Feedbacks, Sensitivity, and Practical application

The global, annual-mean surface temperature is the most widely used measure of climate change. In particular, scientists are very interested in how the globally averaged temperature will respond as a function of changed amounts of greenhouse gases in the atmosphere, changed amount of solar intensity, etc. The term “climate sensitivity” refers to how much temperature change the planet experiences from a given “forcing.” A forcing is an imposed change of the planet’s energy balance with space.

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On the Arctic sea ice

Sea ice extent in the Northern Hemisphere has exhibited large and anomalous declining trends over the last several decades. In particular, there has been over a 20% decline since 1979. Linear trends in arctic sea-ice extent since 1979 are negative in every month.Recently, there has been particular interest recently over a record-breaking year in 2007 which flew by the second-lowest year in 2005. There also has been a foot-race this year, which has kept me particularly interested over the last few weeks. For a while, it seemed that 2008 would clearly not surpass 2007, but due to the drop over the last few weeks, that may not be the case (although it probably will be). Sea ice extent as of September 7, 2008 is 4,739,844 km², while 2007 minima reached 4,267,656 km² on September 16th last year.

Is the atmosphere drying up?

A recent set of posts at Anthony Watt’s blog, particularly this one has sparked some interest over the internet as of late. From a quick glance, it looks like negative trends in specific humidity over the last half a century. Readers were quick to pick up on the connection to water vapor feedback, which is expected to at least double the sensitivity of climate to external perturbations (e.g., by human released CO2).

Physics of the Greenhouse Effect Pt 2

In the preceding post, on the greenhouse effect, I investigated the role of the greenhouse effect and its play in radiative balance, and how the presence of an atmosphere acts to raise planetary temperatures. The take home points should be that for a planet with no infrared absorbing layer above the surface, the fourth power of the surface temperature always approximates a value determined by the incoming solar radiation. The only way the surface temperatures can exceed this value is if there is an atmosphere which acts to be a blanket to outgoing radiation. A planet can also be heated by internal processes such as radioactive decay or rigorous convections from the mantle, but these are rather negligible on the terrestrial planets. Adding greenhouse gases to an atmosphere whose temperature decreases with height must act to warm the surface by making the net downward emission greater than zero. In this post, I will elaborate on specific greenhouse gases, the runaway greenhouse effect, and an antigreenhouse effect.

Physics of the Greenhouse Effect Pt 1

Jean Baptise-Fourier is generally credited with the discovery of a greenhouse effect, which involves the process by which the presence of an atmosphere acts to raise the surface temperature of a planet. This was extremely simplified at the time, and the term greenhouse never appears in his 1827 writing, but he did establish the effect that the atmosphere had on incoming light and outgoing infrared (heat) radiation, and that some heat was absorbed by the atmosphere which was opaque in the infrared but transparent to incoming solar energy. A copy of his essay, translated by R.T. Pierrehumbert can be found here. We’ve made a lot of progress since then, as Svante Arrhenius began to quantify the phenomenon nearly 75 years later, the work of Stefan and Boltzmann established the relationship between an object’s temperature and its outgoing radiation, the role of convection and water vapor and clouds turn out to be important in more complex models developed later, etc. The pionerring paper by Arrhenius, entitled On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground, which is the famous 1896 piece, began to investigate what the effects of doubling the atmospheric CO2 content would be. At this time, most of the interest in the subject was in solving the mystery of the coming and going of ice ages. Like most pioneering efforts, Fourier or Arrhenius did not have the last word, and we still have much to learn today, but they provided a big leap in how we understand planetary temperatures and the role of the atmosphere in radiative balance. Fourier was one of the first to speculate that human activities could influence climate, and such topics are rather important in modern times.

Basic Radiative models/Earth’s climate system analysis Pt. 3

From parts One and Two, we’ve discussed the TOA energy balance as well as the role of the greenhouse effect, and what happens with the addition of more greenhouse gases. In the final piece of this series, I’ll discuss the surface energy budget, and simple principles of atmospheric circulation.

Basic Radiative models/Earth’s climate system analysis Pt. 2

From Part 1 we should be able to calculate the energy balance of a planet, and should be able to calculate the equilibrium blackbody temperature of an isothermal spherical zero-albedo planet, as a function of distance from a sun having a given photospheric temperature (the outer layer of the sun).

Basic Radiative models/Earth’s climate system analysis Pt. 1

I thought I would work backwards a bit and go over some of the basics of radiative transfer/balance in the climate system, and how daily weather systems work. The post is rather broad, and (hopefully) easy-to-read version of what factors determine the Earth’s climate.

The Scientific Basis for Anthropogenic Climate Change

The Scientific Basis for Anthropogenic Climate Change

Note* The graphs that appear here can be clicked for enhanced versions

Climate Science can be a bit like detective work when dealing with an issue such as global warming. First of all, we have a “detection and attribution” process where we spot a problem, and then try to find a suspect. Like they do on CSI, you find a body, and then go and find evidence to put blame on someone or something. Right now, the problem is the globe is warming, and it appears that anthropogenic CO2 plays a large part in this trend.