Measuring the Elements of Climate Change: Tree Canopy

Series: Measuring the Elements of Climate Change

By Mark Tausig

Discussions of climate change often refer to phenomena such as global warming and increased greenhouse gas emissions.  Ever wonder how we measure such things?  How do we know that the global temperature has increased? How do we measure greenhouse gas amounts? What about decarbonization, tree canopies, auto and truck emissions, sea-level rise, glacier melting, and environmental justice? 

In this series, we will learn how climate change indicators are measured and how they change over time.  Reliable, ongoing measurement of these conditions is essential for monitoring climate change and promoting climate action.

Series Part 4: Measuring the Tree Canopy

The Global Tree Canopy

Forest canopies are hotspots of biological diversity, engines of global biochemical processes, and the dynamic interface between organic nature and the atmosphere. Forest and canopy degradation reduces carbon storage, biodiversity, macro- and microclimate regulation, and other ecosystem services, with significant implications for human livelihoods.  

–Nakamura, A., et al. (2017) Forests and Their Canopies: Achievements and Horizons in Canopy Science. Trends in Ecology & Evolution, Vol. 32, No. 6.

Trees combat climate change by capturing carbon dioxide and managing stormwater runoff. They pull carbon dioxide from the air during photosynthesis and store it in their trunks, branches, and roots. Leaves catch dust, smog, and harmful gases like nitrogen dioxide, making the air cleaner. A thick tree canopy can reduce surface temperatures by blocking direct solar radiation. Trees release water vapor that cools the surrounding air (I know that because I run in Prospect Park). Roots and soil filter out dangerous chemicals and keep local streams and rivers cleaner.

Restoring or adding forest cover is often viewed as essential for slowing global warming.  As such, it is important to measure and monitor changes in tree canopy coverage (and other dimensions of forest conditions).  Tree height and structure directly correlate with the amount of carbon a forest stores. Tracking loss and regrowth helps calculate exact carbon releases and uptakes in the global carbon cycle. Canopy health signals overall ecosystem stability and a region’s capacity to support sustained plant life.

Measuring the global tree canopy involves advanced satellite remote sensing, space-based LiDAR (Light Detection and Ranging-a remote sensing method that shoots rapid laser pulses down to the Earth), and artificial intelligence computations. These techniques map tree cover percentage, individual tree detection, and 3-dimensional canopy heights at resolutions down to 1 meter globally.  These maps are used to monitor deforestation, restoration, and global climate effects.  It is estimated that trees and forests cover approximately 30 to 31 percent of the global land mass, which amounts to about 15.7 to 16 million square miles.

     Tree canopy NASA map by Robert Simmon.

The State Tree Canopy

Because tree canopy coverage at the global level is derived from adding together country and regional measurements, we have good measures of coverage at the regional and local levels (e.g, New York State and New York City). 

New York State is heavily forested, with trees covering about 18.6 million acres or 62% of its total land area.  

To manage the tree canopy in New York State, The Department of Environmental Conservation and the Division of Lands and Forests note the use the following strategies: keep forests as forests, plant more trees and forests across New York State, support the use of wood for long-term products and to replace fossil fuels and, promote and use sustainable practices while harvesting timber and manage both State and privately-owned forests with carbon storage, carbon sequestration, and climate resilience in mind.

The Urban Tree Canopy

“The NYC urban forest is a social-ecological system composed of all the trees in NYC and the physical and social infrastructure that supports them. It includes approximately 7+ million trees on public and private property, covering a variety of forest types, from street trees to natural forests, as well as the soil and forest understory. The tree canopy covers 22 percent of NYC (in 2021, 23.4 percent),  with a citywide goal of achieving at least 30% tree canopy cover by 2035 (now 2040).”  Tree canopy is defined as the parts of a tree or group of trees, including the leaves, branches, and stems, that shade the ground when viewed from above. It is a key metric for understanding the extent of the urban forest

—NYC Urban  Forest Agenda, 2021, 2026.

Seven million trees, covering 22 percent of New York City?  How do we measure the content and distribution of all that tree cover?

Measurement methods include technology like LiDAR and aerial imagery, which allows the city to map the three-dimensional layer of leaves, branches, and stems using laser technology to measure light reflection and elevation data from above. There is also the  Street Tree Censuses: thousands of trained volunteers map individual street trees using mobile apps, tape measures, and survey wheels to update public data.  Every 10 years since 1995, NYC Parks has conducted a street tree census to document the city’s urban forest, including tree location, condition, size, and species, and how these conditions have changed over time. Finally, AI and satellite models are used.  Recent studies also apply computer vision and machine learning. 

These measurements also reveal the uneven distribution of the canopy. At the borough level, canopy cover ranges from 19.5 percent to 33.2 percent. Within each borough, pronounced disparities exist across neighborhoods, with canopy cover ranging from less than 10 percent to more than 40 percent.  Measuring the tree canopy has been useful at the “disaggregated” level of neighborhood or even by block.  

Tree canopy is measured using similar methods at the local and state and regional levels.  However, at each level these measurements are used differently.  In New York City, for example, tree canopy measurements are used to plan canopy expansion, monitor and maintain the existing canopy, and address environmental justice issues.  At the state level, it’s used to monitor, maintain, and expand tree cover and forests and to regulate timber-related industries in the state.   

In all cases, accurate measurement is the key to planning, discourse, debate – and progress.

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Mark Tausig, Ph.D., is a volunteer writer for the New York League of Conservation Voters. He is a retired Professor of Sociology, where he studied health disparities, social networks, work and mental health, international health, and population aging in low—and middle-income countries. His latest book, Population Aging in Societal Context: Evidence from Nepal, will be published by Routledge later this year (2025).