• Outgoing longwave radiation from CERES Instrument on NASA Aqua Satellite for March 18, 2011, near Vernal Equinox of 2011

    Outgoing longwave radiation from CERES Instrument on NASA Aqua Satellite for March 18, 2011, near Vernal Equinox of 2011

    Courtesy of NASA

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  • The image shows longwave radiation emitted to space from Earth's surface and atmosphere (left sphere) and shortwave solar radiation reflected back to space by the ocean, land, aerosols, and clouds (right sphere).

    The image shows longwave radiation emitted to space from Earth's surface and atmosphere (left sphere) and shortwave solar radiation reflected back to space by the ocean, land, aerosols, and clouds (right sphere).

    Courtesy of NASA

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  • Reflected solar radiation from CERES Instrument on NASA Aqua Satellite for March 18, 2011, near Vernal Equinox of 2011

    Reflected solar radiation from CERES Instrument on NASA Aqua Satellite for March 18, 2011, near Vernal Equinox of 2011

    Courtesy of NASA

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The missing piece of the climate puzzle

Outgoing longwave radiation from CERES Instrument on NASA Aqua Satellite for March 18, 2011, near Vernal Equinox of 2011

Researchers show that a canonical view of global warming tells only half the story.


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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 work was supported by the National Oceanographic and Atmospheric Adminstration, the James S. McDonnell Foundation, and the National Science Foundation.


Topics: Research, School of Science, Earth and atmospheric sciences, Climate, Climate change, Environment, National Science Foundation (NSF)

Comments

It's nice to see someone taking a step back to see the forest for the trees. The current obsession with the CO2 bogeyman has resulted in deficiencies in various models because they give short shrift to the myriad of other thermal control mechanisms present in nature. When you're funded to show how much a certain GHG is going to increase terrestrial heat adsorption, pesky other factors like the Stefan–Boltzmann law get ignored. And the resulting models diverge from reality. Can we say T to the 4th? That's a pretty hefty increase in power re-transmission if temperatures are actually increasing as well as a significant control loop for planetary equilibrium. Thankfully, not everyone if wearing the same blinders.

Ok about CO2, but in this new study, which are the effects on both radiation types of the the water vapor, known to be the largest greenhouse effect gaz?

"While one would expect the longwave radiation that escapes into space to decline with increasing CO2, the amount actually begins to rise."

Because the error of measurement of TOA energy balance is 5 times larger than the theoretical signal from increased co2, this can't be determined empirically. As Kevin Trenberth said "Our instrumentation isn't good enough".

But if the amount of energy arriving at the TOA from the Sun is constant, and more longwave is leaving for space, then the first law of thermodynamics tells us the Earth is cooling overall. Same energy in - more energy out. This means that rather than accumulating somewhere below surface in the southern ocean, the 'missing heat' is now somewhere past Alpha Centauri.

So if the surface (or more correctly the near surface air) is warming, then what is under discussion is the distribution of energy absorption in the vertical profile. The article mentions two places:

"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."

However, empirical observation tells us global sea ice is at an all time high since the beginning of satellite records, and the last reliable water vapour dataset tells us overall humidity has decreased since 2000, with a small increase near the surface. More absorption of shortwave in the near surface air means less shortwave reaching the actual surface and being absorbed. That may go some way to explaining the increase in sea ice and the cooling (since the mid 1980's according to the Reynolds dataset) of the higher latitude southern ocean surface.

Has anyone factored in the energy released as fossil fuels are burned? What about the weakening of the magnetosphere... What changes will that cause? Could the weakening of the magnetosphere be caused by the extra water seeping through the crust causing a cooling of the mantle?

Release of LWIR from the atmosphere is not increasing as the canon requires, thus the model is wrong.

The rest is just handwaving.

It is looking more and more as if the world is coming around to Dr. Ferenc Miskolcz's view that CO2's effect is saturated and of no consequence to climate.

Reading carefully, the authors are claiming that the atmosphere will absorb more shortwave radiation, and thus less will strike the ground. That means that the ground will heat less and the atmosphere heat more. But that does not change the total heat energy absorbed by the Earth from shortwave radiation; it only changes the ratio of atmospheric to ground absorption.

Then the authors go on to claim that the lower amount of shortwave radiation striking the ground causes more melting of ice, causing the ground to heat more and releasing more longwave radiation. That makes no sense.

The actual energy from the sun is constant (in the short term), so the total of the energy absorbed by the atmosphere and by the ground must be constant. The authors don't say so, but their analysis requires the conversion of shortwave to longwave energy by the atmosphere and the ground to be different, which seems to violate black body science and simple conservation of energy.

These authors seem to be twisting themselves into a pretzel to rescue computer models that consistently fail to correctly comply with reality.

Good thing the science is so very very "settled."

stop burning plastic so that global warming decreases

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