1 Million years ago
Homo Erectus on the plains of Africa and Asia
Ice has advanced and retreated many times, descending into deep, long ice ages, then abruptly thawing to spike up to higher temperatures. Permanent ice sheets are thin and temperatures on average are decreasing very slowly. Homo erectus has already emerged from the forest onto the plains of Africa and spread into Asia, and Europe. They have learned to use fire from their own Prometheus. (more)
Changes in Earth’s orbit drive climate cycles
1.Earth’s elliptical orbit becomes more circular or more elongated on a 100,000 year dominant cycle.
2.Earth’s tilt rotates and wobbles like a spinning top on a 41,000 year cycle
3.Earth’s elliptical orbit rotates, placing the seasons at changing proximity to sun on a 19-23,000 year cycle
7.8 billion in 2021
415 parts per million in 2021
15° C in 2021
measured in hundreds of meters from datum in 1900
Thousands of years
Dotted line is model
Appearance of anatomically modern humans
Eemian interglacial period, slightly warmer than present
Shift to more extreme ice ages
Sea level low ~ -150 meters
Millions of years
One million years ago the Earth is still recognizable, except richer in species diversity, forests, and living things of all kinds. Herds of large, grazing mammals keep the grasslands open beneath flocks of migrating birds that distribute seeds to the edge of the ice. People have little influence, limited to small groups in equatorial Africa, where they are both predator and prey. Ice in the northern latitudes is the normal condition. Relatively short interglacial respites of warmer weather emerge rapidly out of the deepest freezes every 100,000 years, or so.
Dramatic swings in climate are pushed by fluctuations in Earth’s orbit and tilt that change when in the cycle of the seasons the planet receives more or less sunlight. The total amount of sunlight reaching the Earth remains constant, but winter and summer become more or less extreme. Each of three orbital cycles has a specific period, like a pendulum. Together they either reinforce or
dampen one another, resulting in complex pushes and pulls on climate systems. These small variations in seasonal sunlight affect terrestrial phenomena, which amplify climate shifts over their own timescales. Temperature, carbon dioxide concentration, and sea level change in synchrony.
Warming occurs faster than cooling, accelerating when ice melts to expose darker earth or sea, which absorbs more sunlight. Heat builds even more when tundra melts, releasing carbon dioxide and methane that add to the greenhouse effect. Changes in sea level lag behind temperatures, but seas can also rise quickly, leaving beaches behind until the waters stabilize at a new coastline, in some places many miles inland.
In times of cooling, ice sheets grow slowly, locking carbon in frozen tundra and drawing water from the sea with each snowfall.
Image: Composite of Adobe Stock Images
Map: See Milankovitch Cycles.
CO2 : NOAA National Centers for Environmental Information. "Temperature Change and Carbon Dioxide Change " Retrieved 20 September, 2018, from https://www.ncdc.noaa.gov/global-warming/temperature-change. Data from: Jouzel, J., V. , O. et al. (2007). "Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years". Science 317/5839, p. 793-796. https://doi.org/10.1126/science.1141038
Temperature : Snyder, Carolyn, (2016) "Evolution of global temperature over the past two million years" Nature 538 226-8 https://doi.org/10.1038/nature19798 See also: https://www.2degreesinstitute.org/
Sea Level: James Hansen, M. S., Gary Russell, Pushker Kharecha (2013). "Climate sensitivity, sea level and atmospheric carbon dioxide." Philosophical Transactions of the Royal Society A: Mathematical, Physical, and Engineering Sciences 371(20120294). http://dx.doi.org/10.1098/rsta.2012.0294