SAT Reading - Khan Diagnostic Quiz level 2 - reading 1

Questions 1-11 are based on the following

This passage is excerpted from Joseph Mascaro, Gregory P Asner, Stuart Davies, Alex Dehgan, and Sassan Saatchi, “These Are the Days of Lasers in the Jungle,” ©2014 by Joseph Mascaro, et al.

Just as the Moon’s history was disrobed by laser ranging

50 years ago, Earth’s tropical forests are giving up their

secrets to the light. Airborne light detection and ranging—
called LiDAR—has over the last ten years become a key
5 tool that ecologists use to understand physical variation in

tropical forests across space and time. Like an MRI of the

human brain, LiDAR probes the intricate three-

dimensional architecture of the forest canopy, unveiling

carbon that forests keep out of the atmosphere, and also
10 the mounting threats to that carbon storehouse: drought,

fire, clandestine logging and brash gold-mining operations.

Even the quintessential natural disturbance of the sun-

filled light gap—long thought to enhance the incredibly

high species diversity of tropical forests—has been
15 deconstructed by laser technology.
Laser ranging in tropical forests is such a game-changing

technology that science results can scarcely get through

peer-review before they are dwarfed by still larger-scale

studies. In a decade, laser power on commercial-grade
20 LiDARs has skyrocketed and costs have plummeted. These

improvements in LiDAR technology allow airplanes to fly

faster, higher and farther, covering more forest area in a

single day than every ground-based survey that has ever

been collected in the history of tropical ecology. To
25 estimate the amount of carbon stored in a 50-hectare

tropical forest monitoring plot on the ground—the largest

field plot in the world—takes a team of 12 people about

eight months: a slog of rain and mud and snakes with tape

measures and data log books. Today’s airborne LiDARs
30 can get you to within about 10% of the same carbon

estimate in eight seconds.
It is this staggering contrast in scale between LiDAR

and fieldwork that led us here: Before this decade is out, we

could directly assess the carbon stock of every single square
35 hectare of tropical forest on Earth. We could do it just as

well as if we were standing there in the flesh with tape

measures in hand. And we could do it for far less than what

we have already spent to offset carbon emissions from

forests. . . .
40 It is easy in principle, though logistically nightmarish, to

measure carbon in tropical forests. A strict constructionist

would cut, dry and weigh the biomass of the world’s

forests. But this is a self-defeating enterprise. As a result, it

is likely that no one has measured carbon over a single
45 hectare of tropical forest, even with the most detailed field

surveys. For a century ecologists and foresters have relied

on allometric1 estimation in lieu of carbon measurements

to translate field surveys of tree diameters, heights and

wood densities into whole-forest carbon estimates. Given a
50 volume with known dimensions and density, one would

estimate its mass in a similar fashion.
As the new kid on the block, LiDAR has been tacked

onto the back end—initially thought of as kind of large-

scale helper to field surveys. Carbon estimates from the
55 field have been treated as something inherently closer to

the real thing than measurements made by LiDAR—

ground “Truth” with a capital “T”. This is perhaps

understandable historically, but vis-à-vis actual carbon,

there is no such thing as ground truth: both field and
60 LiDAR efforts rely on allometry to convert measurements

into carbon estimates. Prior to using these measurements

for carbon estimation, they exist as standardized, spatially

explicit, archivable and verifiable data—the needed

substrate for a REDD2-type accounting program.
65 Due to the constancy of the underlying measurements,

both field and LiDAR data could provide the needed

information if they covered every hectare on Earth. But, in

the case of field surveys, this is impossible. The surveys

that do exist measure a tiny amount of actual forest, and
70 so what might be verified is widely spaced. And to avoid

fraud and protect landowners, many governments keep

their plot locations secret. Satellite LiDAR data remain

sparse, providing only extrapolated, coarse-resolution

carbon estimates with very high uncertainties, and there is
75 no prospect of wall-to-wall coverage in the near future. By

2020, airborne LiDAR could give us a direct measurement

of 3-D forest structure for every hectare in the tropics: a

standardized database from which to build a carbon


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Question 1 The authors central claim in the passage is that