Study: Loss of Stratocumulus Clouds Could Precipitate Extreme Global Warming
Professor Tapio Schneider discusses a new study examining the relationship between stratocumulus cloud formation and high levels of atmospheric CO2
DIMITRI LASCARIS: This is Dimitri Lascaris, reporting for The Real News Network from Montreal, Canada.
Scientists have known for some time that about 56 million years ago the Earth experienced a brief, cataclysmic hot spell now known as the Paleocene-Eocene Thermal Maximum, or PETM. After heat-trapping carbon leaked into the sky from an unknown source, the planet, which was already several degrees Celsius hotter than it is today, gained an additional six degrees. The ocean became exceptionally hot near the equator, and the earth experienced mass extinctions worldwide. The PETM another episodes of intense warming were more extreme than theoretical models of the climate have been. Even after accounting for differences in geography, ocean currents, and vegetation during these past episodes, paleoclimatologists realized that an important but unknown factor was missing from their models. A new study just published in the scientific journal Nature Geoscience indicates that hitherto unknown factor is cloud cover.
According to the study, entitled Possible Climate Transitions from Breakup of Stratocumulus Decks under Greenhouse Warming, decks of stratocumulus clouds, which cover about 20 percent of the low-latitude oceans and are especially prevalent in the subtropics, become unstable and break up into scattered clouds when CO2 levels rise above 1,200 parts per million. This instability triggers a surface warming of a stunning 8 degrees Celsius globally. The study also concluded that once the stratocumulus decks have broken up, they only reform once CO2 concentrations dropped substantially below the level at which the instability first materialized.
Now here to discuss this study with us is Professor Tapio Schneider of the Faculty of Environmental Science and Engineering at Caltech University. His research focuses on how the climate of Earth and other planets comes about, and may change, for example, by changes in atmospheric circulation or cloud cover. Thank you very much for joining us today, Professor.
TAPIO SCHNEIDER: Thank you for having me.
DIMITRI LASCARIS: So why don’t I start by inviting you to explain briefly what you regard as the major findings of this study?
TAPIO SCHNEIDER: I think you summarized it fairly well. The key is that the stratocumulus clouds become unstable once greenhouse gas concentrations rise to very high levels. The reason that happens is that stratocumulus clouds are quite special. They sustain themselves by repelling air parcels downwards from their cloud top to the surface. The air parcels pick up moisture, return back to the clouds, and thereby nourish and sustain them. And that overturning circulation is driven by cooling at the cloud tops. The cloud tops radiate infrared radiation, heat radiation, upwards, and that cools the cloud tops. The key here is that this cooling becomes less efficient as greenhouse gas concentrations increase. Clouds can’t radiate as effectively upward. There is a point some threshold at which the clouds cannot sustain themselves and break up.
DIMITRI LASCARIS: Now, I understand that the current concentration of CO2 in the atmosphere is about 410 parts per million, so we we seem to be a long way away from the threshold of 1,200 parts per million. If we continue with a business-as-usual scenario, approximately how long in your estimation would it take us to hit that critical threshold of 1,200 parts per million?
TAPIO SCHNEIDER: I should say that the 1,200 parts per million is not a hard threshold. There are a number of uncertainties in our study, and 1,200 parts per million is the lowest CO2 level that we saw this instability occurring. It may occur at higher levels. And we had done some calculations where it occurred at levels up to 2,000 ppm. Now, 2,000 PPM is the CO2 level that, according to the IPCC emission scenarios, we might be reaching in about 100 years after emission scenarios. However, I think it’s unlikely that we’ll actually reach CO2 levels that high. And even if we do, there will be severe adverse climate changes well before the clouds would become unstable.
DIMITRI LASCARIS: But imagining the worst case scenario, that we do hit that threshold and that turns out to be the critical threshold, what would the planet look like, in your estimation, if it experienced 8 degrees Celsius of warming over preindustrial levels?
TAPIO SCHNEIDER: Well, it depends how long that is sustained. But if it’s sustained for a while, it will eventually look like the Eocene, something like 50 million years ago, where we had a very warm climate, extremely hot land surfaces in low lattitudes; crocodiles in the Arctic, and the like.
DIMITRI LASCARIS: Presumably civilization as we know it would be very difficult to maintain, if not impossible to maintain, in those conditions. Is that correct?
TAPIO SCHNEIDER: Well, we are adapted to the climate we’re living in. We have built cities along the coasts that are, of course, vulnerable to sea level change, and the like. There was life on Earth in the Eocene. Life is possible. It just looks very different from what we’re used to.
DIMITRI LASCARIS: Now, there are other known feedback loops in the climate system that are of concern to the scientific community. For example, as I understand it, and I’m a layperson, there are concerns that methane trapped in the Arctic permafrost could be released rapidly due to a little known–a little understood process called abrupt thawing. Could you talk to us a little bit about these other feedback loops, and their potential to accelerate the accumulation of greenhouse gases in the atmosphere so that we end up hitting that critical threshold perhaps more quickly than we had imagined? Is that a real risk?
TAPIO SCHNEIDER: There are as other feedback loops in the climate system, not all of which are fully understood. You mentioned methane in the Arctic. I would say more generally there feedbacks with the biosphere that we know have played a role in, for example, transitions between ice ages and warm ages. CO2 levels change quite substantially in ways that we don’t fully understand. We know that the ocean must be involved in some way, the biosphere. But how, exactly, is–it’s one of the unsolved problems in the geosciences, how do we get from warm periods to ice ages and back. It just says we are perturbing a system right now that we do not fully understand. And that means it can harbor surprises.
DIMITRI LASCARIS: Well, we’ve been speaking to Professor Tapio Schneider about a new study regarding the formation of cloud cover, and how that may affect the climate as CO2 concentrations accumulate. Thank you very much for joining us today, Professor.
TAPIO SCHNEIDER: Thank you for having me.
DIMITRI LASCARIS: And this is Dimitri Lascaris reporting for The Real News Network.