The Journal Gazette
 
 
Tuesday, October 19, 2021 1:00 am

Nobel experiments

Physics winners helped in broadening our understanding of greenhouse effect

Christer Watson

The Nobel Prize for physics was announced a couple of weeks ago. I have always thought it a nice occasion to spend time describing what we understand better as a result of the Nobel Prize-honored work.

To make our way into the topic, I want to consider a trip I am taking this weekend. I will be driving down to Indianapolis next Sunday morning to meet some college friends. Being Sunday morning, the driving time is pretty easy to predict. No traffic means I just need to know the distance and how fast I'm willing to drive.

If instead I were leaving at, say, 4 p.m. Friday, predictions would be a bit more difficult. Rush hour could add quite a bit of time.

In this situation, leaving a little earlier could make a big difference in driving time if it meant avoiding that rush-hour traffic.

That situation is an example of chaotic behavior. A small change in one spot – leaving time – can create a bigger change in some other spot – driving time.

The Nobel Prize in physics this year honors three scientists who improved our understanding of the chaotic system that is the Earth's climate.

The first scientist was Syukura Manabe. His interest, in the 1960s and 1970s, was in describing in mathematical detail how the energy from sunlight flows from the top of our atmosphere, through the air, to the ground and how the Earth's infrared light flows back through the same air and escapes into space.

There are some interactions that make this problem difficult.

The sunlight energy flows down as standard, visible light that can easily pass through the air. When the Earth absorbs that energy and returns it as infrared light, that up-going infrared light is partly absorbed by water and carbon dioxide in the air. This is the greenhouse effect, and it makes the ground warmer than it would be otherwise.

The greenhouse effect depends on the water vapor and carbon dioxide in the air. It is complicated, unfortunately, because the amounts of water and carbon dioxide change.

For example, water from oceans and lakes slowly evaporates, increasing the water vapor. Rain reverses this, of course.

That evaporating and rain depend on the temperature. A higher temperature leads to more evaporation, which leads to more water vapor, more greenhouse effect and a high temperature, which leads to ... well, you get the idea.

Now imagine adding a little extra carbon dioxide into the atmosphere. That extra carbon dioxide will increase the greenhouse effect, leading to a higher temperature, more water evaporation and so on.

That is, a small change in carbon dioxide can lead to a bigger change in temperature because of this whole water vapor process. It is similar to the driving during rush hour traffic problem. A small change here creates a bigger change someplace else.

Manabe helped our understanding by making the first computer simulations that calculated this process throughout the atmosphere. One key step, for example, was making the computer simulation follow the water vapor as it was created on the ground but then rose through the atmosphere, changing temperature as it went.

The second scientist honored, Klaus Hasselmann, worked about 10 years later than Manabe. He, in part, aimed to solve the difficulty of using computer simulations such as Manabe's. Those computer simulations showed that a small change in carbon dioxide could, through the greenhouse effect and water vapor, lead to a bigger change in temperature. That means matching observations of carbon dioxide and observations of temperature was especially hard.

Hasselmann approached the problem from a different perspective. He developed mathematical tricks that looked for patterns in the computer simulations that were more subtle than just starting carbon dioxide and ending temperature. That is, he developed predictions for what patterns would exist in the observations if a change were caused by increasing carbon dioxide or something else, such as volcanic eruptions or changes in solar radiation.

The results of his predictions are, of course, quite well known. Increased carbon dioxide is leading to a warmer climate, along with other climate changes.

It is a little unfortunate that the specific work here is so mathematical that it cannot be described beyond a rough level. This description leaves out some of the beauty and insight of the scientists.

Regardless of that limitation, it is nice to appreciate the progress. We now understand better the very complicated thing that is Earth's atmosphere and the energy flow through it.

 

Christer Watson, of Fort Wayne, is a visiting assistant professor of physics at Purdue University Fort Wayne. Opinions expressed are his own. He wrote this for The Journal Gazette, where his columns normally appear the first and third Tuesday of each month.


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