365 Days of Climate Awareness 19 - Milankovitch Cycles 4: Climatic Significance

 

365 Days of Climate Awareness

19 - Milankovitch Cycles 4: Climatic Significance

Studying Milankovitch cycles leads to the conclusion that changes in the earth's orbit can have significant effects on overall climate, but these are far from the only, or strongest, inflences. Over the past 800,000 years, northern hemisphere glaciations have occured almost with the regularity of tides, but that pattern does not hold further back in the geological record (though large glaciations certainly happened).

As we travel back in geological time, the records become sparser and our dating methods less precise. But the evidence for these regular climatic fluctuations doesn't exist from earlier times. Other conditions have recently set earth on a balancing point where the Milankovitch cycles have outsized effects on regional and global climate.

One idea centers on the formation around 3 million years ago of the isthmus of Panama, connecting North and South America with a complete land bridge where formerly were islands. The newly complete barrier of Central America cut off tropical circulation between the Atlantic and Pacific Oceans. (Ocean circulation will be the subject of many future posts.) This closure limited the transfer of heat around the equator and allowed greater temperature differences to build not only east to west, but also north to south.

A basic function of weather and ocean currents is to transfer heat from the equator to the poles. (Hurricanes play a measurable role in this process.) A barrier to that process like Central America makes the entire planet's poleward heat transfer system less efficient, less robust, and more subject to smaller influences like Milankovitch cycles.

At a moment like this it might seem like the floor drops out from climate science, that no conclusions or results can be trusted, if processes don't seem to operate effectively at all times. Is uniformitarianism valid? The answer, as far as we've yet seen, is yes. Uniformitarianism--the idea that the processes at work now have always been at work--has an important caveat: the conditions must exist for those processes to occur. Ocean tides could not occur before the ocean existed. Ozone and how it shields the earth's surface from UV rays were unlikely to have happened before oxygen existed in the atmosphere. And there are many more examples. Too narrow a focus of inquiry can cut off very relevant information.

These cycles which Milutin Milankovitch identified have operated for essentially all of earth's history (leaving aside the formation of the Solar System). But it is only occasionally that they have come to play a major role in earth's climate. Continental formations, and thereby sea and air circulation, have buried the cycles' effect before. It is very possible that rising the rising CO₂ concentration in the atmosphere will overwhelm them now, for at least a few thousand years.

Tomorrow's post: greenhouse earth.

Be well!