Excerpts from the Introduction in italics with my bolds.

In the past few years, confidence in the AGW theory has declined dramatically. New research points to natural causes of the modern warming, and stabilizing (by some measures, falling) global temperatures have called attention to long-recognized shortcomings of the AGW theory. Tens of thousands of scientists have signed petitions expressing their dissent from the so-called “consensus” in favor of AGW. Opinion polls show a majority of the public in the U.S. and in other countries no longer believes human activity is causing global warming. Evidence of the decline of the AGW theory is presented in the postscript to this booklet.

The demise of the AGW theory makes this a good time to look at other theories of climate change put forward by prominent scientists but overlooked in the rush to judgment. This booklet identifies seven theories – AGW plus six others that do not claim man-made CO2 is a major cause of climate change.

Each theory is plausible and sheds light on some aspects of climate change that were hidden or obscured by too great a focus on the AGW theory.

In some respects these theories are not mutually exclusive: solar variability could be the sustaining force behind what I have called the “cloud formation and albedo” and “ocean currents” theories as well as being its own theory, though the mechanisms in each case differ slightly. Most physicists don’t study biology or chemistry and so don’t pay much attention to biological and chemical feedbacks. If they did, they would probably recognize that such processes play a bigger role in controlling climate than previously believed.

Deeper analysis also reveals that these theories are not all trying to answer the same questions or necessarily achieve predictive power. Trying to discern a human effect on climate is not the primary objective of biologists studying the effect of higher levels of CO2 on plants or of physicists measuring the amount of energy leaving Earth’s atmosphere. While they are “experts” on climate change, they are not part of the search for a “human fingerprint” on Earth’s climate. Nor are they qualified to make predictions based on their narrow expertise, as Kesten Green at the University of South Australia and J. Scott Armstrong at the Wharton School of the University of Pennsylvania have tried to explain.

The six theories of climate change that do not involve
man-made greenhouse gas emissions
are incompatible, though, with the AGW theory.

If evidence exists that negative feedbacks offset whatever warming is caused by man-made greenhouse gases, then the warming during the past 50 years could not be due to the burning of fossil fuels. Similarly, if solar variability explains most or all of the variation in temperatures in prehistoric as well as modern times, then there is no room for speculation about a large role for man-made CO2 .

Over time, the science of climatology will become somewhat more exact, based on examination of the historical record and newly assessed empirical evidence. It probably will not be illuminated much by mathematical models that cannot generate reliable forecasts of a system that even proponents of the anthropogenic global warming theory admit is naturally chaotic. We cannot adequately measure the enormous quantity of data necessary to feed the models, and we are not even sure which variables should be included.

The uncertainty that pervades climate science today,
as climate scientist Mike Hulme has written,
is a function of the limits of science itself.

The object of this essay is not to say which of these seven theories is right or “best,” but only to present them to the reader in a format that allows reflection and balanced consideration. Such dispassionate interest in the subject has been lacking in recent years, and the scientific debate has suffered for it.

PDF of the publication is available from Heartland Institute:  7 Theories of Climate Change