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.
From 1960–2010, winter temperatures on the Antarctic Peninsula rose 6 ºC. That put glaciers into retreat.
Correspondingly, wind patterns shifted. Antarctica’s climate is strongly affected by westerly winds which buffet the Southern Ocean.
Warmer water toward the tropics heats the overlying air, which rises, expands, and meanders south. The subtropical jet stream off eastern Australia gives the warm air mass a push in the same direction as the Westerlies.
Concurrently, high atmospheric pressure over the mid-latitudes pushes air toward the poles. Part of this comes from the El Niño–Southern Oscillation (ENSO) dynamic, which couples temperature changes in the ocean to changes in the atmosphere, and thereby governs tropical eastern Pacific surface water temperature.
Winds rushing south turn eastward with Earth’s rotation. With the warming, air pressure along the Antarctic coast has lowered, strengthening the Westerlies, and driving them farther south. This brings more warm air to the Antarctic Peninsula, reinforcing the dynamic.
Winds drive deep-ocean currents. Over the Southern Ocean, the Westerlies produce the strongest ocean current on the planet: the Antarctic Circumpolar Current. As much as 4 km deep and 1,000 km wide, this current moves 127 million tonnes of water per second.
Less sea ice, stronger winds, and changed currents cause seawater to mix more deeply. This churns sunlight-dependent phytoplankton into the ocean’s depths. As a result, phytoplankton biomass declined 12% from 1980 to 2010.
Loss of phytoplankton means fewer krill and fish larvae. These creatures are also getting hammered by the loss of sea ice, which hides them from predators. Losses on the lower trophic levels diminishes prospects on up the chain, thus warping the food web.
The Southern Ocean is a carbon sink: accounting for 40% of total global ocean uptake of atmospheric CO2. Deeper ocean mixing means stronger upwelling. Stirring carbon-laden waters from the depths toward the surface increases carbonate levels there, leading to carbon outgassing. This too serves as a warming feedback loop.
El Niño & La Niña
El Niño is a periodic climate pattern in the tropical Pacific Ocean which presents high surface air pressure in the western Pacific along with warm ocean surface current in the eastern Pacific Ocean.
The trade winds across the tropical Pacific typically blow from east to west, keeping surface water in the central and eastern Pacific slightly cooler, while warm water accumulates on the western side, toward Indonesia. When these winds weaken, waves of warmer water slowly make their way eastward along the equator toward South America. This can initiate an El Niño or feed one that has already started.
As the ocean is a tremendous reservoir of heat, water even a few degrees warmer can toast the air above it. During an El Niño, the waters in the warmer central-eastern Pacific take over as the engine driving the wind pattern known as the Walker circulation.
Historically, an El Niño has formed every 2 to 7 years, when Pacific winds shift massive pools of warm water, scarcely submerged, eastward. The warm water surfaces, releasing its heat into the atmosphere. This causes global shifts in storms, rainfall, and temperature.
El Niño pushes up global surface temperatures, birthing deluges in southeastern South America and western North America, while bringing drought to India, Australia, Indonesia, and southern Africa.
Global warming has been weakening the Walker circulation and shifting it eastward for the past century. This is decreasing rainfall in the western Pacific, around Indonesia, and increasing it over the central Pacific Ocean.
Winds make all the difference in the intensity of an El Niño. Wind patterns around the world are driven by an intricate complex of factors.
In 2014, an especially intense El Niño seemed to be in the works, as warm Pacific Ocean sloshed eastward. But that July, serious winds pushed westward, thwarting the budding El Niño from building. Those same winds prevented stored ocean heat from being released.
In a sense, we dodged a bullet in 2014 by not having a monster El Niño. But that was short-lived, because the conditions that shut that developing El Niño down set up the big one in 2015. ~ American oceanographer Michael McPhaden
In March 2015, the lingering heat gave that year’s El Niño a jump-start into extremity. Abetted by the right winds, the 2015 El Niño that ended in May 2016 was full strength. The world’s coral reefs suffered terribly.
Global temperature peaks typically occur toward the end of an El Niño, but every event has its own quirks.
With warmer oceans, the world’s basic climate gyre is changing. It is likely strong El Niños will become more frequent, and that weather events unleashed by them will intensify.
La Niña is the opposite oscillation to El Niño: cold water in the eastern equatorial Pacific Ocean (by 3–5 ºC) and low air pressure. A La Niña often follows an El Niño, though not always.
La Niña commonly causes drought in the western Pacific and southeastern United States, flooding in northern South America, and mild wet summers in northern North America.
The cycles of El Niño and La Niña have recurred for many thousands of years. The impact of global warming on these patterns is not known.
The levels of greenhouse gases (carbon dioxide, water vapor, and methane) are high during hothouse. The atmosphere becomes increasing hot and humid. The oceans warm more slowly. Ocean warming begins long before its pronounced atmospheric expression, as the oceans are a vast heat sink.
Volcanic activity can cause short-term variations in climate. Conversely, quick global temperature rises and associated rapid ice melting, as in current times, incites volcanic activity.
Sea level rises as continental glaciers melt. The weight on continents lessens, while the pressure on oceanic tectonic plates increases. This changes the pattern of crust stresses, opening more routes for ascending magma.
This gyre also increases seismic activity. But falling water on land can have its own immediate effect. Heavy rainfall tamps the velocity of seismic waves during earthquakes, slightly lessening their severity.
Icehouse and hothouse are geological time scale trends typically lasting many millions of years. These long-term climatic cycles have had a dramatic effect in shaping the evolution of life, as have shorter climatic cycles when abrupt.