Radiation mainly shapes temperature changes across continents, but evaporation and turbulent heat transfer also play their part. These are inherently complex processes. Following a new physical approach, the observed temperature patterns follow relatively simple and predictable rules.
A new study published in Proceedings of the National Academy of Sciences of scientists from the Max-Planck-Institute for Biogeochemistry in Jena, Germany, used a novel method to determine the contribution of evaporation and turbulent heat transfer to temperatures across continents. Earth temperatures reflect the balance of heating and cooling. The surface is heated by solar radiation and by radiation emitted by the atmosphere, the latter also known as the atmospheric greenhouse effect. This warming is balanced by cooling from radiation emission, water evaporation, and by the transfer of heat to the atmosphere, which is accomplished by convective, turbulent motion.
While radiation is well understood and commonly observed, the extent to which evaporation and motion cool the surface is controlled by many factors, and semi-empirical methods are often used to describe it.
The authors take a different approach to describing these complex processes, focusing on basic physics: An electrical source is needed to drive motion, similar to the way an engine drives a car. In the case of the atmosphere, surface heating drives motion. But the consequences of the move must also be considered.
Sarosh Alam Ghausi, lead author of the study, explained, “The more you move, the more you cool the face. It’s like blowing on hot soup—the more you blow, the faster it cools.” But this cooling, in turn, makes the power generation process less efficient, resulting in a reduction in maximum power. This maximum can be calculated and used to quantify the cooling effects of evaporation and movement of the earth’s surface temperature.
The researchers used satellite-derived radiation data and applied maximum power mathematical methods to estimate the rates of warming and cooling of the continents and seasons. With this, they predicted temperatures, evaporation, and heat changes that closely matched observations. They then used this method to understand why temperatures varied across the continents as they did, looking specifically at the role played by the availability of water. But they did not see what they expected.
“I thought the lack of water would heat up the deserts,” said Ghausi, with his background as a hydrologist. “But we found that maximizing power is more important than the lack of water. The lost water is compensated in such a way that more heat is transferred to the atmosphere.”
The warmer temperatures in deserts are attributed to two effects: Deserts have fewer clouds, so more sunlight warms the surface more strongly than in rainforests. The second effect is less important: deserts are usually located in the subtropics, where the atmosphere transports heat horizontally through the so-called Hadley circulation. This heat is not added to the surface where it drives the engine for movement, but to the atmosphere above.
This makes the surface power generation process less efficient, resulting in less cooling and a hotter surface. With these two factors the authors were able to explain the temperature differences from rainforests to deserts.
Erwin Zehe, professor of hydrology at the Karlsruhe Institute of Technology and co-author of the study sees great potential in this method. “Our findings are very surprising, because usually evaporation is considered to be the key to cooling the environment. And I think that this method can really move things forward by creating a new gold standard, improving current empirical methods for modeling evaporation.”
Axel Kleidon, group leader at the Max-Planck-Institute for Biogeochemistry, and senior author of the study, interprets these results in a more general way. “It is not very clear why it is so simple, but the physical method works so well. One way to understand it is that these processes are so complex that the last limit they encounter is in the physics of power generation .”
The authors hope that they can apply their method to more widely identify the fundamental mechanisms that shape the climate around us and how they respond to global warming.
Sarosh Alam Ghausi et al, Radiative controls by clouds and thermodynamics shape surface temperature and turbulent flows over land, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2220400120
Provided by the Max Planck Society
Citation: Study shows that earth’s surface temperature follows simple physics, probably due to the complexity of the processes involved (2023, July 13) retrieved 13 July 2023 from https://phys.org/news/2023 -07-surface-temperatures-simple-physics- because.html
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