Tag Archives: global carbon cycle

Human Impacts on the Global Carbon Cycle: Signs of Madness and Signs of Hope

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David P. Turner / December 19, 2021

Earth System Science has come to a remarkably good understanding of the global carbon cycle in recent decades.  The various pools (stocks) of carbon have been quantified (e.g. vegetation, soil, and atmosphere), along with the annual fluxes from one pool to another.  A key revelation has been that the quantity of carbon dioxide (CO2) in the atmosphere is increasing and that the increase is driven by anthropogenic factors (fossil fuel combustion and deforestation). 

Since the rising CO2 concentration is associated with a trajectory towards dangerous climate change, humanity has slowly moved towards commitments to reduce CO2 emissions.  Some types of emissions are more glaring than others, and this blog highlights four of the most egregious examples (signs of madness). 

Likewise, there are many technical and policy options for reducing CO2 emissions or speeding CO2 uptake, and this blog highlights four of the most promising (signs of hope).

Signs of Madness

1.  Oil from Tar Sands. Given the goal of reducing anthropogenic CO2 emissions as quickly as possible, an obvious candidate for termination is extraction of oil from tar sands (Figure 1).  The whole process of extracting hydrocarbons from the Earth and refining them has an energy cost, with related CO2 emission.  Unlike conventional oil, which comes out of the ground ready for the refinery, tar sands hydrocarbons must be mechanically extracted in a bulk form that includes many contaminants.  This material is then heated to isolate the oil component, a treatment requiring substantial energy – usually provided by combustion of natural gas.  The net effect is a 15% higher overall emissions of CO2 per gallon of gasoline coming from tar sands compared to conventional oil. 

Human Impacts on the Global Carbon Cycle. Open pit mine in the tar sands oil fields of Alberta, Canada
Figure 1. Aerial photograph of open pit mine in the tar sands oil fields of Alberta, Canada.  Image Credit: Dru Ojo Jay, Dominion

2.  Tropical Zone Deforestation.  About 10% of anthropogenic CO2 emissions comes from tropical zone deforestation.  The driving factors are primarily conversion to cattle ranching, industrial and subsistence agriculture, and tree plantations (Figure 2).  In 2021, the rate of deforestation in Brazil rose 22% and Indonesia cancelled a billion-dollar agreement with Norway to reduce deforestation.  Besides direct CO2 emissions from burning and decomposition of residues, the destruction of intact forests means removal of an on-going carbon sink since most forestland is now gaining carbon as part of normal growth or accelerated growth from CO2 fertilization. 

Human Impacts on the Global Carbon Cycle. Deforestation.
Figure 2.  Deforestation in the area of the Caguan River, Brazil.  Image Credit: NASA

3.  Supersonic Passenger Jets.  United Air Lines has announced plans to operationalize a fleet of supersonic passenger jets around 2029.  Their virtue would be cutting flight times across oceans by about half (they generally aren’t used over land because of sonic booms).  Their downside is a factor of 2.5 to 7 increase in carbon emissions per passenger mile.  In theory, their engines could burn sustainable aviation fuel but there are many issues with scaling up production of this fuel if demand increased substantially.

4.  Bitcoin’s “proof-of-work” Mining Protocol.  Bitcoin is a cryptocurrency that has a particularly energy intensive mode of operation, and much of the energy is from fossil fuels.  New digital coins are mined (created) by a competitive process in which multiple computer processors race to solve a computationally intense problem.  Only one computer wins, meaning that 99.9% of energy use and associated carbon emissions are wasted.  Current Bitcoin electricity consumption is on the order of consumption by a small country.  Alternative “proof-of-stake” approaches used by other cryptocurrencies are much less energy intensive.

Signs of Hope

1.  Natural Climate Solutions (NCS).  The land surface is currently a net sink for carbon dioxide, even after accounting for effects of deforestation.  Most of that carbon accumulation is showing up in live wood (Figure 3), thus it is tracked by global forest inventories.  However, a significant amount may also be accumulating in global soils (in part because of CO2 fertilization of plant growth).  The aim of the NCS strategy is to maintain all existing land carbon sinks and foster new carbon sequestration by way of altered land management.  Besides stopping deforestation, and reforesting large tracts of previously deforested land, NCS (more broadly Nature-based Solutions) will operate in the agricultural sector, wetlands, and grasslands.  Scientists estimate that NCS could provide up to 30% of the reduction in CO2 emission needed to hit net zero emissions at the global scale by 2050.

Human Impacts on the Global Carbon Cycle. Carbon accumulation.
Figure 3.  Old-growth forest at the H.J. Andrews Experimental Forest near Blue River. Image Credit: Oregon State University.

2.  Product Certification.  World leaders made a commitment at COP26 to reduce deforestation and end it by 2030.  A major player in that effort will be nongovernmental organizations that certify forest products as being produced sustainably, notably not in association with deforestation.  Products driving deforestation – and covered to a greater or lesser degree by certification – include wood, beef, leather, soybeans, and palm oil.  The science of sourcing forest products is receiving a big boost from research in remote sensing (e.g. radar detection of land cover change) and genetic analyses.  Individuals as well as buyers for corporate supply chains are increasingly attentive to sourcing issues and now have better leverage to identify products associated with recent deforestation.

3.  Carbon taxes.  Economists have long argued that the fastest and more practical strategy for driving down anthropogenic carbon emissions is to establish taxes on fossil fuel carbon emissions.  That approach of course tends to arouse political opposition, but several case studies prove carbon taxation is possible and effective.  The province of British Columbia in Canada imposed a moderate tax on fossil fuel emission in 2008, which has reduced fuel emissions on the order of 5-15%.  Sweden has one of the oldest and highest taxes on fossil fuel emissions.  Again, follow-up studies suggest emissions have declined, while maintaining solid GPD growth.  Various strategies have been employed to insulate the most vulnerable energy consumers from price increases.

4.  Satellite-based Monitoring of Methane Leakage.  Methane is a strong greenhouse gas in its own right and is eventually oxidized in the atmosphere to CO2.  Unfortunately, methane emissions are on the rise in recent years, with leakage from expanding coal and natural gas mining and infrastructure a significant factor.  Because methane has a relatively short atmospheric lifetime (about 8 years) compared to carbon dioxide, a decrease in methane emissions would have an especially large influence on global warming in the next few decades.  Earth system scientists use satellite borne sensors to track atmospheric methane concentrations and infer regional patterns in methane emissions.  But a new generation of sensors, including one run by the Environmental Defense Fund, is transforming the attribution of leakage sources by increasing the spatial and temporal resolution of the coverage.  These sensors will contribute to monitoring the effectiveness of the Global Methane Pledge recently signed at COP26.

The world is at, or fast approaching, the year of peak carbon dioxide emissions.  The signs of madness identified here serve to push that year farther into the future.  The signs of hope will hasten its arrival and help sustain a multidecadal trajectory towards net zero emissions.

Land Photosynthesis is Increasing

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January 20, 2020/David P. Turner

An image of the global biosphere in which depth of greenness on land represents annual photosynthesis.  Wikimedia Commons

Natural Processes are Slowing the Accumulation of Carbon Dioxide in the Atmosphere − Strategic Land Management Could Boost That Trend

As global climate warms in response to rising greenhouse gas concentrations, various components of the Earth system are responding in ways that amplify or suppress the rate of change.  Most of these feedbacks are positive (amplify warming).  However, a natural negative feedback (suppresses warming) exists and it could be augmented by human actions.

Scientists generally agree that an increase in the concentration of carbon dioxide (CO2) in the atmosphere, precipitated by human activities, is a major driver of climate change.  Hence, any process induced by rising CO2 and climate change in which less CO2 is added to the atmosphere, or more CO2 is removed from the atmosphere and sequestered, constitutes a negative feedback to climate change. 

The most obvious and necessary negative feedback is a rapid reduction in fossil fuel emissions.  The 2015 Paris Agreement on Climate Change points to progress in that direction.  Unfortunately, fossil fuel emissions continue to rise. 

Research in Earth system science is examining the operation of another significant, but naturally occurring, negative feedback to climate change.  Observations suggest that the rising atmospheric CO2 concentration and associated climate change is spurring carbon sequestration by the terrestrial biosphere. 

Earth system scientists speak of the “carbon metabolism” of the terrestrial biosphere, referring to the uptake of carbon by way of photosynthesis and its release back to the atmosphere by way of respiration of plants, animals, and microbes (Figure 1).  When photosynthesis exceeds respiration, carbon is sequestered from the atmosphere.  A critical question concerns the degree to which humanity can purposefully augment this negative feedback and help slow climate change.

Figure 1.  The atmospheric CO2 concentration is a function of uptake by processes such as plant photosynthesis, and release by processes such as respiration and combustion of fossil fuels.  Wikimedia Commons.

The Terrestrial Biosphere is Speeding Up

Laboratory and chamber studies show that plant photosynthesis is generally sped up, and drought stress is alleviated, as CO2 concentration increases.  At the global scale, long-term observations are finding a trend of increasing global photosynthesis in recent decades as the CO2 concentration in the atmosphere rises.  The estimated increase is on the order of 30% based on four independent lines of evidence.

Terrestrial respiration (see Figure 1) also appears to be increasing, but at a slower rate.  The carbon mass difference between global photosynthesis and respiration is accumulating in the biosphere and helping restrain growth of the atmospheric CO2 concentration. 

The dominant reservoir for sequestered carbon is most likely wood.  Note that forests accumulate wood as they recover from disturbances.  Thus, the terrestrial biosphere uptake or “sink” for carbon is a function of both the disturbance history of global forests and the stimulation of wood production by high CO2.

One indication of an invigorated biosphere comes from observations of the atmospheric CO2 concentration at Mauna Loa Hawaii.  The iconic “Keeling curve” (Figure 2) shows an upward trend attributable mostly to fossil fuel emissions, and an annual oscillation, which is attributable to terrestrial biosphere metabolism.  The annual drawdown in concentration is driven by an excess of photosynthesis over respiration in the northern hemisphere spring, and observations of CO2 in recent decades find a strengthening of that drawdown.  Contributing factors include a longer growing season, deposition of nitrogen from polluted skies (= fertilization), and CO2 stimulation of growth.

Figure 2.  Monthly mean atmospheric carbon dioxide at Mauna Loa Observatory, Hawaii (in red).  The black curve represents the seasonally corrected data. NOAA.

Increasing carbon sequestration by the biosphere is evident from the observation that the proportion of human generated carbon emissions that stays in the atmosphere (the airborne fraction) has fallen in the last decade, despite the large upward trend in fossil fuel emissions.  The airborne fraction was 44% for the 2008-2017 period, with the remainder of emissions accumulating on the land (29%) or in the ocean (22%).

Human Augmentation of Terrestrial Biosphere Carbon Sequestration

So, we have a natural brake on the rising CO2 concentration.  And it is one that could potentially be augmented by human intention. 

Thus far, human land use impacts such as deforestation and agriculture have tended to decrease biosphere carbon storage.  However, there is a large potential to deliberately sequester carbon in terrestrial ecosystems by way of several approaches.   

1.  Expansion of the UN-REDD Programme (United Nations Reducing Emissions from Deforestation and Forest Degradation).  REDD consists of intergovernmental agreements that pay developing countries to protect forests.  The carbon benefit is both in terms of reducing carbon emissions and maintaining carbon sinks.  Remote sensing is increasingly effective in monitoring carbon stocks.  Norway has begun to make payments to Indonesia for reducing rates of deforestation.

2.  Making land management decisions in the context of the whole suite of ecosystem services.  Carbon sequestration in biomass and soil is a climate related service that compliments other services such as conservation of biodiversity.  Management of both public and private land could be shifted towards this comprehensive perspective.

3.  Planting trees − something that can be done at the scale of a suburban back yard, whole urban areas, or regions (Figure 3).  Satellite-observed greening in China is attributed in part to large scale tree planting.  Trees affect the absorption and reflection of solar radiation as well as the carbon balance, so care must be taken about planning large scale plantings.

Figure 3.  Forests accumulate large stocks of carbon relative to other vegetation cover types.  Wikimedia Commons.

These human-mediated carbon sinks will all benefit from high CO2 impacts on biosphere metabolism.  In contrast, the impacts of continuing climate change − independent of CO2 impacts − on these carbon sinks and on biosphere metabolism generally are difficult to anticipate.  At high latitudes, climate warming appears to be associated with vegetation greening.  In contrast, increased rates of disturbance in mid-latitudes − such as climate warming induced forest fire − may offset the strength of biosphere carbon sequestration.

In an optimistic scenario, radically reduced fossil fuel emissions along with increased carbon uptake by the land and ocean will cause the atmospheric CO2 concentration to peak within this century, leading to a gradual decline that is powered by biosphere sequestration (natural and augmented). 

Since we are already committed to significant climate change, that CO2 trajectory would still leave us with major − but hopefully manageable − adaptation challenges.  A stabilized CO2 concentration, would also reduce the possibility that the Earth system will cascade through of series of positive feedback tipping points.  That scenario would take hundreds to thousands of years to play out but it could push Earth into a state threatening to even a well-organized, high-technology, global civilization.