Land Photosynthesis is Increasing


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 line 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.

The Teleological Feedback

January 6, 2020/David P. Turner

Earth system scientists commonly refer to feedbacks in the climate system. 

A feedback loop within a system means that a change in one part or component of the system induces a change in another component that either amplifies (positive feedback) or dampens (negative feedback) the initial change. 

The classic positive feedback related to global climate change and the Earth system is that warming of the global climate caused by increasing greenhouse gas concentrations in the atmosphere results in reduction in snow cover and sea ice, which causes less reflectance of solar radiation, and hence more absorption of solar radiation by Earth’s surface, and more warming.  A potential negative feedback is if warming increases evaporation, which causes more clouds, which reflect more solar radiation, and hence cool the climate.  Most of the feedbacks in the climate system are positive.

By burning fossil fuels and pushing up the atmospheric CO2 concentration, humanity is unintentionally warming the global climate and inducing multiple climate system feedbacks.

A big question is whether humanity can collectively begin to purposefully impact the Earth system in the form of a negative feedback to climate change, i.e. begin to slow down the rise in greenhouse gas concentrations and even begin to draw down those concentrations.  This willful action would be a teleological feedback to our unintended warming of the Earth system by way of greenhouse gas emissions.

Teleological feedback. The segmented line indicates the potential for a deliberate societal influence on the Earth system.

A disturbing paradox about current climate change is that by increasing the atmospheric CO2 concentration, humanity has shown that we are the equivalent of a geological force.  But humanity thus far is not organized enough to purposefully shape the Earth system. 

What we don’t have is much political will to reduce greenhouse gas emissions, nor the right international institutions to manage a global scale response. 

Political will comes from lots of sources, but maybe the most likely source is that as more and more people experience extreme weather events, sea level rise, and the other impacts of climate change, they will support mitigation efforts (e.g. a carbon tax).  Australia in 2020 appears to be a test case for this proposition.

Also, we might hope for political leaders who understand the situation and are committed to doing something about it.

Regarding global environmental governance, the size and strength of relevant international institutions are incommensurate with the challenge of global environmental change.  At the very least, a stronger United Nations Environmental Program or a new U.N. World Environmental Organization is needed.

Recommended Reading

Lenton, T. 2016. Earth System Science: A very short introduction. Oxford University Press.

Recommended Audio/Video

Joni Mitchell, They Paved Paradise