In the lead-up to our “Climate Solutions Days” summit/virtual conference at the North Central Climate Adaptation Science Center during “Earth Week” 2022, Science Wednesdays are going to focus on climate change these next few weeks, starting with natural climate change causes: what are they and are they responsible for our current climate change or not?
Chapter 1: Natural Climate Change: Orbital Parameters
The Earth revolves around the Sun in an overall elliptical orbit. No, this isn’t the same thing as the elliptical you find in your gym (because if you Google “elliptical”, those are the first results that show up!) Rather, it’s the shape of an orbit and, in the case of the Earth, the shape can vary through time – sometimes becoming more circular and sometimes becoming more like an oval. The technical term for this is eccentricity and it changes how much sunlight we can receive on planet Earth because there are times of year when the planet is closer to the Sun than at other times. Quite simply, when Earth is closer to the sun, it receives more solar radiation. There are two periodicities that have been identified: one cycle is an average of 100,000 years and another, longer, cycle is about 413,000 years.
The Earth itself is slightly tilted, too (this is obliquity), and the amount of tilt changes with time. More tilt means warmer summers and colder winters; less tilt means cooler summers and milder winters. The periodicity of this change is on the order of 41,000 years. The direction of tilt (precession) also changes – towards or away from the Sun, on a 19,000 – 23,000 year timescale. These shifts and wobbles in the Earth’s orbit can trigger changes in climate, such as the beginning and end of ice ages. But orbital changes are so gradual that they’re only noticeable over thousands of years – not decades or centuries.
The slow changes in the Earth’s orbit lead to small but climatically important changes in the strength of the seasons over tens of thousands of years. Any other climate feedbacks in the system can amplify these small changes. In the shift between glacial and interglacial periods on Earth, this is most related to the severity of summers in the Northern Hemisphere (which has more land than water and land warms faster than water). When summers are mild, enough snow and ice remain throughout the season, maintaining glaciers. When summers are too hot, more ice melts in the summer than can be replenished in the winter. A “perfect orbital storm” for global warming would require Earth’s orbit at its highest eccentricity, Earth’s axial tilt at its highest, and the Northern Hemisphere in “perihelion” (its closest point) at summer solstice. Rather, in our current configuration, the Earth’s Northern Hemisphere currently experiences its summer in aphelion (its farthest point from the Sun), the planet’s tilt is currently on the lower end, and the Earth’s orbit is fairly circular. Earth’s current orbital positions within these cycles, thus, should result in cooler temperatures, but instead, the average temperature of the planet is on the rise. What we’re seeing isn’t natural climate change due to orbital parameters.
To be continued…