The trouble with relying on trees as carbon sink to do our global warming bidding

While a prime source of food, fiber & medicines still, forests may not be our climate change savior, too.

A new study based on satellite data was published this week in Nature Sustainability, claiming that humans have added nearly five and a half million square kilometers in tree cover or leaf area since 2000. China accounts for a quarter of this increase, and India accounts for roughly 7 percent — a disproportionately large contribution considering its land area.

This could be significant when it comes down to matters of reducing atmospheric carbon — a major concern today with countries setting lofty goals to keep global warming below 2 degrees Celcius. [Here's why that's important.]

The trouble with relying on trees as carbon sink to do our global warming bidding

What the comprehensive report by the International Panel on Climate Change warns can be expected from the 0.5C difference between 1.5 and 2 degrees C. Source: IPCC Report

China has become 'greener' over those 20 years as a combination of more forest cover as well as cropland (use of land for agriculture) — both in nearly equal proportions. However, India has primarily grown in its crop cover, with only a tiny increase in forest cover. Still, both countries have managed a 35 percent hike in food production with this increase in leaf cover.

If more croplands could accomplish the same effects on carbon and climate, or even if they weren't bad for carbon levels and climate somehow, this report would be good news. It would be a win for countries struggling to sustain their food production with growing demand and a second win for global goals to contain global warming.

In other words, if feeding the world’s production doesn’t harm reforestation and afforestation goals, then, better options will be available to countries to balance emissions with food security.

Leaf area changes seasonally by almost 50 percent. The factors that decide 'greening' and 'browning' of leaf cover include size of leaves, species, and density of plants in an area, as well as the number of leaves per plant. A complex formula to say the least. Further, growing seasons vary from the tropics to polar regions, but global warming has made these growing seasons longer. Unlike traditional agriculture that relied on rains, irrigation now allows multiple crops per year, which essentially lengthens the growing season beyond monsoon alone. Croplands affect the growing season with multiple cropping in a year. (1)

The question, of course, is whether the increase in leaf area has to be tree cover alone, or if croplands would help too. The relative role of forests vs. croplands in cooling global climate may depend on the extent to which forests actually reduce global warming. They two ways they do this is carbon capture and by direct cooling in their local areas. This means that the more complicated issue is whether forests cause a net cooling effect on the planet or not.

A state-of-the-art analysis on the impact forests might have on the balance of energy received by solar radiation and lost to space by infrared radiation on the planet was published in Nature in January this year. It gives no conclusive answer, only underscoring the need for more research and data before a robust conclusion can be drawn about whether forests actually cool the planet or not.

Climate negotiations have always assumed that forests are a net 'sink' of carbon and a major player in mitigating global warming.


The Kyoto protocol established mechanisms to help countries meet their emissions reduction targets, including a carbon offset to plant more trees. The 'Clean Development Mechanism' allowed developed countries to offset their carbon emissions by paying developing countries to plant trees. It seems intuitive that trees capture carbon via photosynthesis and sequester them in their biomass as well as in soils. One wouldn't be wrong about that, but that's only part of a bigger picture.

The lush greenery that comes with diversity in the tree species in the Amazon appears to be under threat from climate change. Image courtesy: WWF

The lush greenery that comes with diversity in the tree species in the Amazon appears to be under threat from climate change. Image courtesy: WWF

Measurements have shown that trees also emit a number of Volatile Organic Carbon compounds (VOCs). Isoprene — one of these VOCs — can combine with nitrogen oxides to form ozone, a potent contributor to global warming when it's anywhere below its rightful place in our planet's stratosphere. Ozone is also harmful to human health. Some forest studies have discovered that plants emit methane and nitrous oxide as well — greenhouse gases that are way more potent than carbon dioxide in influencing global warming.

Another thing that trees do is reduce Earth’s reflectivity or 'albedo'. With less solar radiation bouncing back where it comes from, the planet absorbs it and up goes the temperature again. Especially on mountains, slopes and high latitudes, darker trees like conifers can also reduce how much solar radiation is reflected from snow cover on the ground. That said, tropical forests grow faster and can capture carbon. They can also increase cloud cover and reflectivity due to their evapotranspiration — the movement of water from land into the atmosphere due to evaporation and transpiration in plants. Tropical forests were, thus, argued to have a net cooling effect on global temperature.

But estimates from satellite data of above-ground carbon sources indicate that tropical forests may also be a net source of carbon to the atmosphere due to deforestation and degradation (e.g., due to logging) and disturbances (e.g., due to fires). Meanwhile, all signatories to the Paris Climate Agreement are counting on reduced deforestation, and higher reforestation and afforestation as important tools for meeting their carbon reduction goals.

Various contributors to Earth's Albedo, or solar reflectivity. Image courtesy: YouTube/ImproveVia

Various contributors to Earth's Albedo, or solar reflectivity. Image courtesy: YouTube/ImproveVia

The scientific community appears to be torn about reporting findings that show forests aren't the big savior we've been counting on after all.

The good news may be that some Earth System Model simulations have shown that all the conversion of forest cover to croplands since the Industrial Revolution has not had a significantly detrimental effect on global warming. Needless to say that some other model simulations have disagreed with this conclusion, and found that forests are, in fact, a significant net sink of carbon. Both sides do agree on one thing though: a more comprehensive representation of 'vegetation' is called for. A representation that considers the impact vegetation has on the reflectivity of solar radiation and greenhouse gases is a far more reliable guide than we have at hand.

Food security will go up worldwide if croplands truly aren't worse than forests in their carbon footprint. However, that still isn't a free pass to turn agriculture into a free-for-all venture. Agriculture has a significant negative impact on groundwater quantity and the global nitrogen cycle. Excess nitrogen from fertilizer use and fossil fuel burning has already had a tangible, tragic impact on biodiversity, human health and climate.

Forests have enduringly been a source of food, fiber, wood, medicines, and more. They process nearly two-thirds of all freshwater in countries like America. The positive impacts from boosting forest cover can be expected to be similarly high elsewhere in the world. Forests aren't likely to run out, or run out of style any time soon, but their net impact on Earth’s climate, the water cycle and carbon uptake have to be studied further. Firm conclusions from these studies are a must for us to safely navigate the future of forests as a means to tackle carbon emission.

The author is a Professor of Atmospheric & Oceanic Science and Earth System Science at the University of Maryland, currently a Visiting Professor at IIT Bombay.


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