Orbiting Carbon Observatory 2 (OCO-2)

OCO-2 uses the spectra of electromagnetic radiation (this technique is called spectroscopy) to measure levels of carbon dioxide in the atmosphere. The original OCO crashed in 2009; the second iteration launched successfully in July 2014. Its measurements are particularly pertinent as carbon dioxide concentrations reached 400 parts per million (ppm) last spring at Mauna Loa, a milestone that will make recalling carbon dioxide levels easier but will also present more serious challenges to humanity. Similar sentiments are better expressed by more thoughtful scientists.

OCO-2 is now orbiting as the head of the A-Train or Afternoon Constellation, a formation of satellites which pass over the same spot on earth within 16 minutes (playing Billy Strayhorn's "Take the A-Train," I’d like to think). Formation flying is incredibly difficult for satellites, and our ability to do this so well represents tremendous technological progress for NASA, and essentially turns the A-Train satellites into one giant satellite with more sensors and instruments than would ever be possible on a single platform.

ISS-RapidScat

Launched September 20, 2014, the RapidScat experiment on the Space Station measures winds using a scatterometer, an instrument which sends out a microwave radar pulse and measures the energy reflected back. Together with ESA’s scatterometers, it is possible to cover over 90% of Earth in a day, which is particularly useful if you want to predict storms or hurricanes.

Cloud Aerosol Transport System (CATS)

A satellite mission fit for the internet: CATS. Launched in December 2014 and hosted on the ISS, CATS senses aerosols by height. This is a critical area for climate scientists. In the latest IPCC report, aerosols represented the biggest source of climate uncertainty, yet ground based measurements cannot resolve their data by altitude. Aerosols alter the energy budget of the air column, depending on how high they are: low aerosols generally warm, but high aerosols cool (depending on the species), so knowing where they are is critical to detecting their climate signal. Beyond a lack of observations and limitations with modeling, the interaction between clouds and aerosols is complex; studying it requires a knowledge of complex chemistry, mineralogy, and physics. And while measuring dust in the air may seem like a dry topic, these little particles do everything from forming clouds (without anything to condense onto, water needs to get to -13°C/8°F for hours in order to spontaneously harden) to fertilizing the Amazon.