Solving the Mysteries of Carbon Dioxide
We know a lot about carbon dioxide (CO2). We know that in 2013, the amount of CO2 in the atmosphere exceeded 400 parts per million for the first time in human history. We know that approximately one out of every four carbon dioxide molecules in the atmosphere results from human activity. We know that greenhouse gases such as CO2 trap heat; and that Earth has warmed 1.1 ˚F since 1880. And we know that global warming has not stopped.
About 50 percent of the CO2 produced by human activity remains in the atmosphere, warming the planet. But scientists don’t know where and how oceans and plants have absorbed the rest of the manmade CO2. Because they don’t fully understand these processes, they cannot predict exactly how the ocean and terrestrial sinks (systems that take in and store carbon dioxide) will behave in the future as the climate changes.
“The oceans and forests are doing a fabulous environmental service for us by taking half of what we emit and storing it,” said Peter Griffith, chief support scientist at NASA’s Carbon Cycle and Ecosystems Office. “But big questions about the future course of C02 emissions from human activities remain. How long will these services continue to be provided if there are changes in the ocean and in forest growth?”
The more CO2 that oceans absorb, the more acidic they become and the more difficult it is for them to store additional carbon; and as surface temperatures of the ocean rise, the ability of oceans to take up CO2 decreases. Forests in the circumpolar regions have increased patterns of browning over the past few decades, likely due to more fires and the encroachment of insects now able to survive at higher latitudes because of warming temperatures. How will these impacts affect the ability of oceans and forests to store carbon? And as the planet continues to warm, will the permafrost of the far North, which has sequestered frozen carbon for thousands of years, begin to thaw, decay and release CO2 and methane into the atmosphere?
To try to answer these questions, on July 2, 2014, NASA launched the $468 million Orbiting Carbon Observatory-2 (OCO-2), its first Earth remote sensing satellite dedicated to studying atmospheric carbon dioxide from space.
The satellite’s 2-year mission is to locate Earth’s sources and sinks of atmospheric carbon dioxide and monitor their change. The data OCO-2 will gather is critically important, because “You can’t manage what you can’t measure,” said David Crisp, the OCO-2 science team leader at NASA’s Jet Propulsion Laboratory in Pasadena, CA.
The satellite’s acronym, OCO, depicts the composition of a carbon dioxide molecule with two oxygen atoms hugging a single carbon atom. In 2009, a previous attempt of the mission failed when the satellite did not separate from its launch vehicle and crashed into the ocean off Antarctica.
OCO-2, weighing approximately 1000 pounds is about the size of a large couch. It will orbit 438 miles above Earth, carrying three high-resolution spectrometers to detect atmospheric carbon dioxide.
OCO-2 is taking its position at the head of the Afternoon Constellation or A-Train, a line of 5 Earth observing satellites that measure aerosols, cloud cover, air temperature and water content in the atmosphere.
OCO-2 should begin operations about 45 days after its launch. The satellite will perform 24 measurements every second—approximately a million every day—but only 10 percent of these, about 100,000, are expected to be cloud free enough to be useable. Its data will be combined with information from ground-based CO2 monitors to help scientists better understand the processes that regulate CO2 in the atmosphere.
Until now, we have been dependent on ground stations and aircraft flight to monitor CO2 emissions, explained Griffith. There are only a few dozen monitoring station towers around the world, which report on CO2 concentration right in their own vicinity. Scientists then run weather models in reverse to show where the wind has blown from to infer the source of the CO2 observed by the tower.
“OCO-2 will be like having 100,000 new towers that are spread around the world, which are being sampled on a daily basis, including over the oceans,” said Griffith.
OCO-2’s spectrometers will detect, in a given location, the distinctive molecular fingerprint of CO2. Sunlight, made up of many different wavelengths or frequencies, travels to Earth’s surface then bounces back into the atmosphere. As it does, carbon dioxide and other molecules absorb specific frequencies in the spectrum of light.
The more light that has been absorbed in a particular column of air, the more dark, narrow gaps in the spectrum, which indicate the presence of CO2. The dark gaps suggest that there is a source of CO2 on Earth’s surface beneath that column, for example, a large industrial city. Less CO2 would suggest that there is a sink absorbing the carbon dioxide, such as a forest. Each OCO-2 reading will sample a column of the atmosphere above approximately 3 square kilometers (about one square mile) of Earth’s surface.
Berrien Moore, Director of the National Weather Center and vice president for the Weather & Climate Programs at the University of Oklahoma, explained that OCO-2 is not going to measure wall-to-wall CO2 concentration, but rather narrow swaths of atmosphere around the planet as we orbit, much as a lawnmower cuts strips across a yard. “It will be able to tell how much CO2 is coming off of North America, or going into the North Atlantic. But we don’t have enough precise information to say how much is coming from a specific city,” said Moore. “Nonetheless, if we can tell how much is coming off or being taken up by the Amazon, it would be a big breakthrough.”
OCO-2’s spectrometers need to calculate the precise length of the path sunlight has traveled before reaching it in order to find CO2 sources and sinks. But often, clouds and aerosols in the atmosphere reflect sunlight back into space before it reaches Earth, shortening the sunlight’s path and confusing the spectrometer. The data from other A-Train satellites about the location and height of aerosols and clouds can correct for these effects and check the accuracy of OCO-2’s measurements.
OCO-2 will also measure fluorescence, an indicator of plant growth. As the chlorophyll molecules in plants absorb sunlight, some of the light is released as heat, and some is emitted as fluorescence, a tiny amount of light invisible to the human eye. Plants depend on water and nutrients, but when these resources are scarce, photosynthesis slows down. Solar-induced fluorescence will be able to immediately detect when plants are stressed or when photosynthesis is robust. OCO-2’s data will thus give scientists better information about how Earth’s vegetation is taking up CO2.
More precise data will lead to more accurate models of climate change, which will give us an enhanced picture of what is happening on Earth. Of the manmade CO2, half stays in the atmosphere, one-fourth is going into the ocean, and approximately one-fourth is being taken up by the biosphere (the realm of all living organisms, i.e. plants and animals). “But more seems to be going back into the biosphere than is leaving it,” said Moore. “Where is the growth occurring and why? Knowledge of where it’s going into the biosphere is important because…if we can’t determine the land and ocean uptake of CO2, we can’t understand why atmospheric CO2 is increasing.”
OCO-2 is not the first satellite to measure CO2 in the atmosphere. Japan’s GOSAT is already measuring CO2, but not as precisely as originally hoped, according to Moore. The European Space Agency, France and China are also planning satellite missions to measure CO2. Moore and Crisp are working on a satellite mission for the future that would use lasers in certain wavelengths instead of reflected sunlight to measure CO2. Lasers would enable measurements to be taken day and night and under lower light conditions. For now, however, OCO-2 will produce the most detailed analysis to date of the sources and sinks of CO2 on Earth’s surface and how they change over time.
“OCO-2 is a tool for scientists to better describe what is happening to CO2 in the atmosphere,” said Griffith. “But we already know that we need to design and implement policies that are different from business as usual.”
Hopefully OCO-2’s data will goad policymakers into action, help them craft more effective responses to climate change, both nationally and internationally; and enable them to determine which countries are keeping their commitments to reduce carbon emissions.
“Two things we know for sure,” concluded Moore. “CO2 is in the atmosphere because of our burning of fossil fuels, and at some stage, we will face a real challenge to industrial society globally…We really need to make these CO2 measurements now because we are going to have to get into the carbon management business at some point. OCO-2 and the other Japanese, French, Chinese and European satellites are evidence that we are going to get serious about this problem.”