REFLECTIONS

REFLECTIONS

The last time I wrote about the balance of energy of the planet as a whole. There is some "effective" temperature at which the output of energy from the Earth balances the input of the energy from the Sun. While this "effective" temperature is an important concept for understanding the Earth's energy balance, it is not as important when we think about climate and climate change. We need to consider the temperature of the Earth in more detail, and, first and foremost, we need to consider the temperature of Earth at the surface. Where we live.

If we return to the idea of a budget that I talked about in the last blog, then the calculation of the energy budget at surface involves more processes than when we want to calculate the average balance of the whole planet. The way climate modelers approach the calculation of this budget is to consider the components of the Earth system. A useful level of granularity is depicted in the figure which shows the Sun and the Earth divided into four major components --the atmosphere, the ocean, land, and ice. Since clouds are so important to the climate, I have also explicitly labeled, surrounding the atmosphere, "cloud-world." To emphasize that we are concerned about the surface, I have placed a thin blue atmospheric layer on the top of the ocean, land, and ice. Climate modelers derive and develop the budget equations for each of these components.




Figure 1: Schematic of the Earth System, which shows the component models of a comprehensive climate model as well as the places that are most important to the reflection of the Sun's energy.


Also marked on the figure are the parts of the Earth system that are most important for reflecting energy from the planet. The Sun is the ultimate source of climate energy. The amount of energy provided by the Sun to the Earth's climate is determined by the total amount that arrives from the Sun take away the amount that is reflected. Clouds and ice are excellent reflectors. That is one reason that ice and snow are so important in maintaining a balanced climate --if ice and snow decrease the Earth will reflect away less of the Sun's energy. Hence, there will be more energy in the climate system--more heat, and more ice and snow will melt. (This is known as positive feedback--change amplifies change.)

Clouds are also important reflectors, and clouds are determined by the amount of water in the atmosphere and temperature. They are also closely related to weather systems, and in particular, air which moves upward. Clouds are difficult to model, and it is also difficult to determine how clouds will change with changing climate. Therefore, it is not as easy to understand the roles of clouds in the planetary energy balance as it is to understand the role of ice. If the atmosphere is warmer, it can hold more water vapor. It's reasonable to expect more clouds, hence increased reflection. Hence, this role of clouds might be to counter some of the warming by increasing reflection. This is, however, only one role that clouds play.

I have also marked in the figure that the atmosphere and land are important reflectors. The gases in the atmosphere scatter some of the Sun's energy back to space. The land is complicated. How the land reflects radiation is determined by what is on the land. If snow is on the land it reflects. If it is a black parking lot, it absorbs. This idea of white things reflecting and black things absorbing is something that people have known and used for centuries. The land is one of places where we change things. In "IPCC's Climate Change 2007" two effects of changing land use are noted. The first is changing the balance of reflection and absorption; the second is contributing to the increase of carbon dioxide.

So these are the most important pieces of how the Earth reflects energy from the Sun. The comments to some of the previous blogs mentioned that clouds balance out the heating. We will look at this more closely in the future, but the short version is that it only partially balances the warming. Other comments have mentioned the urban heat island effect is important. This has been accounted for in the determination of trends, and it has also been accounted for in the temperature observations. And Gaia... to me Gaia implies that there is something about the Earth system that maintains a balance that is comfortable to humans. I know of no reason this would be true--see evidence to the contrary, and personally would not rely on the notion.


If you want to find out more about coupled climate models, get model data, and even download a model here are links to two of the United States models used in the "IPCC's Climate Change 2007." .

National Center for Atmospheric Research Community Climate System Model
Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Climate Models



ricky