Based on measurements from deep ice core samples we know that carbon dioxide concentrations in the atmosphere go up as part of a natural cycle of warming of the Earth following the lengthy cooling that characterizes the ice ages. As the Earth goes through this natural cycle, carbob dioxide concenrations during the cool temperature characteristic of near maximum ice formation in an ice age are around 180 parts per million. These lower concentrations then go up slowly following warming to a maximum of near 290 parts per million and then back down again as the Earth cools into an another ice age. The swings in and out of the ice ages are very slow, taking many centuries to go from relatively warm to cold or from cold back to warm. But, in truth, whether the warming caused increases in atmospheric carbon dioxide levels or carbon dioxide level increases are themselves brought on by increases in warming is not easy to decide.
At the present time the burning of fossil fuels since the beginning of the Industrial Revolution has brought atmospheric carbon dioxide levels to near 400 parts per million, a nearly 40 percent increase over the near steady levels of 290 parts per million during a warming phase at the end of an ice age, which for us is taken to have ended about 11,000 years ago. We may have had some increased warming since the beginning of the Industrial Revolution but that warming has been irregular and in truth may not have begun in earnest until 1960-1970. Whether the advances in warming are due to carbon dioxide elevations or to the slowly advancing warming effects of the Milankovitch Cycles, which have been loosely correlated with ice age cycles, is hard to say.
Undeniably there have been major anthropogenic contributions to carbon dioxide concentrations in the Earth’s atmosphere. As I noted in an earlier post the Milankovitch Cycles can bring us slightly warmer Spring and Summer seasons by extending the relationship and synchronicity between the orientation of the Earth with the Sun, and that certainly over longer term affect the climate making it somewhat warmer year by year, particularly in the Northern Hemisphere. Indeed, the major wildcard here may be a progressively accelerated warming due to an increasingly open Arctic during the summer months. As noted in several previous posts this would likely over time warm the Arctic and accelerate heating of the Earth’s atmosphere.
But, the serious question is, do increased carbon dioxide concentrations in the atmosphere actually elevate temperature? This old idea of carbon dioxide acting as a greenhouse gas evolves from the ideas of Svante Arrhenius and other chemists and physicists of the last century. The idea was that carbon dioxide absorbed and trapped heat radiating from the Earth’s surface subsequent to its heating from the Sun. This idea of a greenhouse like effect is increasingly rejected by modern physics–indeed, physicists often rejecting the whole idea of an atmospheric greenhouse effect.
In our modern understanding of thermal atmospheric physics, there is no role for carbon dioxide or other gases, indeed no atmospheric greenhouse effect. In the warming of the Northern Arctic, the ice melt and the ocean warmed in response to the slowly altering Milankovitch Cycles. Over time this has led to thinning ice in the Arctic and to more accelerated warming as more and more open water absorbs heat energy during the predicted slightly expanded summers. The warming water changes atmospheric conditions and assists in some evaporation of heated water over the ocean. This allows some distribution of the heat over the planet by complex climate mechanisms which are only starting to reveal themselves. But it is no surprise that presently the consequences of warming are concentrated at the northern end of the planet.
Where are we then on the carbon dioxide effect? First, the idea of the atmospheric greenhouse effect due to elevating carbon dioxide concentration may be totally incorrect. Carbon dioxide concentrations may be correlated with a warming Earth, but carbon dioxide concentration are perhaps better thought to be correlated with global warming rather than being strictly causative. At the time (in 1896) the idea that carbon dioxide was a green house gas seemed like a good idea to Arrhenius and others, an idea not at all easy to refute based on the 19th century understanding of atmospheric physics. Clearly concentrations of carbon dioxide are being continuously elevated due to the burning of fossil fuels over the last two centuries, and we are now charting new territory not seen over the last half million years or more. The melting of significant ice in the Arctic may be sufficient to explain current and slowly evolving warming of the Earth and the associated climate changes.
While warming and climate change may be brought on in part or in full by the slow Milankovitch Cycles, it makes sense to continue to reduce our dependence on carbon dioxide emitting fossil fuels. Continuing massive increases in carbon dioxide concentrations reflect a massive imbalance between carbon dioxide and oxygen generation in the atmosphere. Improving the efficiency of our fossil fuel dependent machines and switching to use of renewable, non-polluting energy sources will also be economically important, especially, in longer term, as prices increase on fossil fuel based commodities, which will run very low in the next 1-2 centuries. As we use less carbon-based fuel and improve on the natural rate of fixing carbon dioxide back into forest timber and renewed biomass, concentrations of carbon dioxide in the atmosphere will begin to decline.