IEEE Spectrum April, 2014 - 20

Higher
leakage
rates would
make
natural gas
more carbon
intensive
than coal

20

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aPR 2014

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Not all experts are buying the
latest figures, however. Harvard
University environmental engi‑
neer Daniel Jacob says the latest
inverse modeling study did not
include enough data to reliably
model emissions nationwide.
And Jacob says they conflict with
findings from top‑down estimates
that he and his colleagues assem‑
bled from two months of satellite‑
based measurements from 2004
before the premature failure of a
methane detector on Europe's
Envisat satellite. (The satellite it‑
self operated from 2002 to 2012,
five years longer than it was en‑
gineered for.) Their study affirms
the EPA's national estimate and
attributes larger‑than‑expected
emissions in the south‑central
U.S. primarily to livestock.
Where Michalak and Jacob do
concur is on what they call an ap‑
proaching revolution in top‑down
predictions due to satellite obser‑
vation. For methane, the revolu‑
tion kicks off with the €45 million
(US $62 million) Tropospheric
Ozone Monitoring Instrument
(Tropomi), which the European
Space Agency plans to launch next
year on board the Sentinel‑5 Pre‑
cursor satellite. Like its predeces‑
sor on Envisat, Tropomi measures
sunlight reflected off Earth and de‑
tects absorption of telltale wave‑
lengths to measure atmospheric
levels of methane and other gases.
But Tropomi is at least 10 times
as sensitive as the Envisat instru‑
ment, according to Johan de Vries,
Tropomi's instrument scientist at
Dutch Space, the satellite's man‑
ufacturer. And its resolution is
higher too, promising daily mea‑
sures for the entire globe at 7 by
7 kilometers. "It will mean that
you can see sources of methane
on the subcity level," says de Vries.
The data, predicts de Vries,
will have political and commer‑
cial implications as top‑down
measurements shift the bal‑
ance of responsibility for meth‑
ane emissions from one state
or economic sector to another.
He anticipates another
round of reassessment as top‑
down measurement of CO 2
improves. But that will take
more work, because CO2's ab‑
sorption signal is harder to iso‑
late than methane's from the
signals of other gases. "CO 2
measurements are at least five
years behind," says de Vries.
-peter fa ir ley

nORTh aMERICan

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SPECTRUM.IEEE.ORG

PlanetiQ

Dauria Aerospace

5 eartH-imagiNg
start-ups comiNg
to a sky Near you
A tech boom in cheap, tiny,
yet surprisingly able satellites
Words like "provocative" and "disruptive" are
rarely applied to the staid world of satellite imaging.
But that's exactly the kind of talk the industry is generating,
with the launches of more than two dozen Earth‑imaging sat‑
ellites in the last few months and more planned by year's end.
Those spacecraft, far smaller and cheaper than tradi‑
tional satellites, are being lofted not by the usual aerospace
players but by venture‑capital‑funded start‑ups. In many
cases they'll orbit in large constellations capable of revisiting
sites several times a day. Though their per‑pixel resolution
won't compete with that of satellites from market leader
Digital Globe, they'll provide fresher data at a lower price.
The press has made much of start‑up Skybox Imaging's
projected ability to count all the cars in every Walmart
parking lot in the United States on Black Friday. Other uses

clockwise from top left: plANetiQ; UrthecAst; NAsA; skybox imAgiNg; DAUriA AerospAce

in 2007 and 2008. Anna Michalak,
one of the study's authors and
an expert in remote sensing and
inverse modeling who also hap‑
pens to be at Stanford, says the
team's national methane estimate
is accurate to within 10 percent
and shows that EPA's inventories
have some holes.
"The total from our atmospheric
measurements does not equal
the sum of the parts counted in
the inventories. We need to look
at whether there are line items
that are underestimated by the
inventories, or missing entirely,"
says Michalak.
A regional estimate by the
authors of the PNAS report for
the heavily drilled south‑central
U.S. states, meanwhile, points
to oil and gas operations as a
likely undercounted methane
source. Accounting methods
comparable to those used in
the EPA's inventories (which
do not break down emissions
by region) yield methane esti‑
mates of 3 million metric tons
of carbon per year, only about
a third of what Michalak and her
colleagues found. The report's
authors conclude that the re‑
gion's oil and gas fields leak five
to seven times as much methane
as the EPA inventories capture.
Natural Resources Defense
Council staff scientist Vignesh
Gowrishankar says those higher
leakage rates-at least 7.5 percent
of gas production instead of the
1.5 percent assumed by the EPA-
would make electricity generated
from natural gas more carbon in‑
tensive than coal‑fired power. The
EPA says it is paying attention. In
January, EPA administrator Gina
McCarthy told a U.S. Senate panel
that the EPA was working with
other agencies to "assess emis‑
sions data" for methane and "ad‑
dress data gaps."
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