To quantify and model current and future atmospheric compositions and to better understand tropospheric carbon dioxide (CO2) exchange with the land and oceans, accurate and precise measurements are required. Active sensing of CO2 can be accomplished with light detection and ranging (lidar). Unlike passive spectrometers that rely on sunlight, an active lidar provides significantly better wavelength and spatial resolution and can perform sensing continuously during the day, night, and over the ocean. Lidar instruments are extremely complex optical systems, costly, bulky, and power hungry. Photonic integrated circuits (PICs) on the other hand allow integration of all the required optical functions of a lidar on a single chip. Furthermore, a variety of integration platforms, materials, and devices exist that allow a high degree of flexibility to meet the required stringent specifications. In this work, an integrated path differential absorption lidar seed laser PIC is designed for active sensing of CO2. All optical functions, with exception of a gas cell reference, are integrated onto a chip approximately 10 mm2. First, using an integrated phase modulator, a master widely tunable SG-DBR laser is stabilized to a gas cell reference using a frequency modulation technique. Using an optical phase locked loop, a slave laser is locked to the master laser and stepped across multiple sampling points to map a CO2 absorption line at 1572 nm at offsets up to ±15 GHz. Before processing by the optical phase locked loop, the offset frequency between the lasers is detected with an on-chip high-speed photodiode in the form of a beat note. Finally, a pulse carver generates high extinction ratio pulses for transmission. Using this PIC, we demonstrated a factor of 235 improvement in the master laser frequency stability over a 1-hr period using 1-second gate times. We also demonstrated slave laser locking to the master laser at offsets up to ±15 GHz, albeit we relied on an off-chip detector. Also demonstrated were pulses with more than 40 dB extinction ratio. Finally, we successfully demonstrated continuous wave sensing of the 1572 nm CO2 absorption line. The work illustrated here shows that PICs are a very promising technology for earth science and sensing lidar at significantly reduced cost, size, weight, and power.