I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.
By: Dr. Ricky Rood , 6:03 AM GMT on August 23, 2012
Point of View: Models, Water, and Temperature (6)
This is a series of blogs on models, water, and temperature (see Intro). I am starting with models. In this series, I am trying to develop a way to build a foundation for nonscientists to feel comfortable about models and their use in scientific investigation. I expect to get some feedback on how to do this better from the comments. In order to keep a solid climate theme, I am going to have two sections to the entries. One section will be on models, and the other will be on a research result, new or old, that I think is of particular interest.
Doing Science with Models 1.3: In the previous entry of this series I used the example of balancing a monthly checking account to make the point that studying the Earth’s climate is very much like balancing a budget. Rather than money, we calculate a budget of energy.
Energy is one of the attributes used by scientists to describe the physical world, and it is a basic law of classical physics that energy is conserved. There are the laws of conservation of energy, conservation of mass, and conservation of momentum. Momentum describes how an object is moving: its mass, its speed, and its direction.
I introduced the concept of making a mathematical representation of the real world with this equation for money
Today’s Money = Yesterday’s Money + Money I Get – Money I Spend
and I came to point where I said we have a similar equation for energy
Earth’s Energy Today = Earth’s Energy Yesterday + Energy Gained – Energy Lost
These equations are the most basic models for the process that they describe. In fact, these equations could be said to be the perfect model for your personal budget of money or the Earth’s budget of energy. In the jargon of the scientist who builds models, this perfect model is often called the “analytic” model because it can be solved exactly, or analytically, by arithmetic.
The next idea I want to introduce is point of view. In the first instance, above, the equation represents a personal budget. In the second instance, the equation represents the energy budget of the whole Earth. Recall in the previous entry when I set up the problem of looking at the Earth’s energy, I said to imagine a person not on Earth, but who is observing the Earth. The observer, perhaps on Mars, sees the Earth as a small dot with energy coming in from the Sun, which the Earth then emits back to space from the Earth. If the Earth is in an energy balance, then the amount on energy coming back to space equals that coming in from the Sun.
That’s interesting to think about for a minute. Let’s assume that the Sun is constant. Then if the Earth is in an energy balance, the energy coming back to space is the same no matter the amount of carbon dioxide in the atmosphere. So to the person on Mars, the Earth would look the same. But the conditions on Earth might be quite different if the atmosphere had 600 rather than 300 molecules of carbon dioxide per every million molecules of air. This is because the point of view that we are interested in is from the surface of the Earth.
In 2010 I had a series of blogs called Bumps and Wiggles (here, go back and give it some “likes”). In the third of that series, I introduced Simple Earth. Here is that figure, which is described more completely in the original blog.
Figure 1: Simple Earth 1: Some basic ingredients of the Earth’s climate.
The problem of climate and climate change is important because of our point of view. If we are to continue to build thriving economies in our societies, we need a stable climate. In this case, stable really means that we know what to expect. Therefore, the climate of the Earth that might be of interest to that person sitting on Mars is not especially relevant to the person sitting on the surface of the Earth. Therefore, we need to think of models that are from the point of view of the person on the surface of the Earth. Again, energy and the law of conservation of energy come to the forefront.
In the figure, if the stick man looks around there is energy everywhere. It comes as heat from the Sun. It comes as wind from the air. It comes as waves from the sea. It comes as food from the land. So the accounting problem becomes more complex. We need the budget of energy for the atmosphere, the oceans, the land, and the glaciers and ice sheets. This energy needs to be balanced with what comes in from the Sun and what goes back to space. It is the same simple-to-conceive classical physics, but since we are in the middle of it all, the problem becomes complex. Still though, it is only a matter of balancing the books.
Interesting Research: Warming and Cooling in Ice Sheets - I’m usually not the blogger reporting on the most recent papers and breaking research, but this week I am different. The paper is Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history, which was published online on August 22, 2012 in Nature. Robert Mulvaney is the senior author. The press take on this paper is that ice-core data show that over the previous, approximately, 12,000 years (the Holocene), there have been a number of times when there has been warming on James Ross Island, an island off the Antarctic Peninsula. These periods of warming have been comparable to the warming observed in the last 50 years, and hence, there are examples of warming that are not caused by the recent increases in carbon dioxide. There are scientific and political consequences of this paper. I will try to think like a scientist.
What does this paper say about generalized warming of the planet due to green house gases? First, we have to look at the locality of the data. It is from a single small island, in a part of the world that is known to have substantial fluctuations of temperature. We then need to look at how this knowledge fits in with the body of evidence as a whole. For example, Mulvaney and coauthors found a prominent warming period about 600 years ago. Was this warming at James Ross Island accompanied by warming of the same global extent as the currently observed warming? Are there other existing data that suggest natural internal variability during these previous times of warming? Is there something different in the past 50 years that distinguishes the current warming from the previous times of warming? The list goes on. So this result needs to be placed in context of all of the data and knowledge, and the coherence of this new information with the existing information needs to be evaluated.
This paper highlights the difficulty of extracting the contribution of warming due to carbon dioxide increase for any particular event. Ages ago, I had a blog on the breakup of the Larsen Ice Shelf. This new result makes the easy attribution of that ice-shelf collapse to human-caused warming difficult. As above, that attribution problem requires looking at the ice- shelf collapse in concert with other information. Was the event isolated? Is there evidence of other causes of variability? Is there something now that is different from the past? One attribution question that I can see – can the extra warming from carbon dioxide push the melting of the ice shelf over a tipping point?
Finally, I will bring us back to models, as models are ubiquitous in climate science and science in general. Within this paper, a glaciological model is used to determine the age scale. This model represents the flow of ice in the glacier, and that flow is assumed to remain constant over the time of the study. Another place that a model is used is determining the temperature based on the observations of isotopes of oxygen. This requires a melding of theory and application. Therefore, when you say, “but the observations show …” remember the role of models in making those observations.
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