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Climate-Driven Changes in Clouds are Likely to Amplify Global Warming


bluewave
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Scientists know that global warming is changing clouds, but they haven’t been sure whether those changes would heat or cool the planet overall.

It’s an important question, because clouds have been the main source of uncertainty in projecting just how sensitive the climate is to increasing greenhouse gas concentrations, and because clouds have a huge effect on the climate system. Just a 20 percent change in their extent or reflectivity would have more of an impact than all the greenhouse gases released by human activities.

A new study published today in the Proceedings of the National Academy of Sciences may help find an answer. The researchers analyzed 20 years of cloud data from satellites and found that it was 97.5 percent certain that changes in clouds brought about by climate change will amplify warming. 

Since the cloud effect has been uncertain, its accurate measurement also helps affirm other recent projections that a doubling of carbon dioxide in the atmosphere will warm the planet’s surface by about 5.8 degrees Fahrenheit, said said co-author Paulo Ceppi, a climate scientist with the Grantham Research Institute on Climate Change, London School of Economics and Political Science. 

“Most previous cloud studies focused only on certain regions or regimes, so say they look at places where there are low clouds and they look at low clouds only,” he said. “We did this analysis everywhere, at every point regardless of what type of cloud was there, and that allowed us to get a global picture.”

The new research is an important update to the scientific understanding of clouds in the climate system, said Piers Forster, director of the Priestley Centre at Leeds University. 

“It is a really good step forward,” said Forster, who was not involved in the new study, but has worked on other recent research assessing the climate system’s response to building greenhouse gas levels.

“It really tells us how clouds respond to changes in local surface temperature, especially the reflectance of low clouds,” he said. “This is then used to make an accurate estimate of the total cloud feedback: the amplifying effect that clouds have on global warming.”

To get a sense of how important clouds are in the global warming equation, Ceppi said their effects can be compared to the warming effect of carbon dioxide.

“We calculate that, on average globally, clouds reflect something like 50 watts per square meter of solar radiation,” he said. “You can compare that to the forcing from a doubling of CO2, which would be about 4 watts per square meter, much smaller than the average effect of clouds on sunlight. So even a very small change in how much sunlight is reflected by clouds would be comparable to the effect of a CO2 doubling.”

In general, the new research confirms what some of those other studies have suggested, he said. 

“People have argued that clouds will amplify global warming because of solar impacts, so less reflected sunlight from low clouds, but also because of the greenhouse effect of clouds, where high clouds rise, which makes them have a larger warming effect,” he said. “Our study finds evidence of both. I’m not aware of any other studies that have been able to show that, especially the greenhouse part.”

One recent study, led by University of Oslo researchers, shows global warming will reduce the amount of ice particles in widespread low clouds around Antarctica that currently reflect a huge amount of solar radiation back into space. That would make the clouds less reflective and amplify global warming, said cloud researcher Trude Storelvmo.

Machine Learning

Ceppi said using a machine learning approach is especially suited for complex problems like cloud changes.

“It’s a complex situation because clouds depend on so many factors that all co-vary. 

For example, for a certain change in humidity, you get a certain response from clouds,” he said. “The machine learning method we use is smarter about learning these dependencies. It’s a complex statistical problem, and improved statistical methods can really help. There are so many relationships that it’s hard to calculate them manually. The statistical learning step gives us better predictive power.”

Prior studies showed less strong relationships and thus came up with less reliable projections, he added. 

“One strength of our study is that we show, with 20 years of data from observations, we can really predict the feedback in model worlds where we know the answers,” he said. “Our results will mean we are more confident in climate projections and we can get a clearer picture of the severity of future climate change. This should help us know our limits and take action to stay within them.”

While the research helps narrow the range of cloud responses and feedback to global warming, some uncertainties remain.

“I would like to see a physical process understanding of how clouds respond,” Forster said. “This would add confidence that they are looking at the right statistics. It’s really about how much low clouds reflect sunlight in relation to both the local surface temperature and how quickly the temperature drops with altitude. Both of these temperatures are affected by global warming.”

“Understanding how clouds respond locally to these temperatures,” he said, “builds up a complete picture of how clouds respond to global warming, and thereby how much global warming we expect from increasing levels of CO2.”

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This is another bad break on the climate change front. There had been some hope that cloud changes could somewhat dampen the warming. Research by Jessica Tierney et al., hinted at amplification from the paleoclimate record, but there was no direct evidence. This paper provides that evidence. Its findings underscore the urgency of moving faster rather than more slowly in curbing greenhouse gas emissions.

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Would be more meaningful if clouds were you know...stuck in place, steady in size, of consistent shape and arriving at constant times. I can't see the paper behind the paywall to see the methodology used. But my understanding was part of the reason studying clouds was hard is because most of the Earth is salt water at the surface, and it's hard to get ground observations of clouds that are reliable over a long period over the vast majority of the Earth. Satellites are better than ground observations at some things, but not everything. The other issue with studying clouds is that they'll likely arrive and develop from different places if the climate changes dramatically in a given spot over the coming decades. The changing origin of the clouds (or even introduction of extra sunlight / cloudiness in a over time) is more relevant long-term.

More philosophically, you have different amounts of cloudiness not just by region but by time. So the effects from changing clouds would be by time of day, time of year, and then layered on top of the general pervasiveness of clouds in a region. Presumably, it's not a big deal where I am as we have something like weeks-months of near cloud free days per year, whereas Seattle or Boston would have the opposite, very few true cloud-free days. More generally, these papers usually ignore "ground clouds" like fog, freezing fog, mist, dust, haze, pollution, and so on. Not to mention the differences in terrain with rapid elevation changes where effects like shadows near dawn and dusk take effect and can interact with the "angles" of clouds so to speak. You can be shadowed from a cloud that isn't over head at the right time of day and so on.

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One thing that confused me a couple of years ago is the increase in outgoing longwave radiation (OLR). I naively thought that GHGs would reduce OLR and that's what creates the Earth Energy Imbalance (EEI).

That is what happens initially. But...the warming that results from a positive EEI leads to feedbacks like the cloud feedback. If the cloud feedback is positive then Earth's albedo will drop and the absorbed shortwave radiation (ASR) will also increase. Because EEI = OLR - ASR there two ways for the planet to achieve energy balance after a positive EEI perturbation. The first is if OLR increases (temperature increase). The second is if ASR decreases (albedo increase).  The fact that we observe an increase in OLR while EEI itself continues to increases is a tell that ASR is increasing which means albedo is decreasing. This is consistent with the hypothesis that the cloud feedback is positive. Donohoe et al 2014 has a pretty good explanation for the counter-intuitive OLR increase when the climate system is acted on by increases in GHG. See figure 1c and 1d for how the OLR and ASR respond to pulses of GHG forcing. It is interesting to note that most (not all) global circulation models actually predict this behavior. DeWitte & Clerbaux 2018 point out that OLR is indeed increasing and attribute this to "cloud thinning".

 

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18 hours ago, donsutherland1 said:

This is another bad break on the climate change front. There had been some hope that cloud changes could somewhat dampen the warming. Research by Jessica Tierney et al., hinted at amplification from the paleoclimate record, but there was no direct evidence. This paper provides that evidence. Its findings underscore the urgency of moving faster rather than more slowly in curbing greenhouse gas emissions.

Yeah, that's almost a complete shutout of ECS <= 2C by their calculations. While the mean looks to be around 3.2, that PDF is .... not great. Anything under 2.5 is pretty unlikely and pretty decent probs of something closer to 4.

From what I'm gathering, that's also just from the observational record. Paleo suggests ECS itself might be variable depending on the state (higher during warmer climates for instance). https://advances.sciencemag.org/content/5/9/eaax1874

Having said all of that -- I'm also wondering if the laser-like focus on ECS is a bit troublesome in itself. It's possible, for instance, that we're focusing a bit too hard on global temps and not enough on downstream effects -- like asymmetric hemispheric response. All evidence points towards the NH losing ice much faster than the SH and indeed paleoclimate tends to suggest that the NH and SH can exist in a "decoupled" state where most of Antarctica can still be relatively cold and glaciated and the NH is essentially much closer to something resembling a greenhouse climate. The GIS will still be around for quite a long time but the amount of resistance it can put up pales in comparison to the combination that the circumpolar current and EAIS can put up. It's worth noting that Antarctica glaciated pretty early on (30-40mya at around 650 ppm I think?). I'm sure the isolating power of the circumpolar current helped, but if folks like Tierney are right, that doesn't have as much of an effect as one might initially think.

 

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17 hours ago, raindancewx said:

Clouds are difficult to study. Shape, size, location, color all change very quickly. Even something like a count of the clouds is hard. How many clouds are in the GIF below would you say?

 

There are 2 decades of cloud satellite obs. This study is in-line with others estimating cloud feedback using satellite data. Scientists have been gradually paring down the uncertainty in climate sensitivity with better obs and models, as they do that there is no indication that warming has been overstated. Instead the tightening is mainly from raising the lower-bound.

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On 7/20/2021 at 3:32 PM, csnavywx said:

Yeah, that's almost a complete shutout of ECS <= 2C by their calculations. While the mean looks to be around 3.2, that PDF is .... not great. Anything under 2.5 is pretty unlikely and pretty decent probs of something closer to 4.

From what I'm gathering, that's also just from the observational record. Paleo suggests ECS itself might be variable depending on the state (higher during warmer climates for instance). https://advances.sciencemag.org/content/5/9/eaax1874

Having said all of that -- I'm also wondering if the laser-like focus on ECS is a bit troublesome in itself. It's possible, for instance, that we're focusing a bit too hard on global temps and not enough on downstream effects -- like asymmetric hemispheric response. All evidence points towards the NH losing ice much faster than the SH and indeed paleoclimate tends to suggest that the NH and SH can exist in a "decoupled" state where most of Antarctica can still be relatively cold and glaciated and the NH is essentially much closer to something resembling a greenhouse climate. The GIS will still be around for quite a long time but the amount of resistance it can put up pales in comparison to the combination that the circumpolar current and EAIS can put up. It's worth noting that Antarctica glaciated pretty early on (30-40mya at around 650 ppm I think?). I'm sure the isolating power of the circumpolar current helped, but if folks like Tierney are right, that doesn't have as much of an effect as one might initially think.

 

The differences between the Arctic and the Antarctic are basically because the Arctic has a large quantity of water near the NP while the Antarctic is a continent surrounding the SP correct?

 

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