Tom Crowther garnered international interest in September when his group published an estimate for the number of trees on Planet Earth - 3,041,000,000,000. Here, the Yale University scientist explains how his team came to this figure and how it's being used.
Since
we published the map of the world's trees, my colleagues and I have
been bombarded with inquiries about the implications of this work. "It
is interesting to know the number of trees in the world," people have
told us, "but why is it useful?"
As the political world gears up
for the final few days of COP21, I think that these are exactly the
kinds of questions that people should be asking of scientists.
In the face of climate change - one of the most
significant global threats to life as we know it - it is our duty as
scientists to help environmental stewards and decision-makers by filling
critical gaps in our knowledge.
Climate change is a complex,
global issue, the consequences of which must be addressed across a
multitude of political, societal, economic and scientific fronts.
However, in order to mitigate its effects, we are faced with one clear challenge: we must remove carbon from the atmosphere.
Starting point
Despite
all of our best technological advances, it is nature that provides us
with our single most effective weapons in this fight against rising CO2
concentrations. Trees absorb carbon directly from the atmosphere to be
stored in their biomass and the soil.
Inspired by calls for
action from the International Panel on Climate Change (IPCC), the United
Nations Environment Programme (Unep) and Plant for the Planet designed the ambitious "Billion Tree Campaign".
By
planting and restoring vast numbers of trees, they aimed to promote
environmental stewardship and reduce atmospheric CO2 concentrations at a
global scale.
The problem was that no-one had any idea how many
trees there were to start with. Without a baseline understanding of the
Earth's forests, it was difficult to comprehend the impacts of these
restoration efforts.
Would a billion extra trees add 0.1%, 1% or 50% to the global number?
The global tree study was designed to address this critical knowledge gap.
Listen to Tom Crowther talk about the Yale study on Science In Action
By
combining satellite technology with ground-sourced information
collected by local forestry experts around the world, we generated the
most detailed map of the world's forests to date.
It revealed that the Earth's forests are home to approximately 3.041 trillion trees.
By
using strict criteria for the information that we include (e.g. using
only trees with trunks equal to or larger than 10 cm diameter), and
incorporating a huge amount of data into our predictive equations, the
margin of error around this global estimate was exceptionally small (192
billion trees).
Although there are undoubtedly billions of trees
living outside of the forested regions, their expected total number is
likely to be insignificant at this scale, fitting within this margin of
error.
This has placed the tree planting goals into a new context.
Fresh goal
One billion additional trees would only represent an increase of 0.03% on top of the current global number.
In
addition, the study revealed that we lose approximately 10 billion
trees each year, so even if the billion tree campaign was repeated
annually, it would not get us much closer to the goal of halting the net
global forest loss.
Instead of being discouraged by this news,
Unep and Plant for the Planet have used this new information as the
scientific basis to scale-up their efforts.
The new target is to
restore one trillion trees, and work has begun in earnest - the total
number of trees planted to date already exceeds 14 billion.
But the global tree map does not only tell us about tree numbers.
Carbon patterns
Tree density is an important feature of forest habitat structure.
It
can therefore serve as a guideline to scientists who are interested in
understanding patterns of biodiversity or nutrient processing.
The
ultimate goal is to understand the role of forests as a store of
carbon. To do this, we need to know about the number of trees but also
the size of those trees.
The map of tree density represents a step in the right direction.
The
next step is to generate a global map of the variation in tree sizes.
When combined with information about tree numbers, this can reveal
patterns of forest carbon storage, and help us to estimate how
deforestation or reforestation might impact atmospheric CO2
concentrations and the climate.