Climate Change: The Great and Silly Debate
Climate change is a primary factor in the history and development of human civilization. It caused the earliest migrations out of Africa. It led to the transition from hunter-gather to agricultural society. It gave rise to the development of cities and to the state. It ended many civilizations and allowed others to rise.
Arguments about climate change are what my friend Perry Fisher called the “great and silly debate.” It is great because climate change is serious and affects all of Earth’s inhabitants. It is silly because it doesn’t matter what we think about it. The effect of the debate is to make one side or the other feel better or worse about what is happening whether we like it or not. To say that climate is always changing, that temperatures have been higher during previous periods of Earth history, or that deviations from the warming trend invalidate its truth, ignore geological context and miss the point.
Climate is among the most complex of earth’s natural systems. It is absurd and presumptuous to think that its complexity may be reduced to a few simple debate resolutions which can be judged with a winner and loser declared.
The emphasis on carbon dioxide (CO2) and other gases is both relevant and unfortunate. Emissions are properly the focus for the latest episode of planetary warming and were a factor in historical climate change. At the same time, a preoccupation with CO2 obscures the importance of other factors including solar irradiation, earth’s planetary fluctuations, volcanic activity, and secular variations in ocean temperatures and currents.
Many on both sides of the climate debate believe that the science is settled. Science is not about settling debates among silly humans. Science is not even about solving problems. Engineers solve problems after scientists help them understand the problem.
Science is about describing the present state of things. Scientists attempt to explain what is observed based on its key components, patterns and trends. That explanation is called a hypothesis and there is rarely only one hypothesis for any investigation. Each must be tested and modified until one emerges as a paradigm.
The current climate paradigm is that climate is changing and that the outcome of this change will have important implications for humans and other species. Few dispute this. The great and silly debate is about whether or not human activity is to blame and what if anything humans should to manage the change.
I am not going to write about who is to blame or what should be done. Instead, I hope to provide a geological framework.
Climate and The Agricultural Revolution
Human migrations out of Africa probably occurred in response to climate change caused by Earth’s orbital variations or Milankovich cycles. Waves of hunter-gatherers spread north- and eastward in roughly 20,000 year cycles that began about 106,000 years ago and ended about 15,000 years ago. Migration of pastoral farmers came later.
The agricultural revolution had nothing to do with technology. It was a climate-change revolution. The agricultural revolution took place when climate stabilized and warmed 12,000 years ago (Figure 1).
Humans understood the connection between plants and the seeds from which they grow at least a million years before the agricultural revolution. For most of the Pleistocene temperature changes of up to 8°C over periods of one or two centuries were common making agriculture impossible. Farming was established as early as 37,000 when climate temporarily warmed. It was abandoned after about 2,000 years when colder temperatures returned.
About 5,000 years ago, the mid-Holocene environmental crisis brought widespread drought to north Africa. That resulted in new waves of migration into the Levant but this time many of the migrants were farmers. Agriculture soon spread into sparsely-populated Europe. A dramatic deforestation began there as land was cleared for farming. Forests were largely replaced by grassland and arable land by about 2,200 years before present. The transformation of forests into farmland across the Mediterranean resulted in the beginning of increasing CO2 levels that we see today.
Climatic Evolution of Earth
Temperatures were warmer, not colder, than today through most of geologic time. They key to that statement and to the great and silly debate is geologic time.
Climate change has evolved largely from the interplay between the sun and earth’s atmosphere. Incoming solar radiation has increased over earth’s history. Solar heating is modified as incoming energy is scattered or reflected back out of the atmosphere by clouds, particulates and earth’s surface.
CO2 levels were higher than pre-industrial values (278 parts per million) for most of the last 420 million years (Figure 2). In other words, the long-term decrease in CO2 largely compensated for the increase in solar output.
Figure 3 shows the same data on a logarithmic time scale to compare more recent earth history with its more distant geologic past. Among the various projections on the right-hand side of the figure, RCP8.5 represents the “do-nothing” or business-as-usual scenario. It indicates CO2 values by early in the next century that exceed levels from more than 99% of the last 420 million years. A return to unstable climate would make agriculture impossible again.
Regardless of the reliability of this projection or the ultimate causes for the rise of post-industrial CO2 levels, the message is clear.
What lies ahead during the lifetimes of our grandchildren will most probably not be comparable to anything since the development of multi-cellular life on Earth.
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