Global Climate Change – Update 2015

The topic of man-made global climate change has remained the most widely discussed environmental subject in recent years. I have been writing and teaching about this topic for my entire 20+ years at Ohio State University Extension. I have seen how it polarizes people along a number of lines, including along political lines. That is one of the reasons it continues to hold so much interest for so many people.

My job as an environmental economist though is to sift through the theory and the data. When we take these things together, we might not be able to prove or disprove a hypothesis beyond any doubt – that is beyond the scope of science. But what we can and should do is to try to draw the most reasonable conclusions we can from all the evidence. Essentially that is what scientific method is all about.

Earth’s temperature has always fluctuated. Depending on the time frame considered (millions of years, thousands of years, centuries, etc), the causes of climate change vary. Over the longest term, continental drift (where the continents are positioned on the globe) makes the biggest difference in temperatures. When tropical ocean currents are blocked from getting near the poles, as in the world we occupy today, the earth is cold and snowy, with glaciers, icebergs and permafrost. For most of earth’s history the “ice house” earth we occupy has not been the case. But it has been the case for the entire time in which we (humans) have been around. Our genus (homo) most likely got its start around 7 to 8 million years ago

Milankovitch Cycles - 2015-03-12

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Over periods of tens of thousands of years the dominant driver of climate has been a series of somewhat regular variations in earth’s orbit around the sun called Milankovitch cycles. These cycles brought on the ice ages. Twenty thousand years ago, northern Ohio lay under a mountain of ice more than a thousand feet thick. You can still see evidence of this today at the glacial grooves on Kelley’s Island. Obviously there has been a lot of global warming over the past 20 thousand years to melt all that ice. And this warming was in fact also brought about by the Milankovitch cycle.

Scientists have observed that the warmest period since the last ice age was about 5,000 years ago. They dub this peak the Holocene Maximum, denoting the warmest point in our current geological epoch. Since then global temperatures began to slowly decline, presumably leading earth eventually into a new ice age to occur some time in the future – perhaps within the next few thousand years. There have been some fluctuations around the cooling trend since the Holocene Maximum. Some of these deviations may have occurred as a result of increased solar output, along with changes in ocean currents.

Over the past 150 years however, the post Holocene Maximum cooling trend has not only abated, it has reversed. In fact the rate of global warming has accelerated considerably in the past thirty years, and is currently at .15 C degrees per decade (about ¼ degree F). The most likely cause of this trend is the increased concentration of carbon dioxide (CO2) in the atmosphere that has primarily resulted from the production and use of coal, oil and natural gas, aka “fossil fuels.” Deforestation and the burning of wood have also contributed. Prior to the industrial revolution, atmospheric CO2 concentration was very stable at about 280 parts per million (PPM). It has grown steadily ever since, to a current level of 400 PPM, the highest it has been for any period in which humans have been present on earth. CO2 causes the earth’s temperature to rise by trapping escaping heat in the atmosphere (the greenhouse effect).

Given world population growth, current technology and our reliance on fossil fuels, I do not believe that humans will be able to prevent CO2 concentrations from rising to 450 PPM by mid-century (2050). I think this will lead to an overall warming of about 2 degrees F, and will have enormous ramifications for the entire planet, especially agriculture and lands adjacent to oceans and seas. I think that at a minimum, since we have so far been unable to find appropriate substitutes for the energy sources that cause global warming, we should be investing heavily into finding ways to allow us to cope with the consequences of living on an earth that is significantly warmer than any humans have ever experienced.

If you are interested in learning more about this topic or scheduling a program in your area, please contact me at I currently have two slide shows ready to present. A review of earth’s climate history over the past 542 million years (the Phanerozoic Eon) is the focus of “Global Climate Change: Update 2015.” A more “local” approach is taken in “Climate Change: Outlook for Ohio to 2050.”

(Submitted by Thomas W. Blaine, Associate Professor, OSU Extension)


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