The uncloudy Cretaceous

The planetary albedo (the percentage of the incoming solar radiation that is reflected back to space) plays a large role in the Earth’s radiative balance. The planet currently reflects about 30% of the incoming radiation back to space (clouds, particles in the atmosphere, ice sheets). If the albedo of the planet were 0 (all of it were absorbed) the planet would be over 20 degrees C hotter.

It is very difficult to quantify the effect that aerosols (acting as Cloud Condensation Nuclei) have on cloud albedo. Increased CCN should “brighten” clouds, that is, increase their albedo (for more on the importance of CCN see my last post). Fewer CCN means optically thinner, lower-albedo, and shorter-lived clouds, which should warm the planet by decreasing the amount of incoming sunlight that is immediately reflected back to space before it can interact thermodynamically with the climate system.

During the Cretaceous (~100 mya) temperatures were considerably hotter than today, being around 35 C (nearly 100 F) in the tropics, and the polar latitudes featured well above-freezing winters. Warm climates like that of the Cretaceous tend to have smaller pole-equator temperature gradients than the present, while colder climates like that of the LGM have much larger temperature gradients, what many call ‘polar amplification.’ It is fairly well known that a warming climate (such as one we are getting as we approach a doubling of carbon dioxide) features the poles warming more than the low latitudes. It is believed that CO2 levels were around 4 times what they are today during the supergreenhouse of the Cretaceous, a time when palm trees were in Canada, and crocodiles on Hudson Bay, and at least one published estimate of Arctic waters over 20 C. For those who argue that negative feedbacks are going to save us from an increase in CO2 in modern times, the Cretaceous is not a supporting paleoclimatic example. In fact, temperatures from proxy records are even higher than one would expect from a quadrupling of carbon dioxide. There is still a lot of confusion that the Cretaceous causes amongst scientists today. The very low temperature gradient is one example.

More question marks arose by a paper by André Bornemann and others suggesting that there in fact was some Antarctic ice during the Cretaceous time based on excellently preserved fossils showing that the geochemistry of the ocean had changed in the same way as it should if large scale glaciation takes place. Ray Pierrehumbert has good insight into some of the questions and interesting points going on during this time period, here. But no doubt it was very hot (and we also can’t talk about the Cretaceous as having just one set of conditions since it represents some 60 million years of geological time). Now, in the latest edition of Science, Kump and Pollard provide more insight that can assist CO2 in explaining the steamy conditions– albedo change from decreased CCN caused by biological impacts.

According to the study, reduction in Cretaceous cloud cover stemmed from a drop in cloud condensation nuclei which further amplified temperatures. If CO2-induced warming during the supergreenhouse reduced global biological productivity by temperature stress, and fewer nutrients to rise up to feed algae in the warm surface waters, then this could have resulted in less CCN and clouds, and thus a lower planetary albedo. Kump and Pollard use a GCM to explore the hypothesis, increasing CO2 from 1x to 4x preindustrial atmospheric level and both fail to account for the dramatic warmth (fig 1 a and b below) while 4x CO2 and increases in Cloud droplet radii and precipitation efficiency (leading to fig 1 c) would reduce planetary albedo from about 0.3 to 0.24, substantially enhancing temperatures, and a bit more in line with proxy evidence for paleotemperatures at the time.

Kump and Pollard, Science 2008

Naturally occurring or anthropogenically induced particles that promote condensation or deposition are called nuclei, and those greater than about a micron favor condensation/deposition of water vapor. Once condensation/deposition starts they become comparably sized cloud droplets, or ice crystals. With about 10 to 100-fold global reductions in past aerosol (and so CCN amounts) large increases of droplet radii (to 17 microns maybe) are possible. Decreases of aerosol concentration should lead to fewer and larger droplets, and increased rate at which cloud water is converted to precipitation (larger cloud droplets means faster conversion to precipitation and reduced lifetime of the cloud). In contrast, increases in aerosol concentrations increase the amount of low-level cloudiness through a reduction in drizzle, which regulates the liquid water content and the energetics of shallow marine clouds, and produce brighter clouds that are less efficient at releasing precipitation. This is because large concentrations of small CCN nucleate many small droplets, which coalesce inefficiently into raindrops.

The Cretaceous cloud-to-precipitation conversion rate Kump and Pollard used in the GCM was Pe ~ re3 (precip. efficiency and cloud droplet radii), with a ~30% increase in re, and an increase in Pe by about 2.2x.

In the SOM is a sensitivity experiment for radiative versus precipitation efficiency effects in the modern world. Here, re effects cloud radiative properties (squares) and radiative properties and precipitation efficiency (triangles), and the circle is a modern GCM control with no changes to re.

A 10% reduction in cloud cover, from the decreased CCN concentrations, was caused by the biological impacts from the very high CO2 concentrations. Andreae (2007) explored the sources of CCN before the human era. There are many factors involved in CCN including primary biogenic aerosols (plant particles, spores, microbes, etc.), sources from sea spray and fires, and secondary organic aerosols (from natural hydrocarbons), as shown in the below diagram

Andreae 2007

Humans play a large role in aerosol concentrations in modern times, but in the prehuman days concentrations ranged from a few tens per cm3 in biogenically inactive regions or seasons to a few hundreds per cm3 under biologically active conditions, with similar conditions over the continents and the oceans. If biological productivity of CCN goes down, as Kump and Pollard suggest happened in the high CO2 world, there may have been less CCN and so changes in cloud cover.

Implications for modern times

In modern times, we know that anthropogenic emissions of aerosols have a strong cooling effect (by reflecting solar radiation back to space) and have acted to offset some of the greenhouse warming (Global Dimming), and aerosol rise was partially responsible for a slight cooling trend between 1940-1970 when CO2 was not yet high enough to overwhelm that influence, and environmental regulations in the seventies caused a decline in emissions. Aerosols also modify clouds, and potentially this indirect aerosol effect is much greater than the direct effect of inhibiting solar radiation to reach the surface.

However, separating “natural” from human-induced aerosol amounts is not trivial. The radiative effects of aerosols are complicated because of the direct, and indirect impacts as well as there complicated vertical changes. The “single scattering albedo” is the fraction of energy that a particle scatters compared to the amount that it both scatters and absorbs. For single scattering albedo greater than ~0.89 the aerosol particles will cool at top-of-atmosphere and below 0.89, warm. Any single scattering below 1 means that they warm at the level they reside. Aerosol always cool the surface (Black Carbon has a warming effect though), warm (if they absorb) at the altitude where they reside, and either cool or warm at the TOA.

For potential implications of this paper to modern times, we’re interested in how cloud cover changes in a warming climate, and how biological production of CCN might change. Climate models and observations suggest that cloud feedbacks may be neutral to strongly positive, and if they are negative it won’t be very strong, because observations indicate that low level clouds (which control the albedo more than any other kind) decline in a warmer climate. This is due to various physical processes like increases in precipitation efficiency or decreases in cloud physical extent with temperature. The effects of changes in CCN from changes in aerosol emissions would play a role (if aerosols decline, so should the negative radiative forcing in their direct/indirect impacts). Changes in CCN from responses in land/marine organic life are another interesting factor, and I don’t know of much work that has examined this. If the “background” CCN changes, than so should albedo because of changing cloud cover.

Speaking purely on speculation and non-expert opinion, I do not think the changes in ecology would play a very influential role in the planet’s radiative balance, at least until much different CO2 levels beyond a doubling or even tripling. The responses that shape the carbon cycle would probably play a larger role in determining the rate of change of CO2 concentration in the atmosphere. But, this needs to be further investigated, and the role of all feedbacks into models. I think the main story from the Cretaceous is that there may be some substantial positive feedback lurking in the climate system, from both physical and biological responses. It may not be wise to be playing experiments with greenhouse gases and aerosols.


5 responses to “The uncloudy Cretaceous

  1. Very nice post. I was wondering whether I’d get an understandable rendering of the topic when I first saw the news article.

    We’re already seeing some increased stratification in the water column of the oceans (the expansion of ocean deserts), which would, hypothetically, lead to less cloud cover as there was less organic productivity. I wonder whether this positive feedback will indeed play a role in the modern warming.

  2. It is also important to consider the location of cloud cover in the future since their location determines their effect on planetary albedo.
    We do know that a warmer climate will induce changes in clouds, although the net effect is largely unknown due to the tremendous complexity of the climatic system, not to mention the feedback processes you mentioned concerning a decrease in CCN:
    “If CO2-induced warming during the supergreenhouse reduced global biological productivity by temperature stress, and fewer nutrients to rise up to feed algae in the warm surface waters, then this could have resulted in less CCN and clouds, and thus a lower planetary albedo.”

    We know that clouds have two competing effects on climate:
    1) They reflect solar radiation, increasing planetary albedo and this having a cooling effect.
    2)they trap IR radiation, having a warming effect.

    The cooling effect will be largest where the surface albedo is low, since a lot of solar energy would be absorbed if clouds were not present. In contrast, the cooling effect will small where the surface albedo is high (for example ice or snow cover), since most of the solar energy would have been reflected through the atmosphere back to space anyway.

    The warming effect will be largest for high clouds, since they will be the coldest and will emit the lest IR to space while still trapping the same amount from below.

    The net effect therefore depends on where clouds occur, when they occur, and how high they are. Any of these things could change as climate changes, as well as the property of the clouds themselves, such as their solar reflectivity or geometry.

    Below I outline some of the way in which changes in cloud distribution and properties can affect the equilibrium climatic response to an initial heating perturbation. If the changes listed below occur in response to an initial warming, then those changes that by themselves cause warming constitute a positive feedback, while those changes which, by themselves cause cooling, constitute a negative feedback:

    Increase in amount of cloud: causes cooling effect
    Increase in amount of high cloud: causes warming effect
    Increase in cloud height: warming
    Increase in water content of stratus clouds: cooling
    increase in water content in cirrus clouds: warming
    Increase in ratio of water droplets to ice crystals: cooling through enhanced reflectivity.
    Increase of size of water droplets and ice crystals: warming

    The directions of the cloud changes listed above are given for illustrative purposes only, I admit I could be wrong with these since it is not my field of specialization. Also, the direction of change that would occur in reality is unknown. For example, low cloud amount might decrease rather than increase in some regions as climate warms, in which case the effect would be one of warming rather than cooling. In other cases, the direction of change is known, but the magnitude still uncertain. For example, there are good reasons to believe that average cloud heights will increase, that cloud cover water content will increase, and that the ratio of water to ice droplets will increase as the climate warms.

    The details of the above listing are not important here. The main point of the above table is to illustrate the incredible complexity of cloud radiative effects. Because of this complexity and the large number of competing feedbacks, it is extremely difficult to reliably compute the net effect of cloud changes. The net effect of all of the cloud changes listed above could be either positive feedback – amplifying the warming – or a negative effect, diminishing the warming.

    For this reason, I agree with you that “the changes in ecology would play a very influential role in the planet’s radiative balance”, because the effects they would have of CCN would further complicate the behaviour of clouds – there are so many ways that clouds could influence the climate that it is likely that most would cancel out, leaving only a small net effect on climate change. As you mention, the carbon cycle plays the largest role in anthropogenic climate change.

  3. I quotes you wrongly on my last paragraph, thus changing the meaning of you said and of what I intended to say. I apologize.
    I meant to quote:
    “I do not think the changes in ecology would play a very influential role in the planet’s radiative balance”
    My apologies again


  4. I also apologize for the numerous typos, I was hasty on posting a comment.

  5. Hi! I was surfing and found your blog post… nice! I love your blog. 🙂 Cheers! Sandra. R.

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