PMU199 Week 1: The Discovery of Global Warming

(from the course PMU199H1S: Climate Change - Software, Science and Society)

Ideas we covered this week

Note: you can click on any of the images to expand them

This is our planet. Our spaceship for traveling through the cosmos. It provides a life support system for many different forms of life. Look at all those lights, marking out the places where humans cluster in great cities. Where does all the energy come from to light up those cities? Remember, energy cannot be created or destroyed; it can only be transformed from one kind to another. Right now, most of that energy comes from burning fossil fuels (coal, oil, and natural gas) that have lain buried under the ground for millions of years.

There are many signs that something is wrong with the life support system for planet Earth. For example, temperatures across the planet are now higher than they have been for thousands of years. Here's a temperature series from the Arctic, showing a steady cooling trend over the last thousand years, suddenly reversed with a sharp warming trend over the last century.


Many lines of evidence show the warming. For example, this graphs shows a number of different studies that have analyzed global temperatures over the 150 years since 1850. The further back you go, the more variability there is, because of differences in how temperature readings were taken. But over the 20th Century, the trend is clear: 40 years of strong warming until about 1940, then a slight slight cooling until the 1980s, and finally, a strong warming signal since then. The overall warming is very clear, but it's not the only variability in the graph - a good theory ought to explain the overall warming, as well as the the other, shorter patterns. We'll come back to that pattern of post-war cooling shortly. Note: when we talk about temperature changes ("anomalies"), we have to say change with respect to what. The usual convention is to take the average temperature from the last quarter of the 20th Century, and call that zero; temperatures colder than this will be shown as negative, temperatures warmer will be shown as positive. In this particular graph they used the average from the period 1961-1990 as "zero". We'll need to check carefully what "zero" means on any temperature anomaly graph.


Such warming has happened before. This graph shows the global temperature estimates over the last 800,000 years, reconstructed from the oldest ice core, the Epica Dome C. The troughs in the graph were the ice ages, when ice sheets more than a kilometre thick expanded across North America and Northern Europe. The peaks are the inter-glacials, when the ice retreated. Note that the global temperature difference between the ice ages and the interglacials is only around 4-5°C. The ice ages were triggered by the Milankovitch Cycles, changes in the earth's orbit, which alter the amount of sunlight at the poles. However, the orbital variations aren't big enough to cause this much temperature change. Rather, they act as a trigger. For example, as an ice age ends, the orbital change causes a very slight warming. This in turn triggers two feedback effects: a release of carbon dioxide trapped in the ice, and a reduction of the earth's albedo as the ice sheets melt. Both these effects in turn cause a lot more warming. At the start of an ice age, a similar sequence causes the cooling process.


But human civilization wasn't around last time we had massive climate shifts. In fact, in the last 10,000 years, the period in which humans transitioned from hunter-gathers to an agricultural society, saw a remarkably stable period in the earth's climate. This graph, of Greenland temperature variations, shows just the last 100,000 years. The last ice age ended about 10,000 years ago. Note: This graph, from the Greenland ice sheet project (GISP), plots local temperatures in Greenland, rather than global temperatures. The corresponding global temperature shift at the end of the last ice age was about 5°C.

Greenhouse gas concentrations rose steadily throughout the 20th Century, with the growth much faster in the last few decades. In the post-war economic boom, the warming this caused was offset by increasing industrial pollutions (particles like Sulphur Dioxide, which block incoming sunlight), Then, in the 1970's with the advent of clean air legislation in Europe and N. America, the warming dominates again.

Suggested Readings

Spencer Weart's book, The Discovery of Global Warming.

Note: This book is also available online at the American Institute of Physics

A well research account of the history of climate science, going all the way back to the orginal discovery of the greenhouse effect in the 19th Century, and the early calculations of the result of increasing greenhouse gas concentrations in the atmosphere. The section on the early development of climate models is especially good.

David Archer and Stefan Rahmstorf, The Climate Crisis: An Introductory Guide to Climate Change.

An excellent, readable account of the basics of climate change, written by two of leading climate scientists. Note that David Archer teaches an introductory course on climate change for non-science majors at U Chicago, which you can take online.