Unclouding our view of future climate

May 22, 2014 by Carol Rasmussen
Atmospheric general circulation is expected to change as a result of increasing greenhouse gases. The rising air flow near the equator will grow stronger, lofting air higher into the atmosphere. Mid-latitude clouds will become sparser than is shown here, and these regions will be hotter and drier. Credit: NASA/GSFC/SVS

If we had a second Earth, we could experiment with its atmosphere to see how increased levels of greenhouse gases would change it, without the risks that come with performing such an experiment. Since we don't, scientists use global climate models.

In the virtual Earths of the models, interlocking mathematical equations take the place of our planet's atmosphere, water, land and ice. Supercomputers do the math that keeps these virtual worlds turning—as many as 100 billion calculations for one modeled year in a typical experiment. Groups that project the future of our planet use input from about 30 such models, run by governments and organizations worldwide.

When these models calculate the potential climate impacts of the real-life experiment we are conducting by emitting more into the atmosphere, they all agree that Earth will be warmer 50 years from now. They don't agree on how much warmer.

For a convenient global-change benchmark, climate scientists use "doubled CO2," meaning twice as much carbon dioxide in the atmosphere as before the Industrial Revolution, when humans started adding large amounts of the greenhouse gas to the air. The undisturbed level was about 280 parts per million (that is, 280 molecules of carbon dioxide in every million molecules of air), and today's level is around 400 parts per million. We could reach the doubled-CO2 benchmark as soon as 50 or 60 years from now.

The highest and lowest forecasts of average global temperature when we reach the doubled-CO2 benchmark differ by 5.4 degrees Fahrenheit (3 degrees Celsius). That's greater than the difference between the average global temperature during the last ice age and today. Despite increases in model complexity and sophistication in recent decades, that 5.4-degree range of uncertainty has shrunk very little.

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Modelers have known for decades that clouds are the largest source of the uncertainty. But almost every process in the atmosphere affects clouds. It hasn't been clear which processes are driving cloud changes as the climate warms.

A new NASA-led study suggests that the processes most closely related to the cloud changes are connected with the global pattern of air movements called the atmospheric general circulation. The study also finds that the models with the most realistic representation of the general circulation are those that produce warmer forecasts of the time when we reach the doubled-CO2 benchmark.

Hui Su of NASA's Jet Propulsion Laboratory, Pasadena, California, and colleagues recently evaluated 15 of the world's leading , evaluating links among atmospheric processes, modeled clouds and probable forecast accuracy. "People have been searching for the culprit for these model differences for a long time," said Su. "Our research suggests that how models handle atmospheric circulation in a warming climate is directly linked to how clouds will change—and therefore linked to how much warmer the climate will be." A paper on the research is published online May 21 in the Journal of Geophysical Research.

Scientists expect to see a complicated pattern of changes in the general circulation as the climate warms—prevailing winds are likely to weaken in some areas and strengthen in others, zones of ascending winds will migrate closer to the equator, and descending winds will move closer to the poles, etc. (see illustration).

Su and her research team from JPL; the California Institute of Technology, Pasadena; and UCLA found dramatic variations in the way in which the models simulated these changes. The differences set off a chain of consequences: different environmental conditions within each led it to produce different forms of clouds, which finally led to a different overall response to greenhouse warming.

The atmosphere's general circulation can't be observed directly from space, so the scientists used of clouds and relative humidity as a stand-in for general circulation. They compared the models' simulations of clouds and relative humidity over the last decade with observations from four NASA spaceborne instruments: the CloudSat satellite, Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft and the Microwave Limb Sounder instrument on NASA's Aura spacecraft.

Five of the tested closely reproduced the clouds and observed by the satellites. These same five models all have greater changes in than the other models. The circulation changes decrease the cooling effect of in the future.

The five models predict a relatively warmer future climate than the other 10 models. They forecast that global surface temperatures will rise by 6.5 to 8.5 degrees Fahrenheit (3.6 to 4.7 degrees Celsius) when atmospheric has doubled from its pre-industrial level. That's in the top one-third of the range of all 15 forecasts.

"We've used NASA satellite observations to narrow the range of the climate projections. Based on the observations, we've seen that the models that best represent these observations are at the higher end of the modeled warming. We hope these results, and those of other consistent studies recently published, will have value to policy makers who are responding to this global threat," said Su.

Explore further: Cloud mystery solved: Global temperatures to rise at least 4C by 2100

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Eikka
2.2 / 5 (13) May 22, 2014
But at the same time, haven't these models been over-predicting the warming that has already happened by a pretty gross margin?

http://wattsupwit...obs1.jpg

What is wrong with this picture, if anything?

The models that use the "most" realistic cloud models, predict more warming, are poised to be more in disagreement with actual temperature measurements.
shavera
4.1 / 5 (15) May 22, 2014
Your graphs only have data for surface and troposphere measurements. When you include ocean temps, you find that the heating trend remains ongoing.

http://www.realcl...warming/

Considering the incredible heat capacity of water, and the immense volume of water on our planet, heating the ocean is a far more relevant measure of energy trapping than surface or atmospheric temperatures. That heat will return to the surface in the future (along with damaging the oceanic ecosystems that were already calibrated for the lower temperatures and lower acidity of ocean water that existed not too long ago)
thermodynamics
4.3 / 5 (11) May 22, 2014
From Eikka:
But at the same time, haven't these models been over-predicting the warming that has already happened by a pretty gross margin?


From shavera:
Your graphs only have data for surface and troposphere measurements. When you include ocean temps, you find that the heating trend remains ongoing.


shavera has it spot on. The GCMs need to be improved with respect to the heat distribution on the earth, but they are very accurate on the amount of extra heat being retained. For something as complex as the heat balance of the earth and its distribution, the accurate heat balance is the first step and is right. The distribution of that heat is a more difficult problem but does not take away from the recognition that the earth is being heated over time. It is that heating that is important for long term predictions.
Eikka
3.3 / 5 (4) May 23, 2014
Your graphs only have data for surface and troposphere measurements. When you include ocean temps, you find that the heating trend remains ongoing.


The GCMs need to be improved with respect to the heat distribution on the earth, but they are very accurate on the amount of extra heat being retained.


Alright, that sounds reasonable.

But at the same time, these models predict a 3.6 to 4.7 degrees Celsius rise in -surface- temperatures, which will obviously then be incorrect and over-estimated because the oceans are trapping the heat instead and the models are ill calibrated to account for that because they still turn up higher surface temperatures than reality, and it's the surface temperatures that appear in the media when estimating the severity of climate change.

Not to discount the importance of ocean warming, but that's still using false figures to argue and promote the whole idea while pushing the real issue somewhere where it's less understood (in the oceans).
Eikka
3.5 / 5 (2) May 23, 2014
That heat will return to the surface in the future (along with damaging the oceanic ecosystems that were already calibrated for the lower temperatures and lower acidity of ocean water that existed not too long ago)


But due to the massively larger heat capacity of the oceans vs. the atmosphere and land, the actual average temperature rise in oceans will obviously be much less. If your link is correct, a degree rise in the atmosphere would equal 0.022 degrees in the ocean, or just 2% by virtue of being the bigger heatsink, which is why the ocean is trapping most of the heat. It's the colder object.

That also means, when the energy does surface back, it will do so at a slower pace and lower intensity. This is what I mean by pushing the issue to a lesser understood area: what does the slower release mean for climate change?
kivahut
1 / 5 (1) May 25, 2014
I prefer a warmer ocean. Swimming in the NE is painful.
PinkElephant
3.7 / 5 (3) May 25, 2014
@Eikka,
That also means, when the energy does surface back, it will do so at a slower pace and lower intensity.
Not quite. That heat will eventually come back out into the atmosphere, in the form of increased evaporation. That vapor eventually releases its latent heat back into the air, when it condenses into clouds or rain. Energy is conserved, not lost.

Additionally, simplistic analyses neglect ocean currents and mixing. The warmer waters tend to carry their heat to the poles, before they sink down to the bottom. That tends to melt polar ice sheets, decreasing albedo -- I'm sure you've heard of this feedback on multiple occasions. On a related note, not all the ocean is heating up at once; most of the heat stays in the upper layers (the mixing of heat down into deep waters is a very slow process.)

Lastly, even if colder oceans could keep *global average* temperatures down, that'd be of little comfort or value to *land* temperatures far away from the cooling sea breeze.
PinkElephant
3.7 / 5 (3) May 25, 2014
But at the same time, these models predict a 3.6 to 4.7 degrees Celsius rise in -surface- temperatures, which will obviously then be incorrect and over-estimated because the oceans are trapping the heat instead and the models are ill calibrated to account for that because they still turn up higher surface temperatures than reality, and it's the surface temperatures that appear in the media when estimating the severity of climate change.
The models could be more accurate (and use much more powerful computers), of course. And over time, they will be.

However, it is a common myth that the models predict a straight-line rise in global temperature. When numerous model runs are averaged together, you do get nice smooth-looking curves. However, individual model runs will produce large swings both up and down -- correlating with natural climate oscillations.

That link shavera gave, does talk about those oscillations. We won't be stuck in La Nina conditions forever, one would think.
Eikka
1 / 5 (1) May 26, 2014
Not quite. That heat will eventually come back out into the atmosphere, in the form of increased evaporation.


Some of it will be released by evaporation, some by convection, some by radiation. The intensity of the release however is dictated by the temperature, and since the heat capacity is so large, the difference in temperature is small, which means it takes a long time for the ocean to give up the heat.

The heat doesn't come back "eventually" as if it had dissapeared at some point. It's being continuously removed from the oceans at the same time as more heat is being added, and at some point as the average temperature of the waters rise it reaches an equilibrium where the outgoing energy equals incoming energy.

ubavontuba
1 / 5 (3) May 26, 2014
@Eikka

Consider: The ocean's surface is actually cooling:

http://www.woodfo...14/trend

PinkElephant
5 / 5 (3) May 26, 2014
@Eikka,
The intensity of the release however is dictated by the temperature, and since the heat capacity is so large, the difference in temperature is small, which means it takes a long time for the ocean to give up the heat
No, that doesn't really follow. It's quite possible for the ocean to give up the heat rapidly. The specific heat of water is 4x that of air (and in terms of total heat capacity, the oceans have a hell of a lot more than the atmosphere, not just due to water's higher specific heat, but also because the oceans are much more massive.) If the top 45 meters of global ocean surface waters (roughly equivalent to the mass of the entire atmosphere) were to cool down by 1 degree, that means the entire air column above would get doused in enough energy to warm up by 4 degrees. More regionally, phenomena like that happen quite regularly; for instance in the equatorial Pacific it's called El Nino.
PinkElephant
5 / 5 (3) May 26, 2014
@tootyvonfrooty,

It's back to school time:

http://www.skepti...php?g=47