. However, climate models forced with CO2 reveal that global energy accumulation is, instead, primarily caused by an increase in absorbed solar radiation (ASR). This study resolves this apparent paradox. The solution is in the climate feedbacks that increase ASR with warming—the moistening of the atmosphere and the reduction of snow and sea ice cover. Observations and model simulations suggest that even though global warming is set into motion by greenhouse gases that reduce OLR, it is ultimately sustained by the climate feedbacks that enhance ASR.
In response to increasing concentrations of atmospheric CO2, high-end general circulation models (GCMs) simulate an accumulation of energy at the top of the atmosphere not through a reduction in outgoing longwave radiation (OLR)—as one might expect from greenhouse gas forcing—but through an enhancement of net absorbed solar radiation (ASR). A simple linear radiative feedback framework is used to explain this counterintuitive behavior. It is found that the timescale over which OLR returns to its initial value after a CO2perturbation depends sensitively on the magnitude of shortwave (SW) feedbacks. If SW feedbacks are sufficiently positive, OLR recovers within merely several decades, and any subsequent global energy accumulation is because of enhanced ASR only. In the GCM mean, this OLR recovery timescale is only 20 y because of robust SW water vapor and surface albedo feedbacks. However, a large spread in the net SW feedback across models (because of clouds) produces a range of OLR responses; in those few models with a weak SW feedback, OLR takes centuries to recover, and energy accumulation is dominated by reduced OLR. Observational constraints of radiative feedbacks—from satellite radiation and surface temperature data—suggest an OLR recovery timescale of decades or less, consistent with the majority of GCMs. Altogether, these results suggest that, although greenhouse gas forcing predominantly acts to reduce OLR, the resulting global warming is likely caused by enhanced ASR.
Note: This study was published in November 2014, but was not covered by WUWT then. Thanks to Dennis Wingo for bringing it to our attention.