Taking Earth's temperature

Getting some distance from a problem can provide a new perspective; sometimes that new perspective is from several hundred miles above the Earth. In recent years, debate has been fierce over how much global warming is caused by the increased presence of carbon dioxide, or CO₂, one of the major greenhouse gases. A greenhouse gas is any gas in the atmosphere that absorbs infrared radiation.

By most estimates, the total concentration of CO₂ in the atmosphere has increased by about 25 percent during the past 200 years, from about 280 parts per million to more than 370 parts per million. While the debate over global warming continues, NASA will be collecting data to help the public better understand the issue.

> NASA has 13 satellite launches scheduled for 2008. Although some satellites will study the universe beyond Earth's atmosphere and some are designed for communications, several are going up to get a different perspective on the changes happening to Earth's climate.

Two projects ' the Orbiting Carbon Observatory (OCO) and the Ocean Surface Topography Mission (OSTM) ' are experimental missions designed to deliver information that could help policy-makers deal more effectively with global warming.

NASA is scheduled to launch the OCO Dec. 15, 2008. The Jet Propulsion Laboratory at the California Institute of Technology designed the project to measure the amount of CO₂ in the Earth's atmosphere.

Scientists have been measuring CO₂ in the atmosphere for some time; 120 Earth-based stations measure CO₂ concentrations on a regular basis. However, compared with more than 4,000 Earth-based weather stations, that's not a lot of coverage.

'There are places that you just can't put stations,' said David Crisp, senior research scientist at JPL and principal investigator of the OCO mission. 'One of the best things to do is to go to space. It allows us to see the whole world.'

Although the Earth-based stations have given scientists a fairly accurate view of the total CO₂ levels in the atmosphere, they don't offer much detail about CO₂ sources and sinks, the locations such as forests and oceans that absorb CO₂.

Crisp said scientists need that information to solve one of the major greenhouse mysteries. 'We're getting pretty good at estimating the amount of carbon dioxide that's going into the atmosphere,' Crisp said. 'And we've been monitoring how much CO₂ there is in the atmosphere.' However, those numbers don't agree. 'It turns out that about half of the CO₂ we've dumped into the atmosphere over the last 50 years has disappeared. We don't know where.'

Also, sinks are more difficult to measure than CO₂ sources because carbon absorption is gradual.

'We need to make very precise measurements over a substantial amount of time to figure out what is disappearing,' Crisp said. 'If we don't know what processes are controlling CO₂ in the atmosphere today, it's going to be really hard to predict how much will be accumulating in the future.'

One satellite can measure CO₂ ' the Aqua, part of NASA's Earth Observing System. However, it isn't highly accurate and can only measure CO₂ at altitudes above 4 kilometers.

The OCO project promises more accurate measurements along with the ability to entirely cover Earth's surface every 16 days from its polar orbit.

Hardware and software

However, measuring CO₂ from space isn't easy.

'The method we use for measuring carbon dioxide most precisely at the surface of the Earth [involves filling] a container full of air, and we put it in a FedEx pouch and we ship it to Boulder, Colo.,' Crisp said. 'They hook it up to a mass spectrometer, and we can see how much carbon dioxide there is in there. 'You can't do that from space. So we have come up with a different approach.'

In that new approach, Crisp and his colleagues decided to take a reading of the light bouncing off the Earth to obtain a column measurement of the CO₂ molecules between the planet's surface and space. The reading is then processed by an onboard specialized spectrometer.

Designing the spectrometer was half the battle, Crisp said. Part of the challenge was selecting which parts of the spectrum are most sensitive to the CO₂ levels the team wanted to measure. The other major challenge was engineering the equipment to have the required sensitivity while still being able to withstand the shock of launch and conditions in space. Crisp said the OCO device should deliver a resolving power of 20,000 compared with the current maximum resolving power of 2,000.

The other hurdle was data analysis and processing. Orbital Sciences and Hamilton Sundstrand Sensor Systems will provide support in this realm.

Tom Livermore, OCO project manager, said the raw data from the OCO needs to be analyzed a lot before it's useful. For starters, variations in Earth elevations, surface pressure and cloud densities when the readings are taken can affect measurements and must be taken into account. What's more, he said, 'it's only through modeling and combining our data with other data that we have that we'll be able to estimate how the CO₂ breaks up in the different levels of the atmosphere.'

Improved software programming and faster processors have also played a major role in making OCO possible. 'The method that we originally envisioned using was going to require 256 processors running continuously in order to keep up with the dataflow,' Crisp said. 'The fastest processors we could imagine happening were 3-GHz processors. Basically, we were unable to keep up.'

Better processors and program design have changed that. 'Where it used to take as much as 22 hours to process one spectrum, it's taking 80 seconds today,' Crisp said.

OCO team members stress that theirs is a demonstration project. 'This is basically a demonstration that this can be done from space,' Livermore said. 'We expect to be able to map out the carbon dioxide in the Earth's atmosphere with the system to an accuracy of about three-tenths of 1 percent. Right now, we're probably sitting at about 5 percent or 6 percent' accuracy.

If it works as expected, the project is a bargain, Crisp said. At an estimated cost of $250 million, he said, 'we're talking about a very low-cost mission by NASA standards. If this low-cost method works, then future monitoring satellites can incorporate this technology quite easily and at relatively low cost.'

And, he added, the need for the data is only growing. 'Our primary objective, of course, is to understand the processes that are absorbing CO₂ today,' he said. 'That will help us understand how fast the climate is likely to change. We will be able to identify these sinks everywhere on Earth and send our graduate students there to figure out what's going on with it. Right now, we don't even know how to target our resources.'

While the OCO will be measuring CO₂ the atmosphere, the OSTM will be monitoring the heights of the oceans.

Monitoring the seas

A joint project of NASA, the National Oceanic and Atmospheric Administration, France's Centre National d'Etudes Spatiales, and the European Meteorological Satellite Organisation, the OSTM is expected to launch on the Jason-2 low-Earth orbiting satellite in June. So why measure the ups and downs of the ocean? The rise and fall of ocean regions is crucial in determining ocean circulation, which is crucial in forecasting weather change, hurricane intensity and the impact of global warming.

'We're talking about just a 10-centimeter or so variation,' said Lee-Lueng Fu, project scientist and head of the Ocean Science Research Element of JPL's Division of Earth and Space Sciences. 'But that holds the key to the circulation of the ocean that transports heat, carbon and other chemical properties around and as a profound influence on our lives.'

Bounce back

OSTM will employ a precision radar altimeter on Jason-2 to bounce radar signals from the sea surface back to the spacecraft. 'From the round-trip travel time of the signal, we will know the precise altitude of the spacecraft,' Fu said. 'In the meantime, we also have multiple tracking systems so we know the exact locations of the spacecraft in orbit. From these two measurements, we can calculate the height of the sea surface very precisely.'

The Jason-2 satellite is a follow-up mission to two previous projects ' the TOPEX/Poseidon and Jason-1 satellites ' that were considered to be proof-of-concept missions. 'Here, with Jason-2, we're looking forward to moving this measurement to operational use,' said Parag Vaze, OSTM project manager.

And thanks to newer equipment and better data processing, the team also expects to get more accurate data from Jason-2. 'Before the launch of TOPEX/Poseidon, we thought if we could measure within 10 centimeters of accuracy that that would be successful,' Fu said. 'But after years of effort, now we're approaching 1-centimeter accuracy.' As with the OCO mission, the OSTM team credits both hardware and data-processing improvements for the increases in accuracy.

Vaze said the equipment in orbit is based on technologies that are about 30 years old. With Jason-2, he said, 'we've built essentially all new instruments with the same requirements, but the implementation significantly reduces the mass power volume and increases the performance.'

One result, Fu said, is that measurements not only are more accurate but also can be made closer to coast lines.

'With the previous radiometer, the footprint is fairly large,' he said. 'And when the measurement is too close to the coast, the land in the footprint contaminates the radiometer's measurement so that actually reduces the resolution. We really want to approach the coast within 10 kilometers.' Fu said the new equipment increases the maximum effective resolution by about 50 percent, down to approximately 2.5 centimeters at the Earth's surface. That compares to measurements of 3.3 centimeters with Jason-1.

Better analysis

The team has also learned how to improve software to attain better results even with the old data from earlier satellites. 'We're still reprocessing data from TOPEX/Poseidon and Jason-1,' Fu said. 'In 15 years, we've learned a whole lot more, and therefore we have better algorithms, better ways to process the data.' In addition to delivering improved accuracy, OSTM is expected to extend measurement for at least two decades.

'We have to continue this measurement because the changes in circulation happened on all scales, from days, weeks, years and decades,' Fu said. 'And the longer-scale changes are really important because it tells us how the heat is absorbed by the ocean.' As much as 80 percent of global warming gases are absorbed by the oceans, he said.

'Hopefully, we can predict the rate of global warming over the next decade or century,' Fu said. 'The sea level really holds the key to understand not only the rate of warming but also the consequences.'

And although NASA takes no position on the debate over global warming, it certainly will add some much-needed information on the subject in the decades to come with these new launches and their accompanying technology