Earth cycles between 2 global climates: icehouse (aka ice age) and hothouse (aka greenhouse). Both reflect the supercontinent cycle.
Continents converge during icehouse. Sea level is low, owing to little seafloor production. Climate is generally cool and arid. Continental ice sheets are present, which wax and wane between glacial (ice age) and interglacial (temperate) periods.
Ice sheets build and retreat during glacial and interglacial periods, respectively. These periods owe to Milankovitch cycles: changes in Earth’s orbit, tilt, and proximity to the Sun (Earth has an elliptical orbit).
Icehouse climate tends toward cool and arid. Icehouse has occurred in only 20% of Earth’s history.
Heading to Hothouse
The emergence of humans was largely during a greenhouse interlude (interglacial) in an icehouse period. The world is now rapidly heading toward hothouse. This coincides with the continents coming apart. Sea level rises with seafloor spreading. Oceanic rifting zones release CO2.
Heading to hothouse, glacial melt adds to sea level rise. During hothouse, there are no continental glaciers whatsoever.
There is a feedback loop between sea level and glaciation. Inching into icehouse, lowering sea level, coupled with cooler global temperature, causes marine ice sheets to grow. A cooling climate grows terrestrial ice masses, which initiates sea level fall (regression).
The reverse loop, transgression (sea level rise), also begins with climate change. Marine ice sheets and terrestrial glaciers melt. Sea level rises.
Transgression has been ongoing for the past 18,000 years, since the peak of the last ice age. That trend is now accelerating.
Human activities have instigated warming feedback loops via greenhouse gas emissions and deforestation. The pronounced locuses are the polar regions, most notably ice loss. These are driven by warm currents melting the undersides of ice shelves.
Changes in global patterns cause extreme weather events around the world. Not all correspond with heat. Bitter winter storms bite into North America as the jet stream weakens, allowing frigid Arctic air to dip deep into the United States.
Rising temperatures in the Arctic are causing tundra wildfires to become more common. Smoke from the fires drift over the Greenland ice sheet, where the soot tarnishes the ice with its dark mark.
Soot is a powerful light absorber. As soot settles over the ice and captures the Sun’s heat, it accelerates ice melt.
The atmosphere and ocean are both fluids with thermally driven circulatory systems. As with the atmosphere, ocean circulation patterns change during climate cycles.