Mass Extinctions and Climate Variability

Learning Objectives

  1. Locate the 5 major mass extinction events of the Phanerozoic on the geologic time scale, and recognize that extinctions define major boundaries between time periods.
  2. Describe the effects of specified mass extinctions on biodiversity, including which groups of organisms died and which groups flourished in the vacated niches.
  3. Describe evidence for hypotheses explaining the causes of two mass extinctions, the End Permian (P/T) and End Cretaceous (K/T)
  4. Evaluate evidence for climate change today relative to historical records of temperature and carbon dioxide and predict possible outcomes for species diversity

The Five Major Phanerozoic Mass Extinctions and their Effects on Biodiversity

The information below is modified from Openstax Biology 47.1.

Changes in the environment often create new niches (living spaces) that contribute to rapid speciation and increased diversity events called adaptive radiations. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes that obliterate life, can result in devastating losses of diversity. Such periods of mass extinction have occurred repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve into the empty niches left behind. You are already familiar with the extinctions that occurred during the Great Oxygenation event, when cyanobacteria altered the gaseous composition so dramatically that even species that produced oxygen went extinct from the presence of the very same oxygen. While that extinction was probably massive, it is not on the list of five mass extinctions of the Phanerozoic that are well documented in the fossil record.

Before we discuss mass extinctions of the Phanerozoic, first let’s recall the Eras and Periods of this Eon:

Mass extinctions in the fossil record define the geological periods of the history of life on Earth; these mass extinctions typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. At five points in the Phanerozoic, a large number of taxa were lost in a short geologic time span:

Phanerozoic mass extinctions over geologic time, showing relative extinctions of taxa. Image from Openstax Biology.
  • The end-Ordovician extinction event was the second largest recorded extinction event, when about 85 percent of marine species (land plants and a few groups of animals lived outside the oceans) became extinct.
    • The leading hypothesis is that the end-Ordovician extinction was caused by a period of glaciation and then warming in a rapid (1 million year) timespan, affecting both climate and sea levels, and with both the cooling and warming each causing a round of extinctions.
  • The end-Devonian extinction appears to have affected primarily marine species and not terrestrial plants and animals. The causes of this extinction are poorly understood.
  • The end-Permian extinction (also called P/T or Permian/Triassic) was the largest recorded extinction event so far in the history of life, with an estimated 96 percent of all marine species and 70 percent of all terrestrial species lost.
    • Impact on biodiversity: It was at this time that the trilobites, a group that survived the end-Ordovician extinction, became extinct. On land, many seedless plant lineages disappeared, and the disappearance of some dominant species of Permian reptiles made it possible the dinosaurs to emerge. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The warm and stable climatic conditions of the following Mesozoic Era promoted an explosive diversification of dinosaurs into every conceivable niche in land, air, and water. Plants, too, radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became very large on the abundant food available.
    • Causes: The causes for this mass extinction are not clear, and it has been called “the unexplained catastrophe.” The leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. Evidence for this lies in the massive layers of basaltic rock of the Siberian Traps, which indicate extreme volcanic eruptions that lasted for approximately one-two million years. High global temperatures would have resulted from increased carbon dioxide and methane in the atmosphere from the volcanic activity, and warming of ocean water combined with nutrient runoff from dead and decomposing terrestrial species would have caused the oceans to became largely anoxic (devoid of oxygen), ultimately suffocating much of the oxygen-dependent marine life.
  • The causes of the end-Triassic extinction event are not clear and hypotheses of climate change, asteroid impact, and volcanic eruptions have been argued; these hypotheses are not mutually-exclusive. The extinction event occurred just before the breakup of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.
  • The end-Cretaceous extinction event 65 million years ago was not the biggest extinction event, but it is the most famous because this extinction saw the loss of all dinosaurs except a theropod clade that gave rise to birds. (The end Cretaceous is also called the Cretaceous/Tertiary or K/T; “K” is the traditional abbreviation for “Cretaceous,” and “Tertiary” is a now-obsolete term that describes the Paleogene)
    • Impact on biodiversity: Plants died, herbivores and carnivores starved during this extinction event; in fact, every land animal that weighed more than 25 kg became extinct as a result of the end-Cretaceous extinction event. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in geological time, than for the end-Permian extinction, on the order of 10 million years. As plant biodiversity began to recover in the following Cenozoic Era, the surviving mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs. Birds, descended from the only surviving group of dinosaurs, became aerial specialists. The rapid dominance of flowering plants created new niches for insects, birds, and mammals. Changes in animal species diversity during the late Cretaceous and early Cenozoic were also promoted by a dramatic shift in Earth’s geography, as continental plates slid over the crust into their current positions, leaving some animal groups isolated on islands and continents, or separated by mountain ranges or inland seas from other competitors. Early in the Cenozoic, the evolution of grasses and coral reefs created entirely new ecosystems. Late in the Cenozoic, further extinctions followed by speciation occurred during ice ages that covered high latitudes with ice and then retreated, leaving new open spaces for colonization.
    • Causes: The main cause of the end-Cretaceous extinction event resulted from the cataclysmic impact of a large meteorite, or asteroid, off the coast of what is now the Yucatan Peninsula in Mexico. Evidence in support of this hypothesis includes a sharp spike in the levels of iridium (which rains down from space in meteors at a fairly constant rate but is otherwise absent on Earth’s surface) at the rock stratum that marks the boundary between the Cretaceous and Paleogene periods, and an appropriately aged and sized impact crater. At the time, skies would have darkened and temperatures would have fallen as the meteor impact and tons of volcanic ash blocked incoming sunlight.

Modern Extinctions

A sixth, or Holocene, mass extinction is currently underway, primarily caused by the activities of Homo sapiens. The Holocene Epoch is the name of the most recent ~12,000 years of Earth’s history, or the time since the last major ice age. (Epochs are subdivisions of Periods within geologic time.) Since the beginning of the Holocene period, there are numerous extinctions of individual species that are recorded in human writings. Most of these are coincident with the expansion of the European colonies since the 1500s.

Estimates of Holocene extinction rates are hampered by the fact that most extinctions are probably happening without our even knowing it. While the extinction of a noticeable species of bird or mammal is likely to be observed by humans, especially if it has been hunted or used in some other way, there are many organisms that are of less interest to humans (though not necessarily of less value) and many that are as-yet undescribed.

We measure extinction rates against a background extinction rate of approximately 1 extinction per million species per year (or 1 E/MSY), or 1 extinctions per year for every 1 million species on the planet. Let’s put some numbers to this rate to help give it context. Assume there are about ten million species in existence. We would expect that ten species would go extinct each year. One contemporary extinction rate estimate uses the extinctions in the written record since the year 1500. For birds alone this method yields an estimate of 26 E/MSY. Even this seemingly high value may be an underestimate for three reasons:

  1. many species would not have been described until much later in the time period, so their loss would have gone unnoticed.
  2. the number of recently extinct species is increasing because extinct species now are being described from skeletal remains.
  3. some species are probably already extinct even though conservationists are reluctant to name them as such.

Taking these factors into account raises the estimated extinction rate closer to 100 E/MSY.  More alarmingly, the predicted rate by the end of the 21st century is 1500 E/MSY.

Evidence for a Changing Climate Today in the Anthropocene

The following information is from OpenStax Biology 2e 44.5

Today we are transitioning out of the Holocene Epoch and into the Anthropocene Epoch, named for the outsized influence humans are having on the Earth’s climate and ecology. (The Anthropocene is an unofficial epoch, used to describe Earth’s most recent history; in 2023, it was proposed that the Anthropocene began in the 1950s.) The Anthropocene is noted for human-driven climate change. Climate change can be understood by approaching three areas of study:

  • drivers of global climate change
  • evidence of current and past global climate change
  • documented results of climate change

It is helpful to keep these three different aspects of climate change clearly separated when consuming popular media reports about global climate change; it is common for reports and discussions about global climate change to confuse the data showing that Earth’s climate is changing with the factors that drive this climate change.

Climate refers to the long-term, predictable atmospheric conditions of a specific area, while weather refers to the conditions of the atmosphere during a short period of time. To better understand the difference between climate and weather, imagine that you are planning an outdoor event in Atlanta:

  • You would be thinking about climate when you plan the event in the fall or spring rather than the summer because you have long-term knowledge that any given Saturday in the months of May to August will likely be hot!
  • However, you cannot guarantee temperate and cool weather on any specific Saturday because it is difficult to accurately predict the weather.

A common misconception about global climate change is that a specific weather event occurring in a particular region, like rainstorm in Georgia, provides evidence of global climate change. Instead, a rainstorm is a weather-related event and not a climate-related one. Climate can be considered “average” weather that takes place over many years.

The Earth has gone through periodic cycles of increases and decreases in temperature. The natural drivers of climate change include Milankovitch cycles, changes in solar activity, and volcanic eruptions:

  • The Milankovitch cycles describe the effects of slight changes in the Earth’s orbit on Earth’s climate. These range in length between 19,000 and 100,000 years, which are huge time scales relative to human activity.
  • Solar intensity is the amount of solar power or energy the sun emits in a given amount of time. As solar intensity increases (or decreases), the Earth’s temperature correspondingly increases (or decreases). Changes in solar intensity have been proposed as one of several possible explanations for the Little Ice Age.
  • Volcanic eruptions last a few days, but the solids and gases released can influence the climate over a period of a few years, causing short-term climate changes, generally cooling the climate. The output of gases and solids released by volcanic eruptions into the atmosphere can include carbon dioxide, water vapor, sulfur dioxide, hydrogen sulfide, hydrogen, and carbon monoxide. They establish a haze that block out sunlight and triggers climate cooling globally, often lasting one or more years.

None of these factors, however, leads to rapid increases in global temperature or sustained increases in carbon dioxide. For an explanation of the current upward spike in temperatures, we have to look to anthropogenic drivers of climate change. Greenhouse gases are probably the most significant drivers of the climate. Greenhouse gases absorb and emit radiation and are an important factor in the greenhouse effect: the warming of Earth due to carbon dioxide and other greenhouse gases in the atmosphere. The effect works like this:

  • When heat energy from the sun strikes the Earth, atmospheric gases like carbon dioxide, methane, water vapor, nitrous oxide, and ozone trap the heat in the atmosphere, similar to how the glass panes of a greenhouse keep heat from escaping. These gasses also help protect terrestrial species from the DNA-damaging UV from the sun.
  • The sunlight radiation that makes it to Earth is converted into thermal (infrared) radiation, and then a portion of that energy is re-radiated back into the atmosphere. However, those same gasses reflect much of the thermal energy back to the Earth’s surface, and the more greenhouse gasses there are in the atmosphere, the more thermal energy is reflected back to the Earth’s surface, heating it up and the atmosphere immediately above it.

The burning of fossil fuels is an important source of greenhouse gases, which play a major role in the greenhouse effect.

Historic Evidence for Global Climate Change

Since we cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, we instead indirectly measure temperature through historical evidence of Earth’s past climate which can be obtained from air bubbles trapped in Antarctic ice.

Figure upper panel: Temperatures relative to modern day were determined from the amount of deuterium (a nonradioactive isotope of hydrogen) present in Vostok (Antarctic) ice cores (top). The time scale doesn’t give resolution to the last 200 years (see text). Lower panel: By measuring the amount of CO2 trapped in the ice, scientists have determined that carbon dioxide levels in the atmosphere have historically cycled between 180 and 300 parts per million (ppm) by volume. Image credit: OpenStax Biology 2e 44.5

The 0°C on the Y-axis of the top graph above (blue) refers to the long-term average. Temperatures that are greater than 0°C exceed Earth’s long-term average temperature, and temperatures that are less than 0°C are less than Earth’s average temperature. This figure shows that there have been periodic cycles of increasing and decreasing temperature in the last 400,000 years. Two significant temperature irregularities have occurred during the past 2,000 years:

  • the Medieval Climate Anomaly (900–1300 AD) of 0.1-0.2 °C above normal was a warmer period,
  • the Little Ice Age (1550–1850 AD) showed cooling of 1 °C. During the Little Ice Age, the Industrial Revolution (early 1800s) ushered in fossil fuel use. When a fossil fuel is burned, carbon dioxide is released, and atmospheric carbon dioxide began to rise.
The atmospheric concentration of CO2 has risen steadily since the beginning of industrialization. Image credit: OpenStax Biology 2e 44.5

Direct evidence supports the relationship between atmospheric concentrations of carbon dioxide and temperature: as carbon dioxide rises, global temperature rises. Since 1950, the concentration of atmospheric carbon dioxide has increased from about 280 ppm to 392 ppm in 2011.

Scientists look at patterns in data and try to explain differences (or deviations) from these patterns. In historical carbon dioxide patterns, scientists have concluded that it took around 50,000 years for the atmospheric carbon dioxide level to gradually increase from its low minimum concentration to its higher maximum concentration. Since the industrial revolution, atmospheric carbon dioxide concentrations have increased beyond the historical maximum of 300 ppm. The current increases in atmospheric carbon dioxide have happened very quickly—in a matter of hundreds of years rather than thousands of years. What is the reason for this difference in the rate of change and the amount of increase in carbon dioxide? A key factor that must be recognized when comparing the historical data and the current data is the presence and industrial activities of modern human society; no other driver of climate change has yielded changes in atmospheric carbon dioxide levels at this rate or to this magnitude.

Human activity releases carbon dioxide and methane, two of the most important greenhouse gases, into the atmosphere in several ways. The primary mechanism that releases carbon dioxide is the burning of fossil fuels, such as gasoline, coal, and natural gas (see here). Deforestation, cement manufacture, animal agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide. Methane (CH4) is produced when bacteria break down organic matter under anaerobic conditions. Anaerobic conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines of herbivores of industrially-farmed cattle. Another source of methane is the melting of clathrates, frozen chunks of ice and methane found at the bottom of the ocean and released as ocean waters warm

Scientists have geological evidence of the consequences of long-ago climate change, such as the cause of the Permian extinction. Modern-day phenomena such as retreating glaciers and melting polar ice cause a continual rise in sea level. The Grinnell Glacier in Glacier National Park in Montana is one of many examples of the loss of ice on the surface, which is leading to an increase in sea levels

Figure. The effect of global warming can be seen in the continuing retreat of Grinnell Glacier. The mean annual temperature in the park has increased 1.33 °C since 1900. The loss of a glacier results in the loss of summer meltwaters, sharply reducing seasonal water supplies and severely affecting local ecosystems. (credit: modification of work by USGS)

In addition to changes in temperature, ice, and sea level, organisms are also being affected by the changes in temperature. The geographic distribution is shifting poleward and to areas of higher elevation for many species, and timing of breeding seasons and of animal migrations is shifting. Recent research shows that 385 plant species in Great Britain are flowering 4.5 days sooner than was recorded earlier during the previous 40 years. Insect-pollinated species were more likely to flower earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect pollinators emerged earlier. This mismatched timing of plants and pollinators could result in reduction of plant populations because insect-pollinated plants rely on their pollinators for successful reproduction.