Global climate change has been a natural phenomenon driven by natural processes for 4.5 billion years. Nevertheless, cultural pressures exist to identify a human cause for current global climate change.

As Peter Flawn, President Emeritus of the University of Texas, recently wrote, “All geologists early in their careers are introduced to solving problems through multiple working hypotheses — of deriving solutions from the data rather than, as is common among some social scientists, settling upon a solution consistent with the reigning theory and supporting it with data selectively chosen.” Although many geologists have expressed concern about the paucity of data supporting supposed human-driven climate change, scientific tests to falsify the hypothesis have been lacking.

The first step toward restoring the rigor of science in the global climate debate occurred when, on June 22, a committee of the National Academy of Sciences presented the results of its directed study of the science behind the infamous “hockey stick.” Published by Michael Mann and colleagues, the hockey stick study — which statistically interpreted tree ring data as demonstrating unprecedented current warming relative to the past 1,000 years — failed to capture and denied the existence of the “medieval climate optimum,” a warming event beginning 1,100 years ago. The hockey stick was the basis for the U.N. Intergovernmental Panel on Climate Change’s statement that “there is discernible human impact on climate.”

Vindicating the original criticism of past-published researchers, the National Academy report identifies the failure of the hockey stick to model climate beyond the past 400 years, as evidenced by its inability to reflect the medieval climate optimum. The optimum has been extensively documented by recorded human history and proxies, but cannot be explained by computer models based on equations that assume that greenhouse gases dominate climate change. These same models predict massive increases in Earth’s atmospheric temperature because of the additions of a small percentage of human-derived carbon dioxide.

Data, records and proxies demonstrate that the medieval climate optimum, along with the cyclic and preceding Roman-era warm event and current modern warming event, are natural events. It now remains to be discovered what processes are driving climate change.

Geologists are the scientists who study Earth’s past, and the past can be the key to the present. When insufficient empirical evidence exists to understand climate change in the present, then geologists must visit the past to understand operating processes.

Understanding climate change requires application of the scientific method. Testing hypotheses against data and observations will provide answers that can become the basis of responsible public policy decisions. Resolving the global climate change argument requires testing three hypotheses.

The first hypothesis is that climate naturally and constantly has changed throughout Earth’s history, growing both warmer and cooler, and at many rates and scales. This hypothesis is supported by recorded human history and all paleoclimate evidence, and confirms that natural climate changes are the rule, not the exception. That hypothesis is now established theory.

It is incumbent on the geoscience profession to better communicate this fundamental principle. Statements that the Arctic is warming, or that ice is melting, or that summer temperatures are higher can each be true, but they do not imply any specific cause of such changes. Some in the media, government and even academic organizations follow those statements with, “so that proves that humans are causing climate change.” But that statement does not follow and is not true.

The remaining two hypotheses are contradictory. One is that human greenhouse gas emissions are the major driver of global climate change. The other is that natural processes are the major driver of global climate change. Scientists must use observations and data to test each hypothesis.

Testing greenhouse-gas drivers requires that we first determine in the past how such drivers have worked. Examination of ice-core data documents that in the past, temperatures have risen, and carbon dioxide levels have followed, lagging behind major interglacial warming by up to hundreds of years. Speculation is that the carbon dioxide rise is a result of degassing of the world’s oceans. Mapping carbon dioxide levels against global temperature changes over the last 250 years demonstrates the same relationship — that as temperature rises, decades to centuries later, carbon dioxide rises.

Despite this pattern, many people will argue that a significant portion of the modern temperature rise is due to human greenhouse gas emissions. Yet the human contribution to total global greenhouse gases is so small as to be de minimis. The sum of these observations and correlations falsify the hypothesis that human additions to greenhouse gases are a significant climate driver.

The alternative hypothesis is that natural processes are responsible for most climate change. Much refereed scientific literature documents the role of orbital forcing over millennia to hundreds of thousands of years, solar variability in climate changes over decades to millennia and smaller events, such as volcanism and meteorite impacts over years to decades. The positioning of continents relative to oceans and resulting currents may drive climate over millions of years. Current climate modeling efforts need to accommodate and extend these myriad natural processes. The outputs of global climate quantitative computer models have not been rigorously tested against data except against the medieval climate optimum, against which they have failed.

We don’t yet fully understand how the sun dominates millennial to decadal climate change. But for decades we did not understand how continental drift could occur, so many scientists denied it did occur. We may have a grand experiment ahead of us. NASA researchers have projected a major solar minimum (solar cycle 25) for the period of about 2020 to 2030. If that occurs, and Earth’s temperature decreases, solar energy will have been established as a major driver of climate. If the temperature increases during a major solar minimum, the human impact of climate will have been substantiated. In either case, science will prevail.

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