For the last 540 million years, Earth’s climate has oscillated between three basic states: Icehouse, Greenhouse (subdivided into Cool and Warm states), and Hothouse. The “Hothouse” condition is relatively short-lived and is consequence from the release of anomalously large inputs of CO2 into the atmosphere during the formation of Large Igneous Provinces (LIPs), when atmospheric CO2 concentrations may rise above 16 times (4,800 ppmv), while the “Icehouse” is characterized by polar ice, with alternating glacial–interglacial episodes in response to orbital forcing. The ‘Cool Greenhouse” displays some polar ice and alpine glaciers, with global average temperatures between 21° and 24°C. Finally, the ‘Warm Greenhouse’ lacks of any polar ice, and global average temperatures might have ranged from 24° to 30°C.
Reconstructions of Earth’s history have considerably improved our knowledge of episodes of rapid emissions of greenhouse gases and abrupt warming. Several episodes of global climate change were similar in magnitude to the anthropogenically forced climate change that has occurred during the past century. Consequently, the development of different proxy measures of paleoenvironmental parameters has received growing attention in recent years. Paleobotany, the study of fossil plants in deep geological time, offers key insights into vegetation responses to past global change, including suitable analogs for Earth’s climatic future.
The main forces of climatic change on a global scale are solar forcing, atmospheric composition, plate tectonics, Earth’s biota, and of course, us. Human activity is a major driver of the dynamics of Earth system. Until the Industrial Revolution, the average global CO2 levels fluctuated between about 170 ppm and 280 ppm. But with the beginning of the Industrial Era, that number risen above 300 ppm, currently averaging an increase of more than 2 ppm per year. The average monthly level of CO2 in the atmosphere on last April exceeded the 410 ppm for first time in history. Thus we could hit an average of 500 ppm within the next 45 years, a number that have been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. Therefore, the closest analog for today conditions is the Eocene, meaning greater similarities in continental configuration, ecosystem structure and function, and global carbon cycling.
Some of the best-studied intervals of global change in the fossil plant record include the Triassic–Jurassic boundary, 201.36 ± 0.17 Mya; the PETM, 56 Mya; and the Eocene–Oligocene boundary, 33.9 Mya.The first two events represent rapid greenhouse gas–induced global warming episodes; the last coincides with the initiation of the Antarctic ice sheet and global cooling leading to our current icehouse.
During the PETM, compositional shifts in terrestrial vegetation were marked but transient in temperate latitudes and long-lived in the tropics. The PETM is characterized by the release of 5 billion tons of CO2 into the atmosphere, while temperatures increased by 5 – 8°C. High temperatures and likely increased aridity in the North American temperate biomes resulted in geologically rapid compositional changes as local mixed deciduous and evergreen forest taxa (such as Taxodium) decreased in relative abundance. These suggest that global warming has a marked effect on the composition of terrestrial plant communities that is driven predominantly by migration rather than extinction. However, it’s difficult to draw parallels with Anthropocene warming and vegetation responses because they are occurring at a minimum of 20 times faster than any past warming episode in Earth’s history.
In the early Eocene (56 to 49 Mya), a time of peak sustained global warmth, the Arctic Ocean was ice free, with a mosaic of mixed deciduous, evergreen (Picea, Pinus), and swamp forests, and with high densities of the aquatic fern Azolla. The Azolla bloom reduced the carbon dioxide from the atmosphere to 650 ppm, reducing temperatures and setting the stage for our current icehouse Earth. The eventual demise of Azolla in the Arctic Ocean is attributed to reduced runoff and a slight salinity increase.
The Earth’s poles have warmed and will continue to warm at a faster rate than the average planetary warming, because the heat is readily transported poleward by oceans and the atmosphere due to positive feedback effects involving snow cover, albedo, vegetation, soot, and algal cover in the Arctic and Antarctic. This phenomenon is known as “polar amplification”.
Recent studies about the greening of the Arctic indicates that increasing shrubiness has likely already had an unexpected negative impact on herbivore populations, such as caribou, by decreasing browse quality. Thus, it is important to predict how short-term temporal trends in Arctic vegetation change will continue under CO2-induced global warming. The paleobotanical record of high Arctic floras may provide broad insight into these questions.
Jennifer C. McElwain, Paleobotany and Global Change: Important Lessons for Species to Biomes from Vegetation Responses to Past Global Change, Annual Review of Plant Biology (2018), DOI: 10.1146/annurev-arplant-042817-040405