Deforestation and Carbon
A long and complex history
Land ecosystems hold the largest active pool of carbon in the Earth System, and account for the largest annual exchange of carbon dioxide with the atmosphere. The total amount of carbon in the vegetation and soils of land ecosystems is ~3 times the amount currently in the atmosphere.
The large green and blue arrows in this diagram capture the pre-industrial, balanced global carbon cycle. Of the 11 billion tons of carbon emitted to the atmosphere by human activities (grey arrows), 10 is primarily from fossil fuels, and 1 from impacts on land ecosystems. The fate of these 11 tons is captured in the (+) numbers: 3 dissolved in the oceans, 3 added to land ecosystems, and 5 retained in the atmosphere. That +5 includes carbon dioxide and methane that drive increased greenhouse gas warming.
Forests are the largest actors among land ecosystems, so what we do to and with forests can play an important if limited role in modifying the global carbon cycle and global warming.
Recognition of this importance was reflected by Boris Johnson at the recently concluded COP26 conference when he characterized forests, as “great teeming ecosystems,” “cathedrals of nature” and “the lungs of our planet.” The first agreement to come out of the conference included this text: “We therefore commit to working collectively to halt and reverse forest loss and land degradation by 2030 while delivering sustainable development and promoting an inclusive rural transformation.”
But this Substack site and these essays are about the science of climate change and not the politics, so let’s focus on forests and carbon. How can it be that land ecosystems are experiencing both an increase in carbon gain of 3 units, as shown above, and a simultaneous loss of 1 unit, for a net gain of 2? Is there general agreement on these numbers?
A recent essay in this series described how the carbon balance of forests can be measured, and how different parts of a forested landscape can both gain and lose carbon, depending primarily on whether the forest is allowed to grow, or is instead disturbed by either natural forces like wind and fire, or human disturbances like harvesting or clearing for agriculture.
Using long-term data from the unique set of studies at the Harvard Forest in central Massachusetts, we saw that undisturbed forests can accumulate carbon for decades to centuries, primarily by continued accumulation of wood in the stems of the trees. On the other hand, another stand at the same locale recently infected by an invasive pest began to lose carbon through tree death and decomposition. And another area that had been harvested recently showed a net loss of carbon for 3-4 years as decomposition in soils and of logging debris left on site outstripped photosynthesis by the recovering canopy.
So both inherent rates of wood production and disturbance history determine whether any small forest plot is gaining or losing carbon. The global carbon balance of forests is the sum of what happens on all of these small plots, and rates of both growth and disturbance are subject to change.
At the Harvard Forest, an increase of 93% in the rate of carbon gain by an undisturbed forest over a period of 23 years was attributed primarily to longer growing seasons in a warming world.
But carbon fertilization is also often cited to support increased carbon storage in forests. Carbon dioxide is the carbon source for photosynthesis and the current concentration in the atmosphere, although elevated by human activity, is still below the optimal level for plants. Greenhouse operators sometimes increase carbon dioxide inside their structures to increase rates of plant growth.
Both extended growing seasons and carbon dioxide fertilization can contribute to that +3 for land ecosystems in the diagram above.
But there is a larger force that drives much of the increasing and decreasing carbon storage in forests: Land use and especially land conversion - the processes of deforestation and either reforestation or afforestation.
Deforestation is a hot topic, as evidenced in the COP26 agreement, but as with many other issues, it has a very long history; a history tied to the rise in human population and the need to feed those growing numbers.
Let’s start with a regional example. This figure of the fraction of land in New England covered by forests has become an iconic image in the land use history of the region. At the onset of European occupation, the area was more than 90% forested. As the population grew and spread west and north from the initial settlements, forest conversion accelerated. Agriculture was the only other major land use, and by 1850, more than 40% of the region had been deforested for food production. In four states, conversion exceeded 60%.
When Henry David Thoreau wandered the area around Walden Pond, he saw and described a landscape that was largely devoid of forests.
The coming of the industrial revolution to New England in the mid-1800s and the opening of transportation routes to the growing Midwest broke that clear relationship between population and deforestation. While population continued to increase, food increasingly came from outside of the region, old and less productive farmlands were abandoned, and the forests reclaimed much of the landscape. The legacies of this land use history are everywhere to be seen across the region in old stone walls and in the foundations of long-gone houses with mature trees growing up through them. We used two scenes from the magnificent dioramas in the Fisher Museum at the Harvard Forest in the previous essay to visualize the change from forest to farmland. Two more here capture farm abandonment and the beginning of forest regrowth.
Note that there has been another very recent downturn in the fraction of the landscape in forests in every New England state. This is not conversion to agriculture, but rather to residential and commercial development. Unlike the switch from forests to fields, the conversion to development is very unlikely to be reversed.
This trend has prompted the establishment of an organization and plan termed “Wildlands and Woodlands” formed out of the Harvard Forest and led by the former Director, David Foster. The goal of the group and the plan is to retain 70% of the New England landscape in forest cover; 60% under sustainable management, and 10% as wildlands.
We will get to the carbon implications of this history in a minute.
This New England story is just one manifestation of the impact of population and the need for agricultural land on deforestation. There is a site published by the New York Times that is still active at this writing that captures the extent of global deforestation since the inception of settled agriculture. It is worth a look (link below under Sources). For each continent, the site maps the distribution of forests as envisioned 8,000 years ago, and the current distribution. It’s a sobering analysis that demonstrates the fragility of forests and the impact of population density.
I use this site in my classes, and ask the students to estimate the degree of deforestation in each of the regions, and then plot this against population density for individual countries or subregions. There is a strong relationship. Western Europe, China and India with long histories of cultural and agricultural development, all show ~80-90% forest loss. The Eastern U.S. retains about 50% of its forest area, while the Congo basin and Brazil, with low population densities, have lost ~ 20-30% of original forest cover. This simple relationship results from all of the complex factors controlling the distribution and abundance of humans and forests across the planet.
A fundamental factor in this deforestation history is that the same climate regimes that support forests are also the ones that will best support agriculture and population growth, hence the historical continuity of competing land uses.
How does this history pertain to carbon in the atmosphere? The goal of the COP26 agreement is to stop the loss of forest area and to increase that area if possible, all with the goal of reducing carbon dioxide in the atmosphere.
Richard Houghton of the Woodwell Climate Research Center has been compiling and summarizing changes in carbon storage in native and managed ecosystems due to land use change for decades. His most recent publication on the topic presents a dynamic view of human impact on the rate of loss (and gain) of carbon since 1850.
While the full article presents detailed numbers for 10 countries and regions across the globe, these two figures capture the major dynamic. Non-tropics would include North America, Europe and China. Tropics would be most influenced by the Amazon region in Brazil, Tropical Africa and parts of Southeast Asia.
With the 1850 start date for these figures, much of the loss of carbon from originally forested areas in Europe, Asia and eastern North America has already occurred. The figure above on changes in forest cover in New England from 1600 to the present indicates that the region was a major source of carbon to the atmosphere before 1850. Continued expansion of agriculture and forest harvesting in the western U.S. and other non-tropical regions combined with tropical deforestation to produce net carbon losses in all areas through 1950. Since that time, losses have been dominated by deforestation in the tropics, while forest recovery and plantations in the Northern Non-tropics have driven increased carbon storage.
So carbon sinks currently claimed for forests in the U.S. result in large part from the historical change in forest land cover like that presented for New England. These forests are currently major carbon sinks in part because they were, until 1850, major carbon sources. Continued carbon gains in the currently undisturbed Harvard Forest site reflect disturbances decades ago.
Let’s shift the focus to more recent changes in forest area. International agreements reached in 2004 and 2007 were intended to reduce deforestation, and a Strategic Plan for Forests 2017–2030 was adopted by the U.N. General Assembly in 2017. Have these had an impact?
Reporting on deforestation often concentrates only on forest area lost, often just in the tropics, while ignoring forest area gained over time. For example, one source reports that deforestation has increased from about 14 to more than 25 million hectares per year since 2000.
Another source of information on recent changes in forest land cover and carbon stocks is the United Nations Food and Agriculture Organization (FAO). Based on reports from participating countries and remote sensing data, the FAO tracks both forest area loss and gain. Forest expansion, which has increased forest area substantially, especially in China, is about 50% new plantations. Net loss of forest area (the difference between the red and green bars) has declined from 8 million hectares per year in 1990-2000 to 5 in the three most recent periods. Net forest loss since 1990 totals about 180 million out of about 4 billion hectares of forests globally.
While a net loss in forest land area would suggest a net loss of carbon as well, the FAO report includes increases in carbon storage within forested areas remaining in forest. Results show the same spatial trends as Houghton’s data over the last 30 years based on changes in land use alone, with carbon gain by forests in Europe, China (East Asia) and North America, balanced against losses primarily in Africa, Southeast Asia and South America. However, with carbon gains by existing forests included, FAO reports that global forests have lost less than 1% of total carbon stocks over the last 30 years (a net change of about 6 billion tons out of about 660 billion).
So what can we conclude about the current role of forests in the global carbon cycle? The first figure at the top of this essay says land ecosystems lose one billion tons of carbon per year through land use change but gain 3 through forest regrowth and enhanced photosynthesis. Houghton’s results support a current net loss of 1 billion tons through land use change. The Harvard Forest results suggest an increase in carbon gain through longer growing seasons and perhaps carbon fertilization, and the FAO data report infers that increased growth rates basically offset deforestation to yield a net carbon balance near zero. Zero (FAO) or plus two (Top diagram) – which is it?
Unlike carbon dioxide in the atmosphere, which mixes rapidly and can be measured accurately at a single location (e.g. Mauna Loa in Hawaii - see citation for the Keeling Curve under Sources), forests are immobile and highly variable over short distances, making accurate summaries over large areas very challenging. Zero and two may be statistically indistinguishable.
However, the large stores of carbon in forests and the potential for rapid exchanges with the atmosphere suggests that conserving forests will be a valuable tool for maintaining or reducing future carbon dioxide concentrations. We will need to continue tracking forest extent and carbon storage in detail to capture the impacts of economic and environmental policies, and to continually refine that total net carbon balance number.
So kudos to the delegates at COP26 for prioritizing the conservation and possible recovery of forests. We will have to wait and see if the implementation of this agreement can actually bring the net loss of forested land to zero!
Acknowledgement
Thanks to Dr. Richard Houghton for his review of this essay and the link to the most recent paper on global carbon balances (first citation below).
Sources
Data for the diagram of the global carbon budget are drawn from:
https://essd.copernicus.org/preprints/essd-2021-386/
https://airs.jpl.nasa.gov/resources/155/global-carbon-cycle/
https://earthobservatory.nasa.gov/features/CarbonCycle
The full statement out of COP26 regarding forest conservation and land use is here:
https://ukcop26.org/glasgow-leaders-declaration-on-forests-and-land-use/
The United Nations Strategic Plan for Forests 2017-2030 is here:
https://www.un.org/esa/forests/documents/un-strategic-plan-for-forests-2030/index.html
The figure on changes in forest cover in New England over time, as well as an introduction to the Wildlands and Woodlands project can be found here:
And a wonderful book placing Thoreau’s writings in the ecological context of his time is:
Foster, D. 2001. Thoreau’s Country: Journey through a Transformed Landscape. Harvard University Press. Cambridge, MA. 288pp
The Harvard Forest dioramas can be seen here:
https://harvardforest.fas.harvard.edu/dioramas
The interactive site on historical deforestation over the last 8000 years is here:
http://archive.nytimes.com/www.nytimes.com/interactive/2011/10/01/science/earth/forests.html?_r=0
The Houghton article on historical deforestation is here:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GB005546
One report suggesting much higher rates of deforestation than reported by the FAO is here:
using data from:
https://www.climatefocus.com/initiatives/nydf-progress-assessment
The FAO report is here:
FAO. 2020. Global Forest Resources Assessment 2020: Main report. Rome.
https://www.fao.org/documents/card/en/c/ca9825en
The famous Keeling Curve of atmospheric carbon dioxide content is here:
https://keelingcurve.ucsd.edu/