Note: As the OP, it's not quite fair to say this is "my" answer, but a synthesis of the many comments provided. -- Thank you.
Given the many representations of "Global Warming" and "Climate Change" - what are the "Official Definitions" being used in this debate?
- How these terms are used by the IPCC; and
- The senses of these terms used by skeptics -- especially in the written letters voicing concerns by other Scientists?
- Are people actually talking about different things? The same things?
Source: Climate Change 2013: The Physical Science Basis
1. What is Anthropogenic Climate Change?
As the Original Poster, I had honestly never heard of "Anthropogenic". Understanding what this means clarified the terms, for me.
The context of the objections, and their usage of terms, was explicitly in the context of "policy" and "responses", specifically the significance of "human pollution", that is: Anthropogenic Climate Impact -
Paleoclimate Archives, Chapter 5, pg. 392 - Anthropogenic (human-produced) factors, on the other hand, include changes in the concentrations of greenhouse gases, and emissions of visible air pollution (aerosols) and other substances from human activities. ‘Internal variability’ refers to fluctuations within the climate system, for example, due to weather variability or phenomena like the El Niño-Southern Oscillation, (Chapter 5. pg. 392).
Statistical Use of "Anthropogenic": Google Books Ngram Viewer: 1970 - 2008 Statistical use of Anthropogenic, Climate Change, Global Warming, Greenhouse Effect, Greenhouse Gas."
2. Global Warming - A Discussion of Greenhouse Gasses and Radiative Forcing:
Although Global Warming can be used interchangeably with Climate Change, NASA 12.05.08, Global Warming is in reference to Radiative Forcing and the Greenhouse Effect:
Global Warming Potential (GWP), pg. 1455) - An index, based on radiative
properties of greenhouse gases, measuring the radiative forcing following a pulse emission of a unit mass of a given greenhouse gas in the presentday atmosphere integrated over a chosen time horizon, relative to that of carbon dioxide.
Radiative forcing, (Glossary, pg. 1460) - Radiative forcing is the change in the net, downward minus upward, radiative flux (expressed in W m–2) at the tropopause or top of atmosphere due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun. Sometimes internal drivers are still treated as forcings even though they result from the alteration in climate, for example aerosol or greenhouse gas changes in paleoclimates. (See Chapter 8, Radiative Forcing.)
3. Climate Change - Persistent Change Caused by Internal Processes or External Forcings:
Climate change, (Glossary, pg. 1450) - Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use. ...
...Framework Convention on Climate Change (UNFCCC), Article 1 - Climate change as ... ‘a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’.
The UNFCCC thus makes a distinction between climate change attributable to human activities altering the atmospheric composition, and climate variability attributable to natural causes.
Abrupt Climate Change, Section 22.214.171.124, pg. 1114 - This report adopts the definition of abrupt climate change used in Synthesis and Assessment Product 3.4 of the U.S. Climate Change Science Program CCSP (CCSP, 2008b). We define abrupt climate change as a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems, (pg. 1448, etc.).
4. Relationship Between Global Warming and Radiative Forcing:
Global Warming is associated with the Greenhouse Effect and Radiative Forcing:
Drivers of Climate Change - Natural and anthropogenic substances and processes that alter the Earth’s energy budget are drivers of climate change, (pg. 13).
From Forcing to Attribution of Climate Change (Chapters 8, 9 and 10), (Chap. 1, pg. 151) - In these chapters, all the information on the different drivers (natural and anthropogenic) of climate change is collected, expressed in terms of RF [radiative forcing], and assessed (Chapter 8). As part of this, the science of metrics commonly used in the literature to compare radiative effects from a range of agents (Global Warming Potential, Global Temperature Change Potential and others) is covered.