5 October 2019 | New Scientist | 35is a massive scientific mystery that we’re trying
to solve.” Until we do that, we can’t nurture this
precious carbon sink.
What we do know is that the oceans absorb
some 25 per cent of the carbon we emit. When it
comes to accounting for the rest of the carbon
that vanishes from the air, forests are the prime
suspects. Yet our lack of knowledge about them
was made clear last year when researchers
attempted to quantify the total mass of life on
Earth for the first time. Using measurements
from hundreds of previous studies, they
estimated that nature contains the equivalent
of about 550 billion tonnes of carbon. Bacteria
were expected to account for much of this, so
it was a big surprise to discover that they don’t.
Instead, land plants alone make up 80 per cent.
And most of this biomass is in trees.Trillions of trees
A 2015 estimate put the total number of trees
on Earth at 3.04 trillion, including 1.3 trillion in
tropical and subtropical forests, 0.66 trillion in
temperate regions and 0.74 billion in the boreal
conifer forests encircling the globe below the
Arctic. Despite this, our current knowledge
of how much carbon is contained in forests i
s still so poor that estimates for the Amazon
rainforest range from 60 to 93 billion tonnes,
a difference that isn’t far off the world’s entire
annual carbon emissions.
The most precise way to measure the carbon
in a tree is to chop it down and weigh it, trunk,
branches, roots and all. Of course, that would
kill it, so instead we take field measurements of
tree diameters and then use known densities
of different woods to calculate total biomass.
This method is time-consuming and expensive- and in practice turns out to be near-
impossible, especially in the tropics where
forests are dense and difficult to navigate.
Scanning the forests from the air or from
satellites seems an obvious solution. But
most Earth-observing sensors can take
pictures only of the tops of canopies. GEDI is
different. It uses lidar, a method that sends
thousands of laser pulses towards Earth’s
surface, which bounce back to space after
hitting solid objects. By measuring the time
it takes for a pulse to travel there and back,
the distance can be calculated. So, laser beams
penetrating a forest at different depths from
the canopy down to the ground can be used to
build a three-dimensional map of the forest.
GEDI isn’t the first lidar sensor in space,
but it is different. While others are good at
things like monitoring ice sheets, GEDI was
conceived to give the most complete picture >
“ About half of the carbon
we are pushing up into the
atmosphere disappears into
land systems somewhere”