For reasons that have little or no relation to science the topic of global warming induced by production of greenhouse gases has become a political issue. Most of the greenhouse gases are derived from burning of fossil fuels and the emission of carbon dioxide into the atmosphere. Measures that can be taken to limit fossil fuel consumption produce disruptive effects in our current economic system. Those who would prefer to not endure such disruptions often point to climate changes that have occurred in the past—without human activity as a driver—as evidence that the rise in temperatures being observed can be explained as a natural phenomenon. They may be victims of wishful thinking. Most scientists attribute the current global warming to human-produced greenhouse gases. Many have been calculating global effects and making predictions of how the earth will respond if emissions continue. They may also be victims of wishful thinking. It is useful to review the history of global climate change in order to evaluate the hopes of the deniers and the confidence one should put in calculations of future effects.
An article from Wikipedia provides some data on what scientists conclude has happened in recent decades and what they predict for the future.
“Global mean surface temperature change from 1880 to 2014, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. The green bars show uncertainty estimates.”
The change in global temperature is taken to be about 0.6 ˚C since 1980. Predictions of additional temperature rise by the end of this century based on various emission scenarios vary from 0.3 ˚C to 4.6 ˚C. (from 0.5 ˚F to 8.6 ˚F) Some have predicted that a net temperature rise of 4 ˚C would mean the end of civilization as we know it. The nations of the world have adopted a net rise of 2 ˚C as the value not to be exceeded as they attempt to control emissions.
Let’s put some of these temperature changes in perspective. This source provides a look at how temperature is believed to have changed over geologic time.
The term “ice age” refers only to the five periods indicated in the chart. We are currently worrying about an overheated planet while we are technically living in an ice age. The temperature swings associated with entering and leaving an ice age are rather extreme compared to what is anticipated by the end of the current century, but our temperature rise is taking place at a much faster rate.
It is interesting to note that there are also believed to have been five major mass extinctions. These are times when a significant fraction of plant and animal species have disappeared. This source provides the estimated dates at which these extinction events occurred: 66, 201.3, 252, 375-360, and 450-440 million years ago. The third on the list was the worst and is referred to as “The Great Dying.”
“Earth's largest extinction killed 57% of all families, 83% of all genera and 90% to 96% of all species.”
Note how these extinctions seem to correlate with periods when the earth’s temperature underwent large changes. Perhaps changing the earth’s temperature is a risky business.
What are often referred to as “ice ages” are more properly referred to as glacial cycles which occur much more frequently.
This chart provides temperatures based on studies of ice core measurements. Consequently the temperature scale is consistent with the local temperature at which the ice was formed rather than that of the global average. Note how stable the temperature has been over recent millennia. This is a highly unusual period in history and has been critical in allowing human civilization to develop.
“Currently, we are in a warm interglacial that began about 11,000 years ago. The last period of glaciation, which is often informally called the ‘Ice Age,’ peaked about 20,000 years ago. At that time, the world was on average probably about 10°F (5°C) colder than today, and locally as much as 40°F (22°C) colder.”
What is most astonishing is the evidence that large global temperature changes can occur quite rapidly—within a human lifetime.
“On a shorter time scale, global temperatures fluctuate often and rapidly. Various records reveal numerous large, widespread, abrupt climate changes over the past 100,000 years. One of the more recent intriguing findings is the remarkable speed of these changes. Within the incredibly short time span (by geologic standards) of only a few decades or even a few years, global temperatures have fluctuated by as much as 15°F (8°C) or more.”
“For example, as Earth was emerging out of the last glacial cycle, the warming trend was interrupted 12,800 years ago when temperatures dropped dramatically in only several decades. A mere 1,300 years later, temperatures locally spiked as much as 20°F (11°C) within just several years. Sudden changes like this occurred at least 24 times during the past 100,000 years. In a relative sense, we are in a time of unusually stable temperatures today—how long will it last?”
An excellent description of how scientists have gradually changed their views of the earth from one of an essentially constant temperature model to one in which temperature changes can occur incredibly rapidly can be found in an article provided by the American Institute of Physics: Rapid Climate Change. It describes how the local variations in temperature obtained from ice core data have been correlated with similar observations around the world to verify that it was a global temperature change that had occurred. This article also tells us that when scientists realized these rapid temperature changes were real (in the 1990s) they were able to conjure up a significant number of possible explanations for how this might have occurred. What was scary was the realization that it was unlikely that such rapid changes could have been predicted by the existing climate models.
During the last glacial cycle the earth’s surface temperature dropped and sent glaciers spreading across much of what is now densely populated terrain. Cities and entire nations would have been destroyed. At the height of this period of glacial growth the global temperature was 5 ˚C lower than today. One can only guess what temperature change it took to trigger this glacial period. Scientists are now telling us that we could be on track to raise the global temperature by 5 ˚C by the end of this century. Is it any wonder that they fear the consequences of such an occurrence?
The scientific data tells us that we have raised the earth’s temperature by about 0.6 ˚C since 1980. We have, in recorded history, an event in which a similar sized fall in temperature occurred, presumably due to natural effects. We know of it as the Little Ice Age. Its major effect hit in the 17th century, but scientists view it as extending from 1300 to 1800. This source provides this temperature profile:
The various estimates of minimum temperature place it at a drop of less than 1 ˚C. This is not far in magnitude from where we have pushed the temperature in the opposite direction, and it is a magnitude of change that we seem destined to reach no matter what we do in the future.
What were the consequences observed in the Little Ice Age? David Parrott addresses that question in an article in The London Review of Books: Sad Century. Parrott is reviewing a recent book by Geoffrey Parker: Global Crisis: War, Climate Change and Catastrophe in the SeventeenthCentury
“Contemporary accounts leave little ambiguity about the character of the 17th century. Natural disasters, warfare, political unrest and rebellion combined to bring about levels of mortality, destruction and collective trauma unmatched until the mid-20th century.”
Parker is careful to avoid claiming that the effects of climate change automatically lead to wars and other conflicts, but it is clear that the climatic and political events cannot be unrelated. Climate change exacerbates conflicts and war worsens the effects of climate change.
“These were wars fought on an unprecedented scale and at an unprecedented cost. In France more than a million people died of starvation, cold and disease between 1691 and 1701, and a further 600,000 in the winter of 1708-9. Yet the cold, shortages and crushing fiscal burdens brought despair and hopeless compliance rather than revolt and resistance.”
Parker’s book provides a valuable insight into the effects of climate change.
“As Parker points out, the almost universal population increases of the 16th century had led to a volatile situation in which any reduction or disruption of food supplies, any decline in the availability of marginal employment, any fall in wage levels or increase in rents, pushed large groups into destitution and starvation.”
Rapid population growth left the seventeenth century struggling to gain the economic tools and technologies to provide a stable society with adequate food supplies, shelter, and a source of income. The disruption of climate change was disastrous. In our era we have well-developed technologies and economic tools to provide enough food for the world. However, we have also developed a globalized system of supply and demand that leaves us incredibly vulnerable to disruptions. For example, if higher temperatures lead to lower food production—and eventually they will—we know that food scarcity will lead to food export embargoes by the supplier countries and those who have grown dependent on the global market will suffer. Conflicts of some sort will be inevitable. Parker provides us with this observation:
“….his harsh argument that in many areas the Global Crisis eliminated surplus population and so restored the balance between food supply and mouths to feed. It was this which made it possible for some to survive the continuing depredations of global cooling….”
That is not an outcome that could be considered desirable.
Parrott reminds us that this period of global cooling is most likely to be recalled by global warming deniers who would like to claim that since global cooling came and went global warming is likely to do the same. Parker’s book was intended to counter such claims and serve as a warning that climate change should not be taken lightly because there will be a price to pay.
“Parker’s new book, Global Crisis, responds directly to this type of argument, asserting that humanity survived only at a terrible cost. His epilogue is a plea that the lessons of climate change in the 17th century should not be ignored or misinterpreted. We should be in no doubt that decisions taken now will have an effect on the future impact of natural catastrophes, the resilience of agriculture and the competition for material resources.”
We tend to think of the earth in terms of a “mother nature” that is intrinsically benign and would treat us nicely as long as we behaved. A more appropriate symbol might be a mighty stallion that is frequently angered by the life forms that dare take up residence on its back and periodically tries to destroy them all. We have been incredibly lucky to have enjoyed an exceptionally long period of relatively stable global temperatures. History tells us that could change at any time even without the effects of our activities. Why be stupid enough to dare upsetting the equilibrium that has made our civilization possible.
Let sleeping stallions lie.