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Blackness in the Air - Bumps and Wiggles (7)

By: Dr. Ricky Rood, 4:36 AM GMT on June 02, 2010

Blackness in the Air - Bumps and Wiggles (7):

Introduction: This is the seventh in a series on understanding climate variability, global warming, and what we might do about it. The series focuses on the past 30 years and the next 30 years.

This series started from the perspective that if we take a broad look at the observations and projections of warming of the Earth’s surface, then we see observations, theory, and validation that substantiate the fundamental base of knowledge about the Earth’s climate and how it will change. If we look a little closer, then we start to see some discrepancies. From the point of view of the scientist, these discrepancies are what guides future investigation. From the point of view of the “politician,” these discrepancies provide the foothold on which to base arguments of disagreement, and indeed, to fuel selective doubt and perhaps even discredit the scientists and the body of knowledge. From the point of view of the informed and interested, the discrepancies are the points of discussion, the seeds of interest, and something that needs to be understood.

The blogs that preceded this one started from the notion that the warming that has been measured thus far at the Earth’s surface is not as large as it should be for the amount of carbon dioxide that is in the atmosphere. So digging into this problem a bit, I have tried to say we need to look at the details of our prediction in light of information that is directly relevant to the here and now. (Recall, the simulations were never imagined for this purpose!) So far we have followed some heat into the ocean, worried a little about aerosols (those annoying “particles” of stuff in the atmosphere), and last time, about some simple and complex responses of water vapor to a warming at the Earth’s surface. (From a comment or two I read, I did not do such a good job on explaining the transport of water vapor to the stratosphere. Sorry. To do that well would require several blogs, or one of my compelling lectures, and that just seems too tedious.)

This blog I want to continue down the path of things that change the radiative balance of the atmosphere. That is, things that we are doing to the atmosphere, and things other than carbon dioxide. And, in particular, I want to talk about black carbon. (I refer to an excellent, requires you to know a lot of jargon, article in Nature Geoscience by Ramanathan and Carmichael (a summary.))

Black carbon is the name given to black, or perhaps better dark, particles that are in the atmosphere. These particles are primarily the products of combustion. For many, think soot from diesel engines. “Black” is important because "black" means absorbs solar radiation. For those in Tallahassee, think walking barefoot across the parking lot of the Publix on Ocala on July 4th to buy a watermelon. (Is the old ice house still around in Tallahassee?) OK, seriously.

Black carbon, after carbon dioxide, competes with methane as the most important pollutant that warms the planet. But it is complicated. Black carbon is suspended in the atmosphere, so it warms above the surface. Because the sunlight is absorbed above the surface, there is a net cooling at the surface, below the black carbon. So that warms the planet, and suppresses some of the warming at the surface.

This warming effect of the black carbon particles (that is, aerosols) is opposite of what we usually think about aerosols. Usually we think of aerosols, such as the sulfate aerosols from volcanoes (earlier in this series) as reflecting sunlight and cooling. Also aerosols have an impact on clouds that is a cooling effect. So, black carbon means that the simple construct “aerosols cool” to be untrue.

Here is a picture that conveys this notion of cooling and warming aerosols. It is from a presentation in my class.

Figure 1: Warming associated with black carbon is in contrast to the cooling effect usually associated with black carbon. This figure follows from the Institute for Governance and Sustainable Development’s material on the role of managing black carbon in the environment.

Black carbon has another impact on the warming of the planet that is important. It falls on the snow and ice, darkening the snow and ice. In this role, black carbon is a great accelerator of melting. (Regular “dust” associated with dust storms and plowing also helps to accelerate melting of ice.) In this case, as we look at the amount of warming associated with carbon dioxide and the amount of melting of glaciers and snow on land, we need to disentangle the impact of black carbon and the impact of greenhouse gas warming. Again, this brings attention to needing to examine, more carefully, the role of all of the factors that are causing big changes to the planet.

Another characteristic of black carbon is that it is a signal of very bad air quality. Diesel motors are a source of black carbon, as is “biomass” burning. Cook stoves are a huge source of black carbon, especially in places, like India, where many people continue to cook with traditional fuels on traditional stoves. The entanglement of air quality and greenhouse-gas-caused climate change is complicated, with many instances of improving air quality accelerating the rate of warming. (Think of the relation between coal, carbon dioxide, and coal.) This is not the case of black carbon, which leads to warming (in the atmosphere) and accelerated melting of ice. Another attribute of black carbon and “air quality” is that the compounds that cause “air pollution,” in the traditional sense of Los Angeles in 1968 – smog, are relatively easy to control, regional, only in the atmosphere a few weeks, and more politically appealing to address than carbon dioxide. (This is supposed to suggest near-term ideas on managing warming and geo-engineering – but that is the next series.)

It is worth noting that black carbon is NOT part of the portfolio of pollutants mentioned in the original Kyoto Protocol, which does, still, guide the evolving carbon markets.

For the purpose of this series of blogs, I am conveying the importance of understanding the bumps and wiggles of the current observations and addressing the question of “how does the observed warming compare with the projected warming?” Black carbon is a piece of the puzzle that is important and complicated. It directly impacts the “radiative forcing.” Earlier we followed the heat into the ocean and a cooling effect of water vapor into the atmosphere. There is nothing simple to say about black carbon and how it impacts the current bumps and wiggles, but it is obviously important. (We will need to go back to ocean heating with this new result.)

I am ending this with a complex figure from the 2007 IPCC report as used by some of my students exploring the near-term solutions. This is a version of a picture that has been around for years. It is used by many as the starting point of the climate change discussion, but I have always felt that if you jumped in here, you lost most of your audience. I will offer a brief description. Contributions to the change in the radiative forcing of the Earth’s climate are plotted. This change is in comparison to something prior to the industrial revolution. For example, the carbon dioxide part of the figure is how much more carbon dioxide is warming the planet than in, say, 1750. Reading the plot, the change is about 1.5 watts/meter**2 … that is, watts (energy) per unit area of the Earth. This is 1.5 compared to a total of about 350 – small. (But last time, I talked about the big effect of small things.) Black carbon and methane are each about the same, and second most important in the CHANGE of warming. (Note there is also a cooling component of methane.) Most of the things that act to cool the planet are associated with aerosols and their impact on clouds. That’s enough; this is a picture of how all the pieces play together.

Figure 2: From IPCC 2007: Borrowed from my students: Changes in the energy budget of the Earth due to pollution and changes of the atmosphere and the surface and the sun. Marked up to point out the role of aerosols. See text.


Bumps and Wiggles (1): Predictions and Projections

Bumps and Wiggles (2): Some Jobs for Models and Modelers (Sun and Ocean)

Bumps and Wiggles (3): Simple Earth

Bumps and Wiggles (4): Volcanoes and Long Cycles

Bumps and Wiggles (5): Still Following the Heat

Bumps and Wiggles (6): Water, Water, Everywhere

And here is

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