Milankovitch Cycles

     Few people deny that global warming is happening, the only ones left being the stubborn political pundits who ignore science.  Humans are seeing record high temperatures everywhere, and record droughts.  Some areas that were formerly temperate are becoming semi-tropical.  The question is no longer whether global warming is happening, but to what degree humans are responsible for it. 

    What makes it difficult to quantify the effect humans have on global warming are the three Milankovitch Cycles, each of which contribute to the Earth's climate naturally.  The three cycles are the Earth's eccentricity, axial tilt, and precession.  They all play a role in the amount of sunlight reaching the Earth's surface.  The cycles are pretty convincing when it comes to the natural ebb and flow of ice ages and warmer periods that the Earth has experienced.  What remains to be explained is how they could play such a high role in the rate of warming we've seen in the last 100 years.  Warming trends skyrocketed in the early 1900s.  The increased rate couldn't have been so abrupt if it were caused by a natural cycle, because each Milankovitch Cycle takes many thousands of years: 

 

Eccentricity- a cycle in which the Earth orbits the sun closer than at other times, lasting about 100,000 years.  Current stage: about average, near circular orbit. 

Axial tilt- a cycle in which the Earth tilts more toward than sun than at other times, lasting about 41,000 years.  Current stage: about average, 23.5 degrees. 

Precession- a cycle in which the Earth's wobble brings the Northern Hemisphere closer to the sun than at other times (thought to be the cause of Ice Ages), lasting about 26,000 years.  Current stage: away from sun, period of cold expected. 

 

   Each cycle takes so long that there isn't any "lining of the stars" that would cause the Earth to warm as abruptly as it has in the last century.  Rather, the best explanation seems to be that the rate of warming coincides with the rise in greenhouse gases emitted into the atmosphere by humans.  While both fossil fuel emissions and Milankovitch Cycles may be contributing to the warming trend, I'd estimate that the former far outweighs the latter, just based on climate trends in the last 500 years.  Additionally, only 2 of the 3 Milankovitch Cycles might suggest the Earth is undergoing a warming trend.  The third, precession, should have predicted a cooling period right now, at least in the north.  That simply isn't happening.