1994: The Mount Pinatubo Eruption: Impact on Climate Change

Benjamin Franklin wrote, in May 1784:

During several of the summer months of the year 1783, when the effects of the sun's rays to heat the earth in these northern regions should have been the greatest, there existed a constant fog over all Europe, and great part of North America. This fog was of a permanent nature; it was dry, and the rays of the sun seemed to have little effect tawards dissipating it, as they easily do a moist fog, arising from the water. They were, indeed, rendered so faint in passing through it, that, when collected in the focus of a burning glass, they would scarce kindle brown paper. Of course, their summer effect in heating the earth was exceedingly diminished

Hence, the surface was early frozen. Hence, the first snows remained on it unmelted and received continual additions. Hence perhaps the winter of 1783-4 was more severe than any that happened for many years.

Franklin assumed that the persistent "dry fog" was probably fine particles in the atmosphere arising from either a large volcano or explosion of a comet in the earth's atmosphere ["...smoke proceeding from the consumption by fire of some of those great burning balls or globes which we meet with in our course round the sun, and which are sometimes seen to kindle and be destroyed in passing our atmosphere, and whose smoke might be attracted and retained in our earth...]. We are now confident that this "dry fog" was a mist of fine sulfuric acid particles resulting from a large volcanic eruption in Iceland.

Franklin's discussion is the product of a brilliant inquisitive mind and his personal observations, such as trying to kindle brown paper with a burning-glass. His discussion includes the basic concept of what we call a "climate forcing", an imposed change to the earth's radiation balance which would tend to alter the earth's temperature --- in this case a reduction of solar heating of the earth because of scattering of sunlight back to space by the fine mist, or aerosols, in the earth's atmosphere. His discussion also includes the concept of "climate feedbacks", a response of the climate system to a forcing which can amplify or diminish the effect of the forcing -- in this case increasing snow cover further increases reflection of sunlight, a "positive" feedback.

However brilliant Franklin's speculations and analyses, he could not prove them or develop further inferences about future climate change, because he did not have the data, sufficiently comprehensive theory, and computing capabilities that are needed to quantify his ideas. You can imagine how excited he would be to have the precise global satellite measurements of atmospheric aerosols, such as those we have after the eruption of the large volcano Pinatubo in 1991 -- and the incredible computing power of a modem "workstation" computer, which can perform millions of calculations in one second, thus allowing a proper solution on a global scale of fundamental equations, such as conservation of energy, describing the earth's temperature.

In the Pinatubo project we will study numerical climate experiments on a computer, which Franklin would have given anything to be able to do. We will first have to test the capabilities of the model by examining how well it can simulate average climate. We will also examine the natural variability of climate, the magnitude of "chaos" which exists in the real world and in our climate model, because this "climate noise" is what makes it difficult to observe the effects of climate forcings such as volcanic aerosols.

We will then study the major climate anomalies which occurred during the past three years, and see how well these agree with the effects which the climate model "predicts" to result from Pinatubo aerosols. Eventually, if the model can simulate realistically climate change during the period after Pinatubo, we wi l use the same model to predict climate change of the next several years.

Research Tasks and Scientific Questions

1. Compare climatology of real world and model. Map global climatology of parameter designated to each school research team (Temperature: Bronx Science/Andrew Jackson; Precipitation: York; Winds: CCNYScience) from observations. Do this for both summer and winter seasons. Compare mean observations for the 15-year period 1979-1993 with the climate model simulation for that period. (2 weeks)

• How well or poorly can the model simulate regions of special interest: United States, Indian subcontinent.
• What are the relationships between the different parameters that may effect how well they are simulated in the model?
• What are climate forcing mechanisms, how can they be measured how can we determine whether they are important?

2. Examine the variability of real world and model. Map global variability of the same parameter for (a) observations, (b) model with fixed boundary conditions, (c) model with observed time-varying ocean temperatures. (2 weeks)

• How does the variability in the model compare to the real world?
• How does the variability change from winter to summer?
• Is the model variability significantly larger when ocean temperature is allowed to change?

3. Describe observed climate anomalies of the past three years. Map observed global distribution of the parameter for the past three summers and winters. Also map this quantity as an anomaly from the mean. (1 week)

• How do the anomalies compare to estimates of natural variability and unforced variability calculated in the research task #2?
• Large climate fluctuations have occurred in the past few years, for example the tremendous floods in the midwest last summer; are these just normal fluctuations of the weather or are they caused by some "Forcing" mechanism (such as the large amount of fine particles injected into the Earth's atmosphere by the volcano Pinatubo in 1991? What is the likelihood that the observed climate anomalies are just random variations?

4. Examine Pinatubo climate experiments. Map simulated global distributions of the anomalies for a Global Circulation Model experiment with the forcing of the Pinatubo eruption forcing, for both winters and summer over the past three years. (2 weeks)

• Climate changes have been predicted because of man-made atmospheric changes, such as increasing "greenhouse" gases; what are the implications regarding the causes of recent climate change?

Key Concepts

• Climate parameters: Temperature Precipitation Winds
• Climate forcings and feedbacks: Atmospheric aerosols El Niño, Ozone depletion, Greenhouse gases
• Climate change: Climate sensitivity to a forcing, Climate anomalies, Natural climate variability, chaos
• Global climate models: Numerical solutions of fundamental equations, Capabilities and limitations
• Scientific method: Hypotheses, Observations (errors, uncertainties). Theory alterations or refinements