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The Perihelion, Seasons, & Climate on Earth

Why are temperatures cooler in the winter? The Sun feels more distant and fails to warm our skin, so many assume that the Sun is further away from the Earth during winter, but that is not true for those of us living in the northern latitudes. On average, you are about 93 million miles away from that stellar source of sunlight, and it takes ~8.33 minutes for sunlight escaping from the Sun to reach the Earth's surface. But in January, the sunlight reaches Earth a little sooner due to the perihelion. On January 4th, 2022 you were only 91,406,842 miles (147,105,052 km) away from the Sun, the closest you will be to our star for the entirety of 2022. As Earth orbits the Sun, it follows an elliptical path that is oval in shape. The average distance from Earth to the Sun on this annual trek is 93 million miles, but on the perihelion you are ~1.4 million miles closer than average and sunlight arrives on Earth ~9.65 seconds sooner!

Helios is the Greek name for the Sun, one of approximately 100 thousand million stars in the Milky Way Galaxy, and certainly the most important to those of us on Earth. The root word "peri" means near, so perihelion translates to "the Sun is near". Later, the Romans replaced the word Helios with Sol, the Latin name for the Sun and origin of words like solar, solstice, and parasol. On the perihelion Earth is ~91.4 million miles away from Sol, and on the aphelion (ap= away from) Earth is ~3.1 million miles further away from Sol. In 2022, the aphelion will be on July 4th when Earth is 94,509,598 miles (152,098,454 km) away from Sol and the northern hemisphere is tilted towards the Sun as summer ensues, despite the greater distance.

In January the Earth's north pole is tilted away from Sol, and those of us in the northern hemisphere are in the depths of winter because we receive less sunlight. The southern hemisphere gets the full brunt of solar energy while ~1.4 million miles closer to Sol than average, which drives higher insolation in the southern latitudes. Notice that the blue line on the Insolation vs. Latitude graph reaches slightly higher Average Daily Insolation values than the red line, because Earth is closer to the Sun during the southern hemisphere summer. Insolation is "incoming solar radiation", and can be measured at the top of the atmosphere and on Earth's surface where we experience the heat energy it imparts upon objects. That heat energy is absorbed, reflected, and dispersed by air, land, life, and water.

Sol is the most important influence on Earth's climate. If the Sun stopped shining tomorrow, the processes of weather & life would grind to a slow and tumultuous halt. Insolation plays a leading role in temperature fluctuations on Earth, and therefore is one of Earth's most important climate forcing variables (or climate drivers). As the amount of sunlight that reaches Earth's surface changes over time, atmospheric and surface temperatures change, Earth systems adapt to changing temperatures...and the sum of those innumerable changes is climate change. Insolation is constantly changing, so climate is constantly changing over tens, hundreds, thousands, and millions of years. The map below is a Net Radiation map showing the fluctuations in radiation/energy on Earth that is available to influence the climate, and if you click on the link you can watch a time lapse map of changing radiation on Earth.

Climate is very complicated, EVERY aspect of nature influences climate, including changing: solar activity (due to changes within Sol), rates of volcanism (due to changes within Earth), atmospheric composition (due to elements being added to & removed from the atmosphere), Earth's orbital patterns (the Milankovitch Cycles)...just to name a few. It is a challenge to find any aspect of nature in our little corner of the Universe that does not influence Earth's climate directly or indirectly. Climate is the sum of all the natural processes that are always fluctuating, and therefore the only constant is change. Humans are part of the natural processes on Earth, and we are now a leading climate forcing variable. Our unbridled use of energy is disrupting normal climate change patterns by trapping more insolation.

Today, during the Anthropocene Epoch, Earth's average atmospheric temperature is increasing due to a higher concentration of greenhouse gases produced by human activity. Greenhouse gases are those that absorb infrared radiation, aka heat energy. If you were to put on infrared goggles you would see sources of heat energy, like body heat and sun warmed surfaces. I often refer to greenhouse gases as "heat munchers" because they absorb infrared/heat energy and keep it from escaping into space. During future times of higher insolation due to orbital cycles (perihelion!) and greater solar output due to higher solar activity (solar cycles graph below), more of that incoming solar radiation will be retained in Earth's atmosphere as it is absorbed by the greenhouse gases. Sol is now moving into a more active part of the solar cycle that will cause a moderate increase in insolation....while humans are pumping more & more greenhouse gases into the atmosphere.

The atmosphere and oceans absorb the excess heat, causing rates of evaporation and weather patterns to change as each aspect of the climate (aka nature) re-equilibrates to the higher temperatures. All the natural cycles that influence climate change are still in play, humans have just disturbed the atmospheric composition and surface temperature, causing the climate to change more drastically and rapidly as it attempts to re-equilibrate with the rapid increase in heat retention.

As 2022 began with Earth in relatively close proximity to the Sun, winter storms rage in the northern hemisphere, and we can appreciate that Earth's atmosphere is warming and the departure from once normal weather patterns reflects the process of rapid climate re-equilibration. We can minimize the amount of re-equilibration the climate must experience, if humanity chooses to take responsibility for our role in Earth's ever changing climate. It will take time for the climate to re-equilibrate and we will have to perpetually cultivate equilibrium in recognition that we cannot change the orbital cycles of the planet, the energy that comes from the Sun, or when large volcanic eruptions occur, but we can change how we affect the climate, how we use energy and how much pollution we produce. We can recognize that humanity has the power and choice to change the planetary climate in ways that benefit life and human civilization. A species that travels through space, cures diseases, makes pizza, and invents artificial intelligence is certainly capable of intelligently managing the climate of the planet on which we rely.


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As always, Nicole, your summary is fantastic! Clear, but erudite description and comments ! You are the BEST teacher ever !!!

We thought about you visiting Yellowstone 3 weeks ago - it would be great to do a "Field Trip" there with you !

With our gratitude for all your classes ,

Michaela( Michelle) and Jim Glenn ( OLLI Students)

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