
The IPCC reports are lengthy, detailed, and highly structured reports. In this lesson, we are going to work primarily with the section AR5 Climate Change 2013: The Physical Science Basis called Summary for Policy Makers. To be sure you are prepared to understand the material presented here, I have shopped around the rest of the IPCC document (and other sources too), to provide you with background reading on key concepts. The first of these key concepts, for understanding climate science and the IPCC reports, are uncertainty and radiative forcing.
Reading Assignment
Radiative Forcing explained...
The concept of radiative forcing is fairly straightforward. Energy is constantly flowing into the atmosphere in the form of sunlight that always shines on half of the Earth’s surface. Some of this sunlight (about 30 percent) is reflected back to space and the rest is absorbed by the planet. And like any warm object sitting in cold surroundings — and space is a very cold place — some energy is always radiating back out into space as invisible infrared light. Subtract the energy flowing out from the energy flowing in, and if the number is anything other than zero, there has to be some warming (or cooling, if the number is negative) going on.
It’s as if you have a kettle full of water, which is at room temperature. That means everything is at equilibrium, and nothing will change except as small random variations. But light a fire under that kettle, and suddenly there will be more energy flowing into that water than radiating out, and the water is going to start getting hotter.
In short, radiative forcing is a direct measure of the amount that the Earth’s energy budget is out of balance.
For the Earth’s climate system, it turns out that the level where this imbalance can most meaningfully be measured is the boundary between the troposphere (the lowest level of the atmosphere) and the stratosphere (the very thin upper layer). For all practical purposes, where weather and climate are concerned, this boundary marks the top of the atmosphere.
While the concept is simple, the analysis required to figure out the actual value of this number for the Earth right now is much more complicated and difficult. Many different factors have an effect on this balancing act, and each has its own level of uncertainty and its own difficulties in being precisely measured. And the individual contributions to radiative forcing cannot simply be added together to get the total, because some of the factors overlap — for example, some different greenhouse gases absorb and emit at the same infrared wavelengths of radiation, so their combined warming effect is less than the sum of their individual effects.
(Source: MIT News, Explained: Radiative Forcing, David Chandler, 2010)
Okay, you now have the background needed to follow the information presented in the IPCC's Summary for Policy Makers. For terms that may be new to you, please refer to the IPCC's Glossary and Acronyms (Annex III and IV of the new report). As you read the sections below, focus your attention on the opening paragraphs of each section, especially the statements in the shaded boxes.
Reading Assignment
Visit the IPCC.
Under "Publications and Data," locate the Fifth Assessment Report, select Climate Change 2013: The Physical Science Basis
Review the list of chapters and notice the Annexes, including Annex III: Glossary and Annex IV: Acronyms. You may find these to be useful references!
Download the chapter Summary for Policymakers. Read the following sections (and all subsections):
A. Introduction
B. Observed Changes in the Climate System
C. Drivers of Climate Change
(You may wish to save this document. It will be used throughout this lesson.)