Clouds and Climate Change

Keith Matanachai '22

Global Climate Models (GCM) are essential for predicting the general behavior of Earth’s climate by taking into account natural and human variables. These models contribute to policy-making decisions, allowing for the creation of rough time scales and analysis of the severity of alternative choices. And yet, a recent paper published in Nature Geosciences suggests that there is one crucial factor that many GCMs are inaccurately representing: clouds.

The specific type of cloud that Tapio Schneider and other authors of this study focused on were stratocumulus clouds. They form at relatively low altitudes (1,200 - 6,500 ft) and clump together to create flat sheets (Met Office, n.d.). Modeling and predicting the effects of climate change on stratocumulus clouds is difficult. Their behavior is subject to location, height, temperature, and many more variables, which can determine whether they cool or warm the earth’s surface. However, new research shows that if greenhouse gas emissions go past a tipping point, these clouds might disappear altogether, speeding up global warming.

One reason why scientists believe that clouds could provide more accurate GCMs is that current models, when running simulations of the past, cannot account for the sudden increase from 5 to 8°C during the Paleocene-Eocene Thermal Maximum (PETM) (Haynes & Hönisch, 2020).

“The PETM doesn’t only provide a past example of CO2-driven climate change; scientists say it also points to an unknown factor that has an outsize influence on Earth’s climate. When the planet got hot, it got really hot. Ancient warming episodes like the PETM were always far more extreme than theoretical models of the climate suggest they should have been. Even after accounting for differences in geography, ocean currents and vegetation during these past episodes, paleoclimatologists find that something big appears to be missing from their models — an X-factor whose wild swings leave no trace in the fossil record” (Wolchover, 2019).

Needless to say, accurately modeling the behavior of stratocumulus clouds could provide more insight into the PETM and the rate of temperature increase in the modern world. But what specifically about the behavior of these clouds makes them so difficult to model in the first place?

During the day, clouds can reflect the heat from the sun and cool the Earth. During the night, clouds can trap the residual heat radiating from the earth, warming it. Additionally, “wispy clouds high up in the atmosphere generally make an area warmer. Lower-altitude clouds tend to help an area cool off” (NASA, n.d.).

In general, these changes in cloud-climate feedback occur at a scale too small for current global climate models to account for (Schneider et al., 2019). Hence, these models generalize the behavior of stratocumulus clouds which decreases the accuracy of climate predictions.

Interestingly, these decreases in cloud-climate feedback accuracy have a significant effect on predicting global temperatures: stratocumulus clouds shade 20% of the world's oceans and reflect 30% to 60% of the sun's heat (Schneider et al., 2019). At this moment, they are expected to “cut the amount of energy reaching the Earth’s surface by 4 to 7 percent” (Pearce, 2020).

The research done by Schneider et al. provided more insight into the cloud-climate system stating that “once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred” (Schneider et al., 2019). In other words, there is a specific tipping point in CO2 levels where stratocumulus clouds will quickly disappear. Before they start forming again, it requires CO2 levels to drop down below their original concentration.

The breakup of stratocumulus clouds occurs as a function of two processes: longwave cooling and entrainment. At low atmospheric CO2 concentrations, clouds cool themselves and the earth by reflecting longwave radiation emitted by the sun back into space. This cooling effect makes the previously warm cloud colder and creates condensation which increases the moisture content and makes the cloud heavier. At this point, entrainment occurs where “warm and dry air” diffuses into the cloud, reversing the effects of cooling.

At higher concentrations of CO2, longwave cooling becomes less effective because the atmosphere is now at a higher temperature and so it is more difficult for clouds to reflect the radiation back into a heated atmosphere. Being unable to cool efficiently enough, the stratocumulus clouds break into cumulus clouds.

In order to model this complex behavior, the researchers used a large-eddy simulation (LES) of stratocumulus clouds. While most GCMs attempt to parameterize clouds by connecting stratocumulus behavior with larger patterns, LES focuses mostly on modeling clouds. Due to the specificity of LES, the researchers decided to focus on a particular type of subtropical environment for their simulations. What they found was that at CO2 levels of 400 parts per million (ppm), stratocumulus clouds formed relatively easily; however, at 1,300-1,600 ppm, the clouds broke up completely. For reference, the atmospheric CO2 levels in 2020 were 412.5 ppm ​​(Lindsey, 2020).

Their simulations also showed that “stratocumulus decks only reform once the CO2 levels drop below 300 ppm” (Schneider et al., 2019).

All in all, the LES model used by Tapio Schneider et al. highlights the importance of considering cloud dynamics as a part of climate change. He argues that some current GCM’s model up to 9,000 ppm of CO2, but fail to include any cloud dynamics which could significantly alter the predictions of temperature increase. It is important, now more than ever, that clouds are taken seriously since increasing our atmospheric CO2 from 412.5 ppm to 1,300 ppm could happen in the coming century (Meinshausen et al., 2011). As Schneider says in the conclusion of his research paper, “to be able to quantify more precisely at which CO2 level the stratocumulus instability occurs, how it interacts with large-scale dynamics and what its global effects are, it is imperative to improve the parameterizations of clouds and turbulence in climate models” (Schneider et al., 2019).

References

Haynes, L. L., & Hönisch, B. (2020). The seawater carbon inventory at the paleocene–eocene thermal maximum. Proceedings of the National Academy of Sciences, 117(39), 24088-24095. https://doi.org/10.1073/pnas.2003197117

Lindsey, R. (2020, August 14). Climate Change: Atmospheric Carbon Dioxide (E. Dlugokencky, Ed.). Climate.gov. Retrieved February 4, 2022, from https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide

Meinshausen, M., Smith, S. J., Calvin, K., Daniel, J. S., Kainuma, M. L. T., Lamarque, J.-F., Matsumoto, K., Montzka, S. A., Raper, S. C. B., Riahi, K., Thomson, A., Velders, G. J. M., & Van vuuren, D.p. P. (2011). The RCP greenhouse gas concentrations and their extensions from 1765 to 2300. Climatic Change, 109(1-2), 213-241. https://doi.org/10.1007/s10584-011-0156-z

Met Office. (n.d.). Stratocumulus clouds. Met Office. Retrieved February 4, 2022, from https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/low-level-clouds/stratocumulus

NASA. (n.d.). How Do Clouds Affect Earth's Climate? NASA Climate Kids. Retrieved February 4, 2022, from https://climatekids.nasa.gov/cloud-climate/

Pearce, F. (2020, February 5). Why Clouds Are the Key to New Troubling Projections on Warming. YaleEnvironment360. Retrieved February 4, 2022, from https://e360.yale.edu/features/why-clouds-are-the-key-to-new-troubling-projections-on-warming

Schneider, T., Kaul, C. M., & Pressel, K. G. (2019). Possible climate transitions from breakup of stratocumulus decks under greenhouse warming. Nature Geoscience, 12(3), 163-167. https://doi.org/10.1038/s41561-019-0310-1

Wolchover, N. (2019, February 25). A World Without Clouds. Quanta Magazine. https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/