Why are cycles important?

• Ecosystems are open at a local scale (only the planet is a closed ecosystem)
• Biogeochemical cycles may be influenced by inputs and removals
• The processes involved in cycles are regulated by the biota
• Cycles are involved in the provision of ecosystem services

Why is the carbon cycle unique?

• The carbon cycle is at the centre of the process essential to life
• (Almost) All primary production involves fixing carbon through photosynthesis
• Carbon is obtained from the atmosphere and then cycles through ecosystems

Why is carbon newsworthy?

• The global carbon cycle is influenced by anthropogenic carbon emissions

• This is a concern, as it may impact a wide range of ecosystems

• Initiatives to reduce carbon emissions can impact ecosystems

What is the composition of the atmosphere?

• Dominated by nitrogen (78%)

• Around 1/5th is oxygen (20%)

• Trace gases including argon 1%

• Carbon dioxide at a very low concentration (~450 ppm = 0.045%)

Carbon dioxide is a very small fraction of the atmosphere

What does this imply?

• Nitrogen is an inert gas in the atmosphere. Unusable in this form.
• Oxygen is involved in powering the carbon cycle through respiration.
• Carbon dioxide is distributed fairly uniformly at ground level. Available everywhere
• The available nitrogen (fixed nitrogen) is distributed unevenly
• Human activities that alter available nitrogen tend to act locally.
• Human activities that alter available carbon dioxide act globally

Does this imply that there is no way (now) to increase atmospheric oxygen?

• Essentially yes

• Nitrogen can never be converted to oxygen

• Only carbon dioxide can be converted to oxygen through photosynthesis.

• 20% + 0.045% is still only 20.045%

• So forests do not produce meaningful amounts of oxygen.

Is this a credible message to use to conserve ecosystems?

Is this a legitimate piece of marketing?

Was the atmosphere always like this?

• Lovelock based the Gaia hypothesis on the assumption that the atmosphere was dominated by carbon containing gases (carbon dioxide and methane)
• The current atmosphere is assumed to be the result of early life, which produced oxygen over millions of years.
• However, it has been suggested by some more recent research that the atmosphere of Earth just 500 million years after its creation was “not a methane-filled wasteland as previously proposed, but instead was much closer to the conditions of our current atmosphere” (Trail, Watson, and Tailby 2011)

What were carbon dioxide concentrations in the geological past?

• Very hard to estimate
• Open to much debate
• Most estimates place \({C0}_2\) levels well above current levels, until the quaternary

What were carbon dioxide concentrations in the geological past?

How has atmospheric CO2 changed since the extinction of the dinosaurs?

What was the earth like in the Eocene (50 millon years ago)?

Tropical temperatures have remained fairly stable while the poles fluctuate greatly.

What might have occurred in the Holocene (last 10 thousand years)?

Where are stocks of carbon held?

• Earth’s crust holds about 100 million Pg (Pg = \(10^9\) tons)
• Deep ocean holds about 40 thousand Pg (less than one thousandth of earth crust)
• Fossil fuels holds about 4 to 10 thousand Pg (about a tenth of the deep ocean stock)
• Soils 1.5 thousand Pg (about a fifth of fossil fuels)
• Plants 0.5 thousand Pg (about a third of soils)
• Atmosphere 0.75 thousand Pg (about the same as plants)
• Surface ocean 0.72 thousand PG (about the same as the atmosphere)
• So soils = approximately plant stock plus surface ocean stock

Carbon stocks in thousands of billions of tons

Do the estimated stocks support the Gaia hypothesis?

• Estimated stocks not held in the earth’s crust appear far too low according to Gaia.
• No more than 100 times the current atmospheric concentration.
• Not enough to be the result of conversion of a carbon dominated atmosphere.

How do we change the earth’s atmosphere?

• Carbon is combined with oxygen when fossil fuels are burned (heating,transport,cement making, electricity generation,industry).
• “Dirty” fuels such as coal contain nitrogen and sulphur. Produce particles and acids.
• “Cleaner” fuels such as natural gas produce mainly carbon dioxide (and some methane which escapes)
• Cooling towers release mainly steam.

What would happen if burned all the fossil fuels?

• Carbon dioxide could increase at most from 0.045% (450 ppm) to 0.45% (4500ppm)
• It may have been 0.2% (2000 ppm) in the Eocene when the world was much warmer, but may have been greater than 4000 ppm in the Ordovician when the world was much cooler
• Carbon dioxide would still be a very small component of the atmosphere.
• Oxygen concentrations would remain essentially unchanged.

Schematic diagram

Additions to atmosphere through fossil fuel burning

(1 billion tons carbon = 3.7 billion tons CO2)

Additions to atmosphere through fossil fuel burning

(1 billion tons carbon = 3.7 billion tons CO2)

What does the IPCC predict?

What happens to carbon dioxide emissions through fossil fuel burning?

• Total emissions around 10 Pgs carbon per year (still increasing). Stock in atmosphere ~750 Pgs

• Emissions have added ~270 Pgs since the industrial revolution

• Around 50% is added to the atmosphere ~ 5 Pgs per year

• About 25% (uncertain) dissolves in the sea ~2.5 Pgs per year

• About 25% is absorbed by terrestrial ecosystems ~ 2.5 Pgs per year

Do forests take up carbon?

• Forests release carbon when they are cut down

• Forests absorb carbon when they are growing

• Estimated total forest sink of 2.4 +/- 0.4 petagrams of carbon per year (Pg C year(-1)) (Pan et al. 2011)

• Losses through deforestation may be ~1.1 Pg

• Other terrestrial systems also take up carbon and store it.

How does carbon flow through the system?

What is the greenhouse effect?

How does carbon accumulate in plants?

• Leaves hold carbon for short time scales

• Lignin in trees and woody plants leads to long term storage

• Storage in roots may be substantial

How does carbon accumulate in the soil?

Example of measurements

Measurements of carbon and radiocarbon (C-14) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2-5 years for recognizable leaf litter, 5-10 years for root litter, 40-100+ years for low density humified material and > 100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material.(Gaudinski et al. 2000)

What about agricultural systems?

• Typically only a small fraction of the carbon is harvested as food

• Carbon cycles partly through herbivores in grasslands

• Crop resides and dead grass lead to direct inputs.

• However, studies suggest that roots contribute more to long term below ground storage (Kätterer et al. 2011)

What happens to carbon in the oceans?

What happens when carbon dioxide dissolves in the ocean?

• Oceans tend to sequester more carbon than they release (Feely et al. 2004)

• However, carbon from deep oceans can return to the surface

• Ocean acidification was originally considered to be of concern due to nitric acid (Vitousek et al. 1997)

• Carbonic acid concentrations have not yet risen high enough to affect pH.

• Considerable uncertainty regarding long term effects

• Early predictions from modelling enhanced CO2 do not appear to have been fully validated (Orr et al. 2005)

What happens when carbon dioxide dissolves in the ocean?

“Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluses, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo events may be only imperfect analogs to current conditions.”

(Doney et al. 2009)

Why are we concerned about rising atmospheric CO2?

(Intergovernmental Panel on Climate Change 2014)

Mauna Loa carbon dioxide records

Global temperature anomalies

Temperatures at Oxford

Why is there alarm over rising carbon dioxide levels?

Do all atmospheric scientists share this alarm?

• There is no doubt that \(CO_2\) is a “greenhouse gas”
• However, the tipping point cannot be the result of the effect of \(CO_2\) alone, without additional feedbacks.
• There is no empirical evidence, so far, that a tipping point has been reached.
• The concern is based on results from global circulation models
• Models do not (as yet) predict local climates.
• All models that predict strong warming due to the atmospheric CO2 increase from human emissions do so because they include a strongly positive hydrological feedback (water-vapor, cloud, and albedo feedbacks). If the hydrological feedback is weak the warming would be modest.

Feedbacks are complex

“A feedback occurs when part of the output from a system is added or subtracted to the input modifying the result. Amplifying feedbacks are positive and dampening feed-backs negative. Systems dominated by negative feedbacks are inherently stable and systems where positive feedbacks predominate are unstable. Earth climate system is known to be stable as, unlike other planets, it has maintained a narrow range of life-compatible temperature variability for nearly four billion years, during which all sort of events have pushed it towards both temperature extremes. Climate feedbacks cannot be observed, since by definition they are the effect of one variable on another. Feedback relationship between correlating variables can only be inferred probabilistically or estimated with models. In the same sense feedbacks cannot be deduced from observations, they must be deduced from theory or from model observations. In a complex system such as climate there is a great uncertainty that all relevant feedbacks have been correctly identified” (Vinós 2022)

Why might a 1.5C increase in mean global temperature not lead to a tipping point?

• The Earth is warmest just after the June solstice (when it is actually farthest from the sun), and coldest just after the December solstice, when it is receiving 6.9% more energy from the sun.
• Earth’s average surface temperature is about 14.5°C, which is cold by geological standards, but during the year it warms and cools by 3.8°C
• We might expect warming in the northern hemisphere summer to be balanced by cooling in the southern hemisphere (global mean includes both hemispheres). However its not.
• The global mean temperature changes by more than 1.5C, over the space of each year
• This may change the total amount of evaporation.
• However this annual fluctuation does not tip the balance permanently.

Why might a 1.5C increase in mean global temperature not lead to a tipping point?

Atmospheric cycles

What is meridional transport?

• Areas of the planet above 30 degrees north receive less solar radiation than they emit. Below 30 degrees they receive more. However the tropics don’t warm as the excess is transported.
• Meridional transport moves energy towards the winter pole.
• The winter pole radiates much more energy to space than it receives, since the sun is below the horizon.
• Virtually all energy the winter pole emits is transported there due to winter storms or is latent heat released as summer meltwater freezes

What is the “winter gatekeeper” hypothesis?

• The climate of the planet is characterized by meridional transport of energy along the latitudinal gradient in temperature.
• Meridional transport is poleward, carried out mainly by the atmosphere over the ocean basins and stronger in winter
• The Arctic in winter is the main heat sink for the planet. Part of the energy transported during the warm season is stored in the melting of ice and snow, and is lost through radiative cooling during the cold season after refreezing
• ENSO is a part of the global meridional transport system extracting heat from the equatorial band. The relative numbers of La Niña and Neutral years is set by the solar cycle, while El Niño responds to the build up of residual sub-surface heat.
• The solar cycle, quasi-biennial oscillation, and ENSO, determine the status of the ozone layer in the winter Northern Hemisphere stratosphere and through it, the status of the polar vortex and winter meridional transport.

An alternative view of climate

(Vinós 2022)

http://r.bournemouth.ac.uk:82/Ecosystems/papers/Vinos-CPPF2022.pdf

Conclusions

• The carbon cycle is the basis of all life on earth
• Carbon dioxide levels in the atmosphere are now much lower than they were throughout most geological history
• Different ecosystems cycle and store carbon to different degrees
• Measurements and estimates of carbon stocks no not support the simplest version of the gaia hypothesis
• Carbon dioxide emissions through the burning of fossil fuels will increase atmospheric carbon
• This may (or may not) be of major concern
• Atmospheric proceses are highly complex and not well understood
• Global atmospheric circulation patterns do appear to have an important role in driving changes in the climate over years and decades

Who wants net zero?