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Hank Green: Welcome to SciShow Space News, I'm Hank Green.  In a couple of weeks, NASA will use its awesome powers to observe a new target: Earth.  On July 1st, it will launch the Orbiting Carbon Observatory-2, or OCO-2, the first instrument dedicated to measuring carbon dioxide in the Earth's atmosphere.  Because CO2, as you probably know, is a big deal and the deal's getting bigger.

Carbon dioxide absorbs and reflects longer wavelengths of light, particularly infrared.  This allows it to effectively trap the sun's heat in our atmosphere.  And we humans are so into burning fossil fuels that we've thrown the planet's carbon dioxide levels off and the result is a changing climate.  By adding about 40 billion tons of carbon dioxide into the atmosphere every year, we've elevated its concentration from about 300 parts per million to about 400 parts per million, the highest level in the last 800,000 years.  

Fortunately for us, the planet has tools to maintain some balance, although we don't know much about how they work.  More than half the carbon dioxide we add to the atmosphere every year gets absorbed, some of it by the oceans, some of it on land.  We think that rainforests and other plant-rich places like grasslands do a lot of this absorbing.  We call these places carbon sinks, but we don't understand exactly where they are, what proportion of the job they're doing, and what mechanisms they're using to save us from ourselves.  

So OCO-2 aims to increase our understanding of these sinks as well as the biggest sources of CO2 so we can better predict the changing climate and plan our defense accordingly.  Up until now, the Mauna Loa Observatory in Hawaii has been our primary tool for measuring carbon dioxide in the atmosphere by analyzing the levels of infrared light being absorbed in the air, but OCO-2 will be able to measure carbon dioxide from everywhere.  After launch, it will establish a polar orbit at 705km above the Earth, passing over both poles every 99 minutes.  A cycle of 233 of these orbits will cover every latitude on Earth every 16 days, making about 16 million measurements along the way, and it'll do it for two years.  For this terrific data dump, we can thank OCO-2's three high resolution spectrometers, which are sensitive enough to focus on an area smaller than three square kilometers.  Together they will, in effect, read the Earth's reflected sunlight, spread it into its component colors and precisely measure the intensity of each component.  By sensing variations in the intensity of infrared and other wavelengths, the spectrometers will be able to detect relative concentrations of CO2 at any place at any time.  Repeat that millions of times and you'll get a pretty good idea of what's going on with global carbon dioxide.  

Now I'm excited, I don't want to jinx anything here, because OCO-2 isn't the first attempt to get this important data.  This new mission actually replaces the OCO, an instrument of similar design that crashed during launch in 2009.  So keep your fingers crossed on July 1st, or whatever superstitions you find helpful, and in the meantime, happy solstice, if you're in the Northern Hemisphere!  June 21st marks the beginning of summer, and in the South, it's now winter.  

But the solstice isn't really a day, it's a specific celestial event.  You probably know that the Earth's axis of rotation from pole to pole is tilted.  It's tilted at exactly 23.5 degrees and the direction of the axis remains fixed as it rotates the sun.  So it's like eeeeeh.  The June, or Northern Summer Solstice, is when the North pole reaches its maximum tilt toward the sun.  Here on Earth this means that the sun appears straight overhead of the Tropic of Cancer, 23.5 degrees North latitude.  Now, if you live south of the Tropic of Capricorn, the sun appeared at its lowest point in the sky, and the 21st was the shortest day of your year.  But if you live north of the Tropic of Cancer, the sun's waking up early in the morning and in the evenings it's lingering longer.  In fact, the days this time of year measured from solar noon to solar noon are more than 24 hours long, only by about 15 seconds or so, but if I were you, I'd make the most of them.

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