The Human Epoch: How we are destabilising the Earth
In this post from NET-ZERO:
What is the Anthropocene and why it may already be upon us? Given the changes we have already made to the planet, many experts believe the geological Epoch of humans has already arrived.
How is human activity is disrupting the carbon cycle? Whilst annual human emissions may be ten times smaller than natural flows, they have no balancing flow in the short term - so CO2 is accumulating in the atmosphere and has already warmed the Earth by over 1⁰C.
What is the Paleocene-Eocene Thermal Maximum? The PETM occurred 56 million years ago when temperatures rapidly increased by about 6⁰C. We are currently emitting CO2 ten times faster than this event.
The Human Epoch
Not long ago, humans were a tribe of subsistence co-operative farmers tending to the land. But fossil fuels have turbo-charged productivity and turned humanity into a collective superpower with planet changing consequences.
We have reduced the number of trees on Earth from six trillion to just three trillion to make way for crops and livestock. We now move more rock, soil, and nitrogen every year than all the natural processes put together. If you weighed all large mammals on Earth, just 3% of the mass would be wild animals, 30% is human flesh, and the remaining 67% is domesticated livestock bred to sustain our energy hungry bodies.
We account for just 0.01% of life yet have destroyed 80% of wild animals and 50% of plants.
We have altered the rules of evolution through selective breeding, genetic engineering, health, welfare reform, and conservation. Global trade has reconnected the supercontinent Pangea and mixed once discrete species and diseases. We have learnt how to release, but not yet harness, the energy from atomic fusion with world-changing consequences.
We have also come to understand that humans are tampering with the global thermostat. Once thought to be the innocuous and invisible by-product of the industrial revolution, carbon dioxide has become the very definition of anthropic change to the planet.
The idea of the Anthropocene has been proposed and popularised by atmospheric chemist Paul Crutzen as the next geological epoch, one characterised by the human impact on Earth’s geology and ecosystems. Given the changes we have already made to the planet, many experts believe the Anthropocene has already arrived.
Destabilising Earth’s Systems
Our fossil-fuel-powered endeavours over the last few hundred years have pumped over 2,000 billion tonnes of additional CO2 into the atmosphere, increasing CO2 concentrations from a stable 250-280 ppm to over 410 ppm. As humans turn up the thermostat, the Earth is now moving away from the stable temperature of 14-15⁰C that we have enjoyed for the last 12,000 years.
To better understand how our activities have tipped the natural balance we must understand the flows of the carbon cycle and solar intensity today and into the future:
The Fast Carbon Cycle: Every year land biomass will exchange 120 billion tonnes of carbon with the atmosphere and oceans exchange around 90 billion tonnes of carbon.
Anthropic Emissions: We now release 9 billion tonnes of carbon (33 billion tonnes CO2) per year from the combustion of fossil fuels and another 5.5 billion tonnes of carbon equivalent emissions (20 billion tonnes CO2e) from burning forests and other greenhouse gases. Human emissions may be just one tenth of land or ocean flows, but they have no natural counterbalance.
The Slow Carbon Cycle: The slow carbon cycle exchanges less than one billion tonnes of CO2 emissions between atmosphere and rock each year. This exchange will move to counterbalance the higher CO2 concentrations from human activity because higher temperatures speed up rock weathering, but this will take hundreds of thousands of years: the rate at which rock grows.
Earth’s Orbit: Over the next 30,000 years the intensity of sunlight will reach its cyclical minimum at a level similar to the last ice age where global average temperatures were 3-4⁰C below today. A cooling rate of 0.01⁰C per century: too slow to offset anthropic warming in the near term.
Simplified depiction of the Carbon Cycle. Grey text shows carbon stores in billion tonnes (Gt). Black text shows annual carbon flows in billion tonnes (Gt) (multiply by 3.7 for CO2). Natural flows balance each other, human emissions into atmosphere are only half offset by the enhanced uptake of the land-ocean sink processes. Adapted from NASA Earth Observatory and NOAA carbon cycle data.
Prior to the industrial revolution, the amount of CO2 leaving the atmosphere was balanced by emissions from volcanoes, oceans, and land. The detailed reconstructions of the last 2,000 years show CO2 concentrations between 270 and 280 ppm. Temperatures were below today, with small fluctuations during the medieval warming when temperatures increased by 0.5⁰C. Then the little ice age or maunder minimum in the late seventeenth century cooled the planet by 0.5⁰C as sunspots became rare and solar intensity dropped. Even this small change saw the River Thames occasionally freeze over with ice thick enough for Londoners to hold a winter festival on the water.
Reconstructed temperature (dashed line) and atmospheric CO2 concentrations (solid line) for the last 2,000 years. Temperature data from NOAA and IPCC 5th report.31 CO2 from Macfarling Meure et al. (CO2 Law Dome Antarctica ice core).32 Temperatures are relative to the pre-industrial average through the years 1850-1900.
Changing the Climate
Global temperatures started to respond to higher CO2 concentrations by the 1920s increasing by about 0.2⁰C, first recognised by Guy Stewart Callendar. The temperature rise stalled from 1940-1970 as aerosol pollution in the atmosphere, due to the post-war industrial boom, reflected more sunlight away from the Earth, but pollution reduction and further CO2 increases led a return to increasing temperatures from the years 1970 to 2000. The temperature rises slowed in the 2000s as the oceans took on more of the heat energy for a period, before atmospheric temperature increases re-accelerated once again over the last decade. The planet is now over 1⁰C warmer than it was 100 years ago with another increase of nearly half a degree to come as temperatures play catch up with CO2 concentrations already in the atmosphere.
Temperature (dashed line) and CO2 concentrations (solid line) since 1860. Data from NASA Land-Ocean Temperature Index and NOAA ESRL CO2 data (Antarctic ice core data to 1959 Law Dome (D.M Etheridge et al.) and Mauna Loa, Hawaii direct measurements thereafter).33, 34 Temperatures are relative to the pre-industrial average between 1851-1880.
So, whilst annual human emissions may still be ten times smaller than the flow of carbon from land or sea, they have no balancing flow in the short term. The land and the oceans have done their best to accommodate some of the additional CO2; the ocean has turned from a net carbon emitter to a net absorber. This change has removed 50-60% of the additional atmospheric carbon at the expense of an increasingly acidic ocean.
Humans have emitted over 2,000 billion tonnes of CO2 since the year 1750 and raised atmospheric concentrations by 135 ppm; that’s a 1 ppm increase for every 18 billion tonnes.
But the rate of change is too fast for the land and oceans to accommodate. Carbon is entering the atmosphere faster than any other event over the last 66 million years, according to work published in Nature by Richard Zeebe and his colleagues.
The closest historical period of change was 56 million years ago during the Paleocene-Eocene Thermal Maximum (PETM) when carbon dioxide was pumped into the atmosphere at a rate of up to 4 billion tonnes per year for 4,000 years and temperatures increased by about six degrees. The total CO2 emitted over this period was roughly the equivalent of burning half our remaining fossil fuel resources. Climate scientists consider this event a good comparison for the changes taking place today, except that we are emitting greenhouse gases more than ten times faster, at over 50 billion tonnes per year.
Temperatures and the Earth’s systems have started to respond to the unprecedented CO2 increases. With the planet already over 1⁰C warmer through the last 100 years, the rate of temperature increase is accelerating towards the fastest rate of the last million years. If no change is made to the existing fossil fuel system then CO2 and temperature will further climb, natural dampening mechanisms such as ocean uptake will saturate, and amplifying feedback mechanisms become increasingly dominant.
