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Fawwaz Ulaby - How Radar Connects to Carbon Economics

Last Thursday I went to a Electrical Engineering colloquium at the UW. The speaker was Fawwaz Ulaby of the University of Michigan, and he presented a project based on using radar satellites to audit carbon sinks.

The talk started with a long setting of the scene by explaining why global warming is both likely and undesirable. It's a topic close to my heart, but it's also one that's been done to death with the recent IPCC report, so instead of relaying the argument in all its detail I'll just list a few statistics from Ulaby's presentation:

  • We have historical records going back about 400,000 years from Antarctic ice cores (the longest of which is 3 kilometres).
  • These records tell us the average temperature over a year (from the ratio between two isotopes of oxygen dissolved in the ice), and the CO2 concentration in the air (proportional to the concentration dissolved in the ice).
  • The highest atmospheric CO2 concentration before the industrial revolution was around 280ppm; right now it's 370ppm.
  • Projections for the atmospheric CO2 concentration in 2100 range from 478-971ppm.
  • Warming so far has been around 1°C, and credible predictions are for a further 2-5° warming this century.
  • Sea level rise projections by 2100 range from 1 to 3 feet
Personally, I find all of this rather frightening, but Ulaby's point (much like Jared Diamond's in Collapse) was less to instil fear than to underline that this is a problem we must do something about. Just like Diamond, he anchored his argument with examples of successful management of environmental problems; in this case how SO2 and NO emissions trading have had considerable success in reducing acid rain in North America. There is now talk of instituting similar emissions trading schemes for carbon, but carbon-related schemes have the additional complication of allowing for carbon sinks.

A carbon sink is a development that is intended to take carbon out of the atmosphere. The difficulty with such a project is that it can be very hard to monitor; forests are the most obvious sort of carbon sink, and even without considering the danger of fraud it's very difficult to determine how much carbon is locked up in a given tract of forest. For an emissions trading scheme that allows carbon sink owners/managers to monetise their contribution, we need a way of auditing for fraud and valuing the carbon sink correctly, which is where radar comes in.

It turns out that there are several useful properties of radar as an imaging technology for surveying a forest remotely:

  • It's possible to build up a composite image from several images taken as the satellite travels over the surveyed area, to get a resolution of 4 inches from orbit.
  • The choice of wavelength determines which materials are opaque or transparent, allowing the creation of images of the forest canopy and of levels below that.
  • Interferometry allows the construction of 3D images which are accurate enough to reveal very small changes between surveys.
  • Polarimetry (comparing the reflected images from two snapshots polarised perpendicularly to each other) gives a very detailed picture of the shapes of water droplets
This last property is particularly important: because a leaf is about 80% water, it's possible to build up a detailed picture of exactly which trees are in the surveyed tract, and both how big and how healthy they are, by considering each species's leaf shape as a very odd-shaped drop of water. To use this insight, Ulaby's team [i.e. his grad students] have conducted a detailed survey of 64,000 trees on and around the UMich campus, to come up with reference radar signatures. These can now be used to interpret images from satellites, allowing detailed forest auditing without ever having to physically visit the forest being audited.

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