12.2: A Sixth Mass Extinction- the Anthropocene

The geological record contains information about the dramatic changes that have occurred on our planet. The lessons from the five known mass extinctions shows that the impact to climate and life is greatest when change is more sudden.

During the middle to late Cretaceous period the overall temperature of the Earth increased by 5°C. However, this change took place slowly, over the course of millions of years. The warming is thought to have been caused by the steady release of greenhouse gases from volcanic eruptions. Because the rate was so slow, the oceans were able to absorb the greenhouse gases without causing significant acidification. As a result, most life was able to adapt or migrate and survive.

In contrast, during the Palocene-Eocene thermal maximum (PETM) temperatures rose by 0.025°C every 100 years. In just a few thousand years, the temperature had increased by 5°C. The warming was caused by a combination of volcanoes, methane release from the seafloor, and peat/coal fires. As heating progressed, the permafrost began to thaw, leading to the release of even more greenhouse gases. The oceans began to acidify, leading to the extinction of many organisms living on the seafloor.

Today, humans are exerting a pressure on the Earth system, so extreme that it is triggering the start of a new geological epoch: the Anthropocene. Modern-day global warming appears to be occurring at a rate of 1-4°C per 100 years, orders of magnitude faster than the rate during the PETM. The Earth is expected to gain 2-10°C in the next 100 years. There have been times in the history of the Earth when there was as much - or perhaps more - carbon in the atmosphere. There has probably never been a time in the past 4.5 billion years when the rate of change of carbon in the atmosphere has been so rapid. We have pulled coal from the carboniferous period out of the ground and we are burning it to fuel our industrial revolution. Like it or not, our mark will be left in the geological record of out planet, the first stage of a mass extinction, a rapid decrease in biodiversity has already started.

The Sixth Extinction

Since the industrial revolution, we have had a front row seat to the sixth major mass extinction. Human activity has led to widespread habitat loss, climate change, pollution, and rising ocean acidity. Vertebrates are dying out at a rate estimate to be a hundred times faster than they would be without human interference.

Amphibians that interact with polluted land and water are most impacted, with a current extinction rate of tens of thousands times greater than the background extinction rate. Rising temperatures are threatening most species with changes that are now happening on the timescale of generations rather than thousands of millions of years as before.

The Energy Budget for Earth climate

An "airless Earth" will absorb higher energy photons from Sun in the top 1-meter of soil and then re-radiate that energy back into space in the form of infrared radiation. The incoming energy comes from a 5500 K black body - the Sun - and an airless Earth would radiate as a blackbody with a temperature of about 253 K.

Figure \(\PageIndex{1}\): The energy from the Sun peaks at optical wavelengths and would be absorbed and re-emitted by an airless Earth as infrared wavelengths. Recall that the Sun and Earth will emit as black bodies with a peak wavelength that is inversely proportional to their effective temperature.

However, it is more difficult for the incoming solar energy to escape when the Earth has an atmosphere. Molecules with three atoms are particularly effective at absorbing the infrared radiation released from the surface of the Earth. The absorbed energy is transformed into energy for vibrating and bending the molecular bonds of these greenhouse gases, as shown in the short TED-Ed video below.

Ultimately energy is re-emitted and adds heat into the atmosphere. The energy budget (incoming higher frequency solar energy and absorbed-released lower frequency energy) is summarized in the image below. Not all of what comes in is released again.

Figure \(\PageIndex{2}\): The energy budget of the Earth, including greenhouse gases.

The solar flux above the Earth's atmosphere is 1360 Watts per square meter. However, only one hemisphere of the Earth faces the Sun. Furthermore, the surface of the Earth is curved not flat, so there is a geometric reduction, such that an average of 340 Watts per square meters is intercepted above the atmosphere over the entire surface of the Earth. Without greenhouse warming from the Earth's atmosphere, our planet would be a frozen world. The presence of methane, water, carbon dioxide and other natural greenhouse gases transformed the Earth to a habitable world with oceans of liquid water.

Anthropogenic climate change

In 1896, the Swedish scientist Svante Arrhenius calculated that the burning of coal by humans would add carbon dioxide to the atmosphere causing a small amount of warming. This was a curiosity, but it was hard to imagine that the then-world population of 1.6 billion people would have much impact. By the 1930's, the precise monitoring of climate for military purposes provided data that showed that the global mean temperature on Earth was indeed rising. Some people speculated that changes in the solar irradiance would also affect the temperature of Earth. Whatever the underlying reason, by the 1960's scientists estimated that the planet would warm by a few degrees over the coming century. Even though the physical feedback was poorly understood, scientists in the 1980's agreed that increasing the amount of carbon dioxide would increase the temperature of our planet. Data from ancient ice cores showed that a doubling of CO₂ was correlated with a 3 degree rise in the global average temperature. Climate research intensified. With faster computers, it is now possible to carry out sophisticated simulations of the effect of rising carbon dioxide on the temperature of Earth.

There are some effects of greenhouse gases outlined in the 2018 U.S. National Climate Report that are undisputed by scientists.

1. The concentration of carbon dioxide (CO₂) has increased from pre-industrial levels to 391 ppm in 2012 and is now rising at a rate of 1.8 ppm per year.
2. The present level of CO2 is higher now than at any time in the last 15 million years. This information comes from paleoclimatic and geological evidence.
3. Global mean temperature is about 1 degree C above pre-industrial levels and increasing.
4. The oceans have also been warming; 90% of the excess heat from increased greenhouse gases is stored in the oceans.
5. The loss of sea ice has tripled in the past 20 years.
6. Sea levels have risen by about 20 centimeters around the world and are now increasing at a rate of about 3 cm per decade. Melting from the continents of Greenland and Antartica could add another 15 cm by the end of the 21^st century.
7. There has been a tenfold increase in extreme heat waves since the 1950s.

The consequence of 2-4 degrees of warming

Projecting the risks forward in time, every degree of warming exacerbates the stress on our planet and on civilization. The greatest warming occurs over land, bringing extreme heat waves, widespread drought, and increasing fires and deforestation. The resulting large-scale displacement of populations threatens security and economic systems. The higher temperatures and flooding of low-lying delta areas reduce crop yields and increase malnutrition and disease. Increasing carbon dioxide results in acidification that threatens marine life and ecosystems. A large-scale loss of biodiversity is already occurring with the loss of ecosystems. The predictions are unacceptably dire if the global temperature increases by four degrees Celsius, as predicted for 2100 if we continue business as usual. Despite decades of warnings by climate scientists, we continue to miss our targets for reducing the use of the fossil fuels that are directly responsible for increasing green house gases (carbon dioxide and methane).

The data are unequivocal. We know what to do to mitigate climate change. But, this is a problem that we have to solve together. Will humanity answer this call to action?