365 Days of Climate Awareness 20 - Greenhouse Earth

 

365 Days of Climate Awareness

20 - Greenhouse Earth

Greenhouse Earth is a climatic condition which has existed periodically throughout the planet's history, in which no continental (as opposed to alpine) glaciers exist. It is not to be confused with the intervening period between two glaciations. Greenhouse (and the opposite, icehouse) conditions persist for several million years and are due to longer-term causes than Milankovitch cycles.

Changes in carbon dioxide concentration are one component, but earth's atmosphere has changed tremendously since the planet's early existence (a future post). The sun's output--known as the solar constant--has not been constant over the last five billion years, but has increased by nearly 50% from its early level. And atmospheric carbon dioxide level itself is controlled by other factors, one in particular.

The major influence on carbon dioxide concentration over the earth's history is tectonics: the breakup, movement and collision of earth plates. The weathering of rock consumes carbon dioxide, and sequesters it in seafloor sediment. So when conditions arise exposing large amounts of bedrock to the environment, a drawdown of CO₂ occurs. When less weathering happens, carbon dioxide does not act to decompose rock as much, and atmospheric concentrations can rise.

There are a few ways this can happen. At times the earth is more tectonically active than at other times: plates are being generated and destroyed at faster rates, and mountain-building is more pervasive. The arrangements of continents matters too. Smaller continents in the tropical zones will be exposed to a great deal of weathering, being subjected to large amounts of rain. When tectonic activity is lower, or continents around the planet are in colder areas, or are larger and more arid, less CO₂ is consumed by rock weathering, and volcanic outgassing can continue to warm the planet.

Earth has been in greenhouse conditions for roughly 85% of its existence.

Tomorrow: icehouse earth.

Be well!