As a kid, I was always astonished by the clouds in the sky. With their unique shapes, that I, and others so it seemed, evermore tried to deduce particular shapes from. Their ability to float in the sky, like nothing else on Earth seemingly could, sparked an immense scientific interest in me. Especially considering that drops of water have a larger density that the surrounding gas. Clouds are formed as moist air moves upwards and becomes colder. As a result, the water vapor inside condenses to tiny water droplets. Clouds can form at altitudes ranging from just above the ground to very high in the atmosphere. Their distinct shapes are determined by the different nearby air flows that create them. Exactly because every cloud is so unique, it is incredibly difficult to analyze their effect on system Earth. They have an important, but rather mysterious impact on our planet. This blog post aims to uncover part of the mystery of how clouds will be affected by and affect climate change.
Clouds are one of the main drivers behind the uncertainty of climate sensitivity estimates. Computer models have a wide range on global warming, they predict an increase from 2 to 5 degrees Celsius globally. But this doesn’t really mean anything: these upper and lower bounds will have vastly different consequences for life on Earth. As a result, we would want to have a better estimate. But is there a way to fix the uncertainty originating from cloud simulations? Clouds are poorly simulated in any climate model. The impact of clouds on the climate differs largely depending on their composition, thickness, and height. This impact consists of a few different aspects. On the one hand, clouds reflect incoming sunlight back into space, more effectively than Earth’s surface. Because of that, the amount of sunlight actually reaching the Earth (to then be reflected and absorbed by the atmosphere) is reduced, cooling it. On the other hand, they themselves absorb the reflected light from Earth’s surface. Since clouds are colder than our planet’s surface, the light is then radiated away from the cloud at a lower energy than before. Unlike what you might expect, the amount of cloud cover itself will have only a minor impact on warming: if cloud cover decreases, more sunlight reaches and heats up the surface, but at the same time, more reflected light has the chance to escape and cool it down. Yet here, their elevation is significant. As clouds lie higher in the sky, their temperature decreases, and this cooling effect becomes stronger. If the average elevation of clouds increases, warming will likely accelerate, and vice versa. If, due to climate change and greenhouse gases, the vertical configuration of clouds in the sky changes, this must create a positive feedback. However, it is not entirely clear yet how strong this feedback will be. In any case, taking into account that together clouds cover more than half of the sky on a global average, this effect could be enormous.
The temperature of clouds influences the amount of light they radiate into space. In this picture, the darker regions represent colder clouds and vice versa.
Unfortunately, scientists find it very difficult to model the small-scale movements of clouds. There are a lot of uncertainties involved, sometimes even leading to competing outcomes. To simulate all small- and large-scale physical processes that form clouds, scientists will need a lot of money, time and a huge computing power. This is currently not feasible considering that most climate models employ data sets covering several decades. But it is essential to assess their climate impact: the three most important types of clouds, stratus, cirrus and cumulus, all have different properties dependent on their thickness and height, and all react differently to the incoming (reflected) sunlight.
The scientists Mark Zelinka, Kate Marvel and their colleagues have identified a few processes that help simulate the climate impact of clouds. First of all, it is likely that changes in cloud configurations will influence precipitation globally. Because warmer air holds more water vapor, regions that were already wet, will probably become wetter. In combination with that, what is dry will become drier, and clouds will concentrate towards the wet regions. Secondly, since temperature will rise quicker at the South and North Pole than at the equator, the global air circulation will change. It is expected that cloudy regions will shift from the equator towards the poles. Thirdly, they anticipate the troposphere to expand towards a higher altitude, which would result in an upwards movement of the tops of clouds as well. There is some evidence that clouds are indeed rising, which would mean that global warming will intensify. This is partly due to the presence of aerosols. Anthropogenic absorbing aerosols have a unique forcing distribution: even though they cool the atmosphere at the Earth’s surface, they heat it at its higher levels.
Sadly, these three general predictions do not remove the uncertainties of climate change estimates. And at this moment in time, it is essential to have exact numbers on what humanity awaits if no climate action is taken. Whenever a statement is uncertain, humans have a tendency to diminish it altogether. The expected positive feedback of clouds on the warming climate is rather unclear, and results in a high uncertainty in the overall climate sensitivity. This, combined with the other difficulties preventing the necessary global action, does not help the fight against climate change whatsoever.
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